Compressed Air
Content
How to save energy and money
Guide Book 3
COMPRESSED AIR SYSTEMS
STRATEGY
ENERGY EFFICIENCY EARNINGS
3E
STRATEGY
Netherlands Ministery of Economic Affairs
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Technical Services International
TSI
EUROPEAN COMMISSION
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HOW TO SAVE ENERGY AND MONEY IN COMPRESSED AIR SYSTEMS
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This booklet is part of the 3E strategy series. It provides advice on practical ways of improving energy efficiency in compressed air systems. Prepared for the European Commission DG TREN by: The Energy Research Institute Department of Mechanical Engineering University of Cape Town Rondebosch 7700 Cape Town South Africa This project is funded by the European Commission and co-funded by the Dutch Ministry of Economics, the South African Department of Minerals and Energy and Technology Services International , with the Chief contractor being ETSU. Neither the European Commission, nor any person acting on behalf of the commission, nor NOVEM, ETSU, ERI, nor any of the information sources is responsible for the use of the information contained in this publication The views and judgements given in this publication do not necessarily represent the views of the European Commission
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HOW TO SAVE ENERGY AND MONEY IN COMPRESSED AIR SYSTEMS
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HOW TO SAVE ENERGY AND MONEY IN COMPRESSED AIR SYSTEMS
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Other titles in the 3E strategy series: HOW TO HOW TO HOW TO HOW TO HOW TO HOW TO SAVE SAVE SAVE SAVE SAVE SAVE ENERGY AND ENERGY AND ENERGY AND ENERGY AND ENERGY AND ENERGY AND MONEY:THE 3E STRATEGY MONEY IN ELECTRICITY USE MONEY IN BOILERS AND FURNACES MONEY IN STEAM SYSTEMS MONEY IN REFRIGERATION MONEY IN insulation
Copies of these guides may be obtained from: The Energy Research Institute Department of Mechanical Engineering University of Cape Town Rondebosch 7700 Cape Town South Africa Tel No: (+27 21) 650 3892 Fax No: (+27 21) 686 4838 Email: [email protected] Website: http://www.3e.uct.ac.za/introduction.htm
ACKNOWLEDGEMENTS
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The Energy Research Institute would like to acknowledge the following for their contribution in the production of this guide: • • • Energy Technology Support Unit (ETSU), UK, for permission to use information from the “Energy Efficiency Best Practice” series of handbooks. Wilma Walden of Studio.com for graphic design work ([email protected]). Doug Geddes of South African Breweries for the cover colour photography.
Guide Book Essentials:
QUICK ‘CHECK-LIST’ FOR SAVING ENERGY and MONEY IN COMPRESSED AIR SYSTEMS
This list is a selected summary of energy and cost savings opportunities outlined in the text. Many more are detailed in the body of the booklet.These are intended to be a quick ‘checklist’. EFFICIENT COMPRESSED AIR DISTRIBUTION: • Establish and audit the compressed air needs of the plant. Identify system oversupply in terms of air quality or pressures. • Remove/isolate dead lags and minimise pressure drops. • Ensure the compressor plant is operating according to manufacturers/suppliers specifications. • Minimise the air leakage rate. A planned maintenance program to cover the air distribution system is useful. COMPRESSED AIR TREATMENT: • Keep air treatment to the minimum possible. If only one user in the system requires high quality air, consider treatment of air at that point. COMPRESSED AIR GENERATION: • Make sure the air intake is cool and clean. Use outside air for compression where possible. • Generate compressed air at the lowest possible pressure that will meet site requirements. • Ensure that the design of the plant compressed air distribution system does not produce an excessive pressure drop between generation plant and end user. • Recover heat of compression from inter-coolers and after-coolers wherever possible. • Ensure that the control systems installed result in efficient operation. Investigate the possibility of sequencing a plant with more than one compressor unit. • Control generation to meet demand closely and efficiently. Make sure the control system ensures that energy consumption is minimised during low or no-load conditions for individual machines. MONITORING: • Set up a targeting and monitoring program. This will help ‘benchmark’ consumption and indicate future irregularities. AUDITING: • In order to establish the status quo, and quantify wastage an audit – as described in chapter 8 of this booklet - should be carried out.
Table of Contents
UNITS ............................................................................................................................................................................................................................1 1. INTRODUCTION............................................................................................................................................................................................3 2. ECONOMICS OF COMPRESSED AIR PRODUCTION.........................................................................................................5 2.1 Calculating electricity costs – a simple calculation ...........................................................................................................5 2.1.1 For a compressor running at full load.........................................................................................................................5 2.1.2 A Calculation using Measurements ..............................................................................................................................6 2.2 A Calculation for Part Load Operation ...................................................................................................................................6 2.3 Savings from Performance Improvements...............................................................................................................................6 3. REDUCING COMPRESSED AIR DEMAND TO SAVE ENERGY .....................................................................................7 3.1 Compressed Air Usage........................................................................................................................................................................7 3.2 Assessing Air Needs ..............................................................................................................................................................................7 3.2.1 Air Quality...................................................................................................................................................................................7 3.2.2 Air Quantity - Capacity .......................................................................................................................................................8 3.2.3 Load Profile.................................................................................................................................................................................8 3.3 Compressed Air Misuse ......................................................................................................................................................................9 3.3.1 Unregulated End-Uses..........................................................................................................................................................9 3.3.2 Abandoned Equipment.....................................................................................................................................................10 3.3.3 Periods when Compressed Air is not Required ................................................................................................10 3.4 Leaks .........................................................................................................................................................................................................10 3.4.1 Estimating Amount of Leakage.....................................................................................................................................11 3.4.2 Leak Detection ......................................................................................................................................................................12 3.4.3 How to Fix Leaks .................................................................................................................................................................12 3.4.4 A Leak Prevention Program...........................................................................................................................................13 4. EFFICIENT COMPRESSED AIR DISTRIBUTION .....................................................................................................................14 4.1 Introduction..............................................................................................................................................................................................14 4.2 Distribution Main..................................................................................................................................................................................15 4.3 Water Drainage .....................................................................................................................................................................................18 4.4 Drain Traps................................................................................................................................................................................................18 4.5 Air Receivers ...........................................................................................................................................................................................19 4.6 Regulators..................................................................................................................................................................................................20 4.7 Distribution Network........................................................................................................................................................................20 4.8 Maintenance .............................................................................................................................................................................................21 4.9 System isolation.....................................................................................................................................................................................21
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COMPRESSED AIR TREATMENT........................................................................................................................................................22 5.1 Introduction..............................................................................................................................................................................................22 5.2 Dryers ........................................................................................................................................................................................................24 5.2.1 Desiccant Dryers..................................................................................................................................................................24 5.2.2 Sorption Dryers ....................................................................................................................................................................25 5.2.3 Deliquescent (Absorption) Dryers............................................................................................................................25 5.2.4 Refrigeration Dryers ..........................................................................................................................................................25 5.3 After Filters ..............................................................................................................................................................................................25 5.4 Air Intakes ................................................................................................................................................................................................25 5.4.1 Air Inlet Cooling....................................................................................................................................................................26 5.5 Installation, Configuration and Sizing ......................................................................................................................................26 5.6 Treatment Systems Maintenance................................................................................................................................................27 5.7 Potential saving areas ........................................................................................................................................................................27 6. COMPRESSED AIR GENERATION ..................................................................................................................................................28 6.1 Positive Displacement Compressors ......................................................................................................................................29 6.1.1 Reciprocating Compressors...........................................................................................................................................29 6.1.2 Rotary Positive Displacement Compressors........................................................................................................29 6.2 Dynamic Compressors ....................................................................................................................................................................30 6.3 Energy Efficient Compressor Selection..................................................................................................................................30 6.4 Energy Efficient Compressor Control ....................................................................................................................................31 6.4.1 Individual Compressor Control ........................................................................................................................................32 6.4.2 Multiple Compressor Control ......................................................................................................................................34 6.4.3 The Importance of Pressure Control ......................................................................................................................35 6.5 Sizing ........................................................................................................................................................................................................36 6.6 Maintenance ............................................................................................................................................................................................36 6.6.1 Compressor Package..........................................................................................................................................................37 6.6.2 Compressor Drives ............................................................................................................................................................37 6.7 Heat Recovery........................................................................................................................................................................................39 6.7.1 Heat Recovery with Air-cooled Compressors....................................................................................................39 6.7.2 Heat Recovery with Water-cooled Compressors ............................................................................................40 6.7.3 Calculating Compressor Heat Energy Savings ....................................................................................................40 6.7.4 Use of Hot Compressed Air for Process Duties ..............................................................................................40 6.8 Site Integration to Save Energy ..................................................................................................................................................41
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7. MONITORING OF COMPRESSED AIR SYSTEMS..................................................................................................................42 7.1 Monitoring ................................................................................................................................................................................................42 7.1.1 Compressors ..........................................................................................................................................................................42 7.1.2 Air Flow Meters ....................................................................................................................................................................43 7.1.3 Compressor House Airflow Metering......................................................................................................................44 7.1.4 Distribution Line Airflow Metering ............................................................................................................................45 7.2 Monitoring and targeting ................................................................................................................................................................45 8. COMPRESSED AIR AUDITS ..................................................................................................................................................................46 8.1 End Usage Audit ....................................................................................................................................................................................46 8.1.1 Leaks ............................................................................................................................................................................................46 8.1.2 End Users..................................................................................................................................................................................47 8.2 Distribution Network Audit..........................................................................................................................................................47 8.3 Air Treatment Audit ............................................................................................................................................................................47 8.4 Compressor House Audit ..............................................................................................................................................................47 APPENDIX 1: METER DETAILS ................................................................................................................................................................49 A1.1 Pitot Tube ............................................................................................................................................................................................49 A1.2 Orifice Plate........................................................................................................................................................................................49 A1.3 Turbine Meters..................................................................................................................................................................................50 A1.4 Vortex Shedding Meters ............................................................................................................................................................51
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UNITS
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Since South Africa mainly uses metric units, these are the first choice in this guide. However, Imperial units are often given as well in parenthesis. The units used are given in the table below:
Table 1: Unit Conversions Pressure absolute Pressure gauge Flow, volumetric l/sec Power Energy Specific energy Metric bar barg l/sec cfm kW kWh J/l Imperial psi psig cfm 1 l/s = 2 cfm (approx) hp Btu Conversion 1 barg = 14.7 psig 1 barg = 14.7 psi 1 l/s = 2 cfm (approx) J/l 1 kW = 1.34 hp 1 kWh = 3412.4 Btu
ABBREVIATIONS: psi: pounds per square inch psig: pounds per square inch gauge l/sec: litres per second cfm: cubic feet per minute J/l: Joules/litre kW: kilowatt hp: horsepower kWh: kilowatt-hour Btu: British thermal units Pressure absolute = pressure gauge + 1 bar 1 bar = 100 kPa Standard atmospheric pressure = 1.013 bar
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1. INTRODUCTION
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This guide has been written to provide those responsible for energy management and maintenance of compressed air systems with opportunities for optimising system performance in terms of cost. Significant cost savings can be realised as a result of implementation of the energy efficient measures outlined in this booklet. avoidable wastage. This can be done without compromising production at all. Another factor that should be taken into account is the awareness of end users, who often tend to regard compressed air as a free and convenient resource and usually overlook the high cost of compressed air relative to other energy mediums. With typical energy savings of 30%, the annual potential savings are illustrated in the table below for the various compressor capacities. This table has used Eskom's Megaflex tariff rate for energy charges only. The 40 hr week assumes an annual three-week shutdown period and the 168 hr week assumes continuous operation. These figures are approximate, and differences due to electricity tariffs and hours of operation as well as the utilisation of the full compressor capacity should be expected. This guide provides the compressed air user with information on how to improve or optimise the design, performance and operation of compressed air systems from an energy efficiency (and therefore cost) perspective.
Compressed air as an energy-transmitting medium is versatile, flexible and safe, making it a popular choice and ensuring its continued use in industry.
Typically, compressed air accounts for about 10% of the total electrical power consumed by industry. For a time frame of 10 years, the cumulative costs of compressed air comprise of 10% maintenance, 15% capital and 75% energy. Therefore the implementation of a program to reduce energy consumption in compressed air systems (as outlined in this booklet) can translate into significant financial sayings.
Generally 30% of the total electrical energy used to compress air is wasted, and potential savings could be reaped through the introduction of simple cost effective measures that minimize this
Table 2: Potential Savings Compressor Capacity KW 18 90 160 315 litres/sec 55 250 500 1000 Cfm 110 500 1000 2000 Annual Potential Savings (Rands) 40hr/week 168hr/week 1,704 4,118: 8,520 23,590 15,146 41,937 29,818 82,563
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The key areas that savings can be derived from are: • The use/misuse/demand of compressed air (Chapter 3). • The distribution of compressed air (Chapter 4). • The treatment of compressed air (Chapter 5). • Compressed air generation (Chapter 6). • Compressed air control/monitoring (Chapter 7). The best approach to take to obtain these savings would be to tackle the main areas mentioned above sequentially and in a structured manner. For example leakage (coming under compressed air distribution, Chapter 3) and control (Chapter 6) of
compressed air systems are the biggest waste areas and could account for up to 70% of the total wastage. The guide looks at the recovery of the considerable amounts of waste heat generated via the thermodynamic process of air compression. Sections on system management and auditing are incorporated into the latter sections of guide. At the end of the guide, the potential savings that can be derived from the implementation of some of the energy efficient techniques described are illustrated in the presentation of three South African case studies.
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2. ECONOMICS OF COMPRESSED AIR PRODUCTION
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The delivery of compressed air requires costly equipment that consumes large amounts of electricity and needs frequent maintenance. However many users are not aware how much money they could save annually by improving the performance of their compressed air systems. Electricity is the largest cost in producing compressed air. Initial capital costs are usually exceeded by electricity costs after a short period, about one-year (depending on the type of compressor, manufacturer and the electricity charges). Maintenance costs could be ten percent or more of the initial cost of the system depending on the system quality and usage. The following sections of this chapter deal with a simple calculation estimating annual electricity costs.
2.1.1 FOR A COMPRESSOR RUNNING AT FULL LOAD
The following data is needed for this calculation: i. Motor rating ii. Motor efficiency (from the nameplate or manual) iii. Annual hours of operation iv. Costs of electricity i.e. tariff for consumption and maximum demand v. Power factor (PF) of the motor For a motor efficiency of 90% and using Eskom's Standard tariff of 0.0746 R/kWh and R41.33 / kVA, (assuming this motor does not significantly contribute maximum demand): Annual Electricity Energy Cost = kWx (1) x 24hrs x 7 days x 52 weeks x 0.0746 kWh = kWx (0.9) x 24hrs x 7 days x 52 weeks x 0.0746 R/kWh 0.9) x 24hrs x 7 days x 52 weeks x 0.0746 For newer, more efficient motors, this percentage could be higher but for older and rewound motors it could be 80%. The performance of electric motors should be monitored, and they should be serviced or replaced if their efficiency drops below an economically acceptable level. For a demand tariff of R41.33/kVA and assuming that the compressor maximum demand coincides with the peak demand for the whole factory: Monthly demand charge = Max kW / 0.9 / PF x R41.33
2.1 CALCULATING ELECTRICITY COSTS – A SIMPLE CALCULATION •
Electricity consumers may be charged just on the energy (kWh) they consume or they may be charged on the energy and on a maximum demand in kVA. Typically the maximum demand would be highest averaged kVA demand for any half hour in the month.
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2.1.2 A CALCULATION USING MEASUREMENTS
Another more accurate way of determining costs is through a relationship that involves taking electrical measurements of both full load amps and volts. The motor rating and efficiency are not needed, but the full load power factor is required and if not available, could be obtained from the manufacturer. Data needed: i. ii. iii. iv. v. Full load amps Full load volts Full load power factor Annual hours of operation at full load Costs of electricity tariff
loaded as a percentage of the electrical consumption when fully loaded. For a compressor that runs fully loaded for 65% of the time and at 0.3 of full load for 35% of the time: Annual Electricity Energy Costs = 52 weeks Motor rating (kW) x 24 hrs x 7 days x 0.9 x 0.0746 R/kWh x (0.65 + 0.3 x 0.35) Electrical utilities such as Eskom bill industrial customers at different rates and have more complicated rate structures such as Megaflex. These charges include energy (R/kWh) and demand charges (R/kVA) and have different rates depending on the season, time of day and level of consumption. It may be possible to restructure demand to cheaper ‘off-peak’ tariff times, this must be kept in mind when calculating potential savings.
Therefore, Annual Electricity Energy Costs = Full load amps x volts x Power factor 1000
2.3 SAVINGS FROM PERFORMANCE IMPROVEMENTS •
The higher the pressure of the compressed air required, the more expensive it is to deliver. For a system at 7 barg (l00psig), an additional onepercent in operating energy costs is required for every additional 0.14 bar (2psi) in operating pressure. Life cycle costing should be used because the lifetime electricity costs are much greater than the initial costs of the compressors. Overall system efficiency should be considered and not only the motor or compressor efficiency. To achieve this least cost solution for compressed air systems, users should select systems based on appropriate economics to determine compression, treatment, distribution, control and monitoring.
x annual hours of operation x electricity cost per kWh
2.2 A CALCULATION FOR PART LOAD OPERATION •
This calculation is done using the percentage of the time that the compressor is running at full load and the average fraction of the load at which the compressor runs for the rest of the time. Data needed: i. ii. iii. iv. v. vi. vii. Motor rating Motor efficiency - from nameplate Annual hours of operation Cost of electricity Percentage of time running fully loaded Percentage of time running not fully loaded The electrical consumption when not fully
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3. REDUCING COMPRESSED AIR DEMAND TO SAVE ENERGY
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3.1 COMPRESSED AIR USAGE •
Compressed air use should be constantly monitored and re-evaluated. Investigating where and how compressed air is used around site will reveal the areas in which major savings can be achieved. By the time compressed air reaches the end user, its cost as an energy source is very high, around 50c/kWh. It is vitally important that each use is investigated in detail to establish whether: 1. 2. 3. it needs to be operated by compressed air at all; the supply pressure is greater than necessary; there is adequate facility for isolating the supply line when it is not in use. compressed air system. As a general rule, compressed air should only be used if it improves safety, increases productivity or reduces labour. Typical overall efficiency when compressed air is used is around 10%. So the air used should be of minimum quantity, pressure and duration.
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ASSESSING AIR NEEDS
Air needs are defined by the air quality, quantity and pressure required by the end uses in your plant. Assessing needs carefully will ensure that a compressed air system is configured properly.
3.2.1 AIR QUALITY
As illustrated in the following table, compressed air quality ranges from plant air to breathing air. Industrial applications typically use one of the first three air quality levels.The quality is determined by
Two commonplace examples of misuse are using compressed air for cleaning or cooling duties: alternatives exist for cleaning benches, and cooling duties can generally be carried out using high pressure fans or a lower pressure
Table 3: Ranges of Air Quality Plant Air Air tools, air-actuated valves, general plant air Instrument Air Process Air Laboraties, paint spraying, Food and pharmaceutical powder coating, process air, electronics climate control Breathing Air Hospital air systems, diving tank refill stations, respirators for cleaning and/or grit blasting
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the dryness and contaminant level required by the end-uses, and is accomplished with filtering and drying equipment.The higher the quality, the more the air costs to produce. Higher quality air usually requires additional equipment, which not only increases initial capital investment, but also makes the overall system more expensive to operate in terms of energy consumption and maintenance costs. One of the main factors in determining air quality is whether or not lubricant-free air is required. Lubricant-free air can be produced with either lubricant-free compressors, or with lubricantinjected compressors that have additional separation and filtration equipment. Lubricant-free rotary screw and reciprocating compressors usually have higher capital costs, lower efficiency and higher maintenance costs than lubricantinjected compressors. However, the additional separation and filtration equipment required by lubricant-injected compressors will cause some reduction in efficiency, especially if systems are not properly maintained. Careful consideration should be given to the specific end-use for the lubricantfree air, including the risk and cost associated with product contamination, before selecting a lubricant-free or lubricant-injected compressor.
effective. In most cases, a thorough evaluation of system demand may result in a control strategy that will meet system demand with reduced overall compressor capacity. Oversized air compressors are extremely inefficient because most systems use more energy per unit volume of air produced when operating at part-load. In many cases it makes sense to use multiple, smaller compressors with sequencing controls to allow for efficient operation at times when demand is less than peak. If a system is properly designed and maintained but is still experiencing capacity problems, an alternative to adding another compressor is to re-examine the use of compressed air for certain applications. For some tasks, blowers or electric tools may be more effective or appropriate.
3.2.3 LOAD PROFILE
Another key to properly designing and operating a compressed air system is assessing the plant’s compressed air load profile, that is, its requirements over a period of time.The variation in demand for air over time is a major consideration in system design. Plants with wide variations in air demand need a system that operates efficiently under partload. Multiple compressors with sequencing controls may provide more economical operation in such a case. Plants with a flatter load profile can use simpler control strategies.
3.2.2 AIR QUANTITY - CAPACITY
The required capacity for a compressed air system can be determined by summing the requirements of the tools and process operations (taking into account load factors) at the site. The total air requirement is not necessarily the sum of the maximum requirements for each tool and process, but the sum of the average air consumption of each. High short-term demands should be met by air stored in an air receiver. Systems may have more than one air receiver. Strategically locating air receivers near sources of high demand can also be
3.2.3.1 ARTIFICIAL DEMAND
Artificial demand is defined as the excess volume of air that is required by unregulated end uses as a result of supplying higher pressure than necessary for applications.This should be eliminated.
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3.2.3.2 PRESSURE
Different tools and process operations require different pressures. Pneumatic tool manufacturers rate tools for specific pressures, and the process engineers should specify process operation pressure requirements. Required pressure levels must take into account system losses from dryers, separators, filters, and piping. As mentioned before, a rule of thumb is that every 0.14 bar (2psi) increase in operating pressure requires an additional 1% in operating energy costs.
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3.3 COMPRESSED AIR MISUSE •
Compressed air is heavily misused in industry. Careful thought should be given to all uses to see whether another, more cost-effective, method might be used instead of compressed air. For example, at a steelworks, measurements found an average air demand of 5,000 l/s at 7 barg. After investigation, it was established that only 2.000 l/s were actually being used appropriately. 2,000 l/s were used at 3 bar and 1,000 1/s were used for non-compressed air duties such as water-jetting and bearing cooling. Compressed air is probably the most expensive form of energy available in a plant. It is also clean, readily available, and simple-to-use. As a result, it is often chosen for applications in which other energy sources are more economical. Users should always consider more cost-effective forms of power before considering compressed air. Many operations can be accomplished more economically using alternative energy sources. For example, plants should: • Use air conditioning or fans to cool electrical cabinets instead of compressed air vortex tubes.
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Apply a vacuum system instead of making a vacuum using compressed air Venturi methods that flow high-pressure air past an orifice. Use blowers instead of compressed air to provide cooling, aspirating, agitating, mixing, or to inflate packaging. Use brushes, blowers, or vacuum systems instead of compressed air to clean parts or remove debris. Use blowers, electric actuators, or hydraulics instead of compressed air blasts to move parts. Use low-pressure air instead of highpressure compressed air for blowguns and air lances. Use efficient electric motors for tools or actuators where safe and appropriate. (Electric tools can have less precise torque control, shorter lives, and lack the safety of compressed air powered tools). Use dedicated high-pressure blowers rather than compressed air for air knives. On conveyors, these can be automatically switched off if the product stops passing beneath the knife.
Other improper uses of compressed air are unregulated end-uses and supply air to abandoned equipment, both of which are described below.
3.3.1 UNREGULATED END-USES
A pressure regulator is used to limit maximum end-of-use pressure and is placed in the distribution system just prior to the tool. If a tool operates without a regulator, it uses full system pressure. This results in increased system air demand and energy use, since the tool is using air at this higher pressure. High-pressure levels can also increase equipment wear, resulting in higher maintenance costs, and shorter tool life.
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3.3.2 ABANDONED EQUIPMENT
Many plants undergo numerous equipment configuration changes overtime. In some cases, plant equipment is no longer used. Airflow to this unused equipment should be stopped, preferably as far back in the distribution system as possible without affecting operating equipment.
3.4 LEAKS •
This subject, which comes under the general heading of misuse, is treated on its own in this booklet because it is the single, biggest waste area and yet one of the simplest and cheapest to control and obtain savings. A system with 5% of demand as leaks is said to be excellent and one with 10% is good. Systems with leaks as high as 70% of demand have been measured. Leakage is not only a direct source of wasted energy, but is also an indirect contributor to operating costs. As leaks increase, system pressure drops, air tools function less efficiently and production is affected. Often the only solution is to increase generation pressure to compensate for the losses. Increased running time can also lead to additional maintenance requirements and increased unscheduled downtime. Leaks can lead to adding unnecessary compressor capacity.
3.3.3 PERIODS WHEN COMPRESSED AIR IS NOT REQUIRED
Another common malpractice is to generate compressed air at times when it is not needed. In many cases there is no need for compressed air at all during non-production hours, but compressors are often not switched off.
Table 4: Power wastage through leaks Hole Diameter mm 0.4 (pin head) 1.6 (match head) 3.0 Air leakage at 7barg l/s 0.2 3.1 11.0 Cfm 0.4 6.2 22.0 Power required to compress air being wasted kW 0.1 1.0 3.5
Table 5: Cost of Leaks in South Africa Compressor Capacity [kW] Annual Compressed Air Costs [R] 4Ohr/week utilisation [R] 75% 50% 18 90 160 315 4260 21300 7865 74545 2840 14200 25244 49697 l68hr/week utilisation [R] 75% 50% 11795 58975 104843 206408 7864 39317 69895 137605
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While leakage can come from any part of the system, the most common problem areas are: • • • • • • Couplings, hoses, tubes, and fittings, Open condensate traps and shut-off valves, and Pipe joints, disconnects, and threadsealant. Leaking pressure regulators: Air cooling lines left open permanently; Air-using equipment left in operation when not needed.
test should be carried out to establish the percentage leakage from the system.
3.4.1.1 NO-LOAD METHOD 1
This method applies to compressors that are operated in on/off-load i.e. when the compressor is on-load it produces a known amount of air. • • Close down all the air operated equipment. Start the compressor and operate it to full line pressure. After it off-loads air leaks will cause the pressure to fall and the compressor will come on-load again; Over a number of cycles make a note of average on-load time (T) and average offload time (t). Total leakage can then be calculated:
It is estimated that UK industry wastes up to £100m (a billion Rand) each year on air leaks. Table 4 gives an example of how even very small holes can contribute to energy wastage. Leaks can be a significant source of wasted energy in an industrial compressed air system, sometimes wasting 20-30% of a compressor’s output. A typical plant that has not been well maintained will likely have a leak rate equal to 20% of total compressed air production capacity. On the other hand, active leak detection and repair can reduce leaks to less than 10% of compressor output. The first step in tackling leaks is to recognise the costs involved and make a commitment to a plantwide awareness program. Regular, continuous attention to the compressed air system coupled with proper maintenance will lead to effective progress in minimising leaks.
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Leakage (l/sec) = (Q x T)/(T + t) Leakage (%) = [(T x 100)/(T + t)] Where: Q = air capacity of the compressor (litres/sec).
3.4.1.2 NO LOAD METHOD 2
For modulating compressors the test is more difficult as the compressor output is unknown.The following method can be used if you have a pressure gauge down-stream of the receiver. • Calculate the volume V (litres) of air mains downstream of the receiver isolating valve, including all the pipework (25 mm and above) and the receivers. Pump up the system to operating pressure (P1), and then close the isolating valve. Record the time (T) for pressure to drop to P2. Leakage can then be calculated as follows:
3.4.1 ESTIMATING AMOUNT OF LEAKAGE
It is necessary to take measurements to establish the leakage rate, for which several methods exist. The best way to establish the amount of leakage in a system is by measurement. However, in the absence of suitable measuring devices, a no-load
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Leakage (litres/sec) =
{V (litres) x [P1-P2] (barg)} {T (secs)}
Derivation of the equation above: System volume = V (m ) Initial pressure of system = P1 (barg) Final pressure of system = P2 (barg) Time for pressure decline = T (secs) Atmospheric pressure = Pa 1.013 bar Initial quantity of free air = {(P1+Pa) 3 Initial quantity of free air = {(P1+Pa)/ x V (m ) Initial quantity of free air = Pa} Final quantity of free air = {(P2+Pa) 3 Final quantity of free air = {(P2+Pa)/ x V (m ) Final quantity of free air = Pa} Therefore, flow-rate Therefore, flow-rate Therefore, flow-rate =(V P1+Pa - P2+Pa 3 = (m /sec) = T Pa Pa =(V/TP1-P2)/Pa} 3(m /sec) =(V/T) x {(P(m3 /sec) =(T PPa (m /sec) ≈ The above temperature. ≈( V/T) x (P1-P2) since Pa ≈ 1 ≈(V/TP1 – P2 since Pa ≈ 1 T ) x (P P2) since Pa ≈ 1 assume constant
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An Ultrasonic Leak Detector is a more effective leak detection method as it can detect leaks against a background of other equipment noise. The detectors work by picking up the very high frequency sound emitted by a leak, inaudible to the human ear.They are simple to use and do not pick up frequencies emitted by the mechanical actions of machines. These portable units consist of directional microphones, amplifiers, and audio filters, and usually have either visual indicators or earphones to detect leaks
( ) ( ) ( )
3.4.3 HOW TO FIX LEAKS
Once leaks have been detected they should be repaired as soon as possible. Simply tightening components and shutting off valves, which have been left open, can repair most leaks. One method of motivating people to fix leaks is to offer a reward for the person who finds the most leaks. (The motivation chosen should not inspire people to make leaks though !) Leaks occur most often at joints and connections. Stopping leaks can be as simple as tightening a connection or as complex as replacing faulty equipment such as couplings, fittings, pipe sections, hoses, joints, drains, and traps. In many cases leaks are caused by bad or improperly applied thread sealant. Select high quality fittings, disconnects, hose, tubing, and install them properly with appropriate thread sealant. Non-operating equipment can be an additional source of leaks. Equipment no longer in use should be isolated with a valve in the distribution system Another way to reduce leaks is to lower the demand air pressure of the system.The lower the pressure differential across an orifice or leak, the lower the rate of flow, so reduced system pressure
equations
Having established the size of the problem, a realistic target for leakage rate (say 10%) should be set. No-load tests should then be carried out regularly, approximately every two to three months, with inspections when necessary during shutdown conditions.
3.4.2 LEAK DETECTION
During shutdown of the whole factory, the detection of larger leaks within the compressed air system is simple, as they are audible. Once the compressor is started the exact leak positions should be marked with tags. In addition, checking joints and unions with soapy water is recommended to identify the smaller leaks, which invariably develop with time. An organised approach is required to obtain good results.
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will result in reduced leakage rates. Stabilising the system header pressure at its lowest practical range will minimize the leakage (as well as minimize compressor load and thus electrical demand). Where a system cannot be shut down to test for leaks this method is recommended. Once leaks have been repaired, the compressor control system should be re-evaluated to realize the total savings potential.
A leak prevention program should be part of an overall program aimed at improving the performance of compressed air systems. Once the leaks are found and repaired, the system should be re-evaluated. Having established the size of the leakage problem, an aggressive campaign of leak reduction should take place. Targets should be set and careful monitoring of results conducted. This work will involve inspections during silent hours checking for pipe work and tool leaks, and checking hoses and couplings for air tightness. Following this program the leakage rate should be re-measured and the work continued until the target is met. This is an ongoing exercise, which must be repeated every six months at least, otherwise problems, and energy and money losses will return.
3.4.4 A LEAK PREVENTION PROGRAM
A good leak prevention program will include the following components: identification (including tagging), tracking, repair, verification, and employee involvement. All facilities with compressed air systems should establish an aggressive leak program. A crosscutting energy management team involving decision-making representatives from production should be formed.
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4. EFFECIENT COMPRESSED AIR DISTRIBUTION
•••••••••
4.1 INTRODUCTION •
Having investigated possibilities for reducing the end usage of air and having determined the extent of leakage, the next step is to examine the distribution network. The main factors that affect energy consumption at this stage are: • • Pressure losses due to inadequate pipe sizing; Water condensing in the lines, causing • damage to components and also reducing pipe cross-sectional areas which leads to additional pressure losses; Air leakage from pipework and end-using equipment, due to poor maintenance and in many cases permanently open condensate drain valves.
A typical compressed air system is outlined in Figure 1 below.The following sections refer to this diagram, describing the main design criteria and components.
Figure 1: Compressed air system components (Source: ETSU)
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4.2 DISTRIBUTION MAIN •
Generally, a ring main is the most effective method of supplying compressed air to a point of use.The main advantages of this type of network are that: • Velocity to any one point is reduced, since air can converge from two directions, thereby reducing pressure drop over the system; Automatic zone valves can be fitted to
•
isolate areas operating different working patterns; Alteration or extension of the distribution system is made easy.
Ideally a ring main should be placed around each building and a single branch point should feed the main, enabling each area of the network to be metered by a single meter. An example of such a network is given below.
•
Figure 2: Preferred distribution line layout (Source: ETSU)
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Airline diameters are usually based on calculations of throughput velocities. A figure of approximately 6.0 m/s is the accepted target value, because this is sufficiently low to prevent an excessive pressure drop.Table 6 utilises the maximum recommended flow in various line sizes based on a 7 barg output pressure. For a quick reference the nomographs outlined in Figures 3 and 4 are a useful guide to sizing distribution lines. The distribution system should be designed to cause no more than 0.2 bar pressure drop at full demand at the point of use.
Table 6: Maximum recommended flow rates Pipe Bore mm 10 25 50 65 80 100 150 Max Flow Cfm 10 55 220 375 500 875 1,900
l/s 5 25 100 180 240 410 900
Figure 3: Pipe Carrying capacities at varying velocities (Source: ETSU)
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Figure 4: Pressure Drop in Steel Pipes (Source: ETSU)
When sizing ring mains it is advisable to err on the high side as future demand may increase. The consequence of under-sizing is very significant in energy efficiency terms, because air velocity will be excessive leading to high-pressure drops and nonseparation of condensed water. The table below shows the power wasted for different pipe diameters with a flow rate of 500 l/s (1,000cfm) at 7barg pressure. Compressor house to ring main feeds should be
oversized to avoid pressure drop problems. As a guide it is recommended that a pipe of twice the cross-sectional area of that used for the ring main be selected. Higher air velocities (up to 20 m/s) are acceptable where the distribution pipe-work does not exceed 8 meters in length.This would be the case where dedicated compressors are installed near to an associated large end user.
Table 7: Distribution line power wastage (500 l/s, 7barg)
Pipe Nominal Bore (mm) 50 65 80 100
Pressure Drop per 100 m (bar) 2.6 0.9 0.2 0.1
Equivalent Power Lost (kW) 18 5 0.8 0.4
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4.3 WATER DRAINAGE •
For undried compressed air, system problems may occur through water condensation in the distribution network. It is good practice to remove as much of this water as possible. Condensation typically occurs where the air main travels outside the buildings and is therefore subject to temperatures below those in the compressor house. The accumulation of condensed water and scale in the system can lead to pressure drop. To prevent this occurring, the air mains should slope to strategically located drain legs equipped with automatically operated drain valves (avoiding air wastage, which frequently occurs with manually operated drain valves). Building layout will dictate the best position for drain points, but in general the main should be installed with a fall of not less than 1 meter in 100 meters of ring main pipe-work in the direction of air flow. The recommended distance between drain points is approximately 30 meters. Any branch line should be taken off the top of the main to prevent any water in the main pipe from running into it. In addition, the bottom of a falling branch line pipe should be drained.
4.4 DRAIN TRAPS •
For the sake of energy efficiency, automatic drain traps could be fitted to the bottom of the drain line within a compressed air system. Reliable electronic condensate traps are available that ensure water is regularly drained away. Manual traps left open account for a substantial percentage of total wastage. The ball float type of drain trap is the most common because it gives a positive shut off, opening only if water is present and closing immediately the water has cleared. If, however, there is a possibility of large quantities of water at the trap, then air binding can take place and a balance pipe must be fitted.With this arrangement, the water will flow freely into the trap, displacing air, which then passes through the balance pipe and into the main system.
All drain traps require occasional maintenance, to remove any build-up of oils or emulsions, which may be present in the condensate. If oil or emulsion build-up is heavy, consideration should be given to using drain trays, which have a blast action discharge.
Figure 5: Air binding of water traps (Source: ETSU)
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4.5 •
AIR RECEIVERS
Air receivers are designed to provide a supply buffer to meet short-term demand spikes, which can exceed the compressor capacity. They also serve to dampen compressor pulsation, separate out particles and liquids, and make the compressed air system easier to control. Installing a larger receiver tank to meet occasional peak demands can even allow for the use of a smaller compressor. In most systems, the receiver will be located just after the dryer. In some cases it makes sense to use multiple receivers, one prior to the dryer and one closer to points of intermittent use. Storage can be used to control demand events (peak demand periods) in the system by controlling both the amount of pressure drop and the rate of decay. Storage can be used to protect critical pressure applications from other events in the system. Storage can also be used to control the rate of pressure drop in demand while supporting the speed of transmission response from supply. Many
systems have a compressor operating in modulation to support demand events, and sometimes strategic storage solutions can allow for this compressor to be turned off. Storage can also help systems ride through compressor failure or short energy outages. Receivers have three main functions:
Providing storage capacity; Acting as a secondary cooler; Creating more stable pressure conditions, effectively acting as pulsation dampers. On most installations, the receiver is fed from the after-cooler and further cooling will take place in the receiver. On installations where the compressor plant is small, an after-cooler may not be fitted, making the receiver the point at which most condensed liquid will be found. If liquid is allowed to build up in the receiver, carry-over into the ring main is likely, with resulting efficiency implications. The receiver, therefore, needs to be fitted with both automatic and manual drain traps, in order to remove the condensate and any carryover solids such as dust, scale, carbon and so on.
• • •
Figure 6: Air binding of water traps (Source: ETSU)
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4.6 •
REGULATORS
Some applications, such as control and instrumentation, require air at a pressure lower than the main supply and in these circumstances, if a separate pressure system cannot be installed, then regulators should be fitted. Figure 6 illustrates a direct-acting regulator. Excellent savings can be achieved by reducing pressure at the usage point as opposed to ‘over supplying’ the pressure. The air consumption of most air-using devices, such as air tools, spray guns and air knives, increases in proportion to the operating pressure ratio. Reducing pressure to the minimum acceptable saves energy.
and other air line component sizing which cause high flow velocities and pressure drops. All systems should be designed to minimise pressure drops. It is worthwhile obtaining a flow/pressure profile of the distribution network to establish where the bottlenecks occur and how they can be overcome. Air velocities should not exceed 6 meters per second in the main components of a distribution system. The distribution system should be designed to cause no more than 0.1-0.2 bar pressure drop at full demand at the usage points. The nomograph shown in Figure 7 is a useful method of arriving at pipe sizes. In conjunction with establishing the correct pipe diameters for the flow, a check on the pressure drops caused by the major valves and fittings should be made.Table 8 gives the equivalent pipe length for these components at the relative nominal pipe diameter. The equivalent lengths should be added to the actual pipe length to be used, and the total length should then be used with the nomograph shown in figure 7 for finalising the pipe diameter.
4.7 DISTRIBUTION NETWORK •
Following compression and treatment, compressed air is distributed to the usage points by a piping network. Much energy can be wasted during distribution by having to generate overpressure to overcome incorrect pipe, valve
Figure 7: Nomograph (Source: ETSU)
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Table 8: Pressure loss through steel fittings
Equivalent Pipe lengths in meters Inner Pipe Diameter (mm) Item Gate valve Fully open Gate valve Half closed Diaphragm valve Fully open Angle valve Fully open Globe valve Fully open Ball valve (full bore) Fully open Ball valve (red. bore) Fully open Swing check valve Fully open Bend R=2d Bend R=d Mitre bend 90° Run of tee Side outlet tee Reducer 15 0.1 20 0.2 32 0.6 1.5 2.7 0.5 3.4 1.0 2.6 4.8 0.2 4.9 1.3 0.1 0.2 0.6 0.6 0.2 0.3 1.0 0.3 1.0 0.3 25 0.3 5 1.3 4 7.5 0.2 2.4 2.0 0.3 0.4 1.5 0.5 1.5 0.5 40 0.5 8 2.5 6 12 0.4 2.2 3.2 Ø.5 0.6 2.4 0.8 2.4 0.7 50 0.6 10 3 7 15 0.3 5 4 0.6 0.8 3 1.0 3 1.0 80 1.0 16 4.5 12 24 0.4 2.6 6.4 1.0 1.3 4.8 1.6 4.8 2 100 1.3 20 6 13 30 0.3 4.1 8 1.2 1.6 6 2 6 2.5 125 1.6 30 8 18 38 0.5 3.3 10 1.5 2 7.5 2.5 7.5 3.1 150 1.9 40 10 22 45 0.6 12.1 12 1.8 2.4 9 3 9 3.6 30 60 0.6 22.3 16 2.4 3.2 12 4 12 4.8 200 2.6
4.8 MAINTENANCE •
Piping systems need regular inspection and maintenance. Inspections for leaks, checking drains and blowing down contamination are all worthwhile measures to avoid energy losses.
4.9 SYSTEM ISOLATION •
Sometimes it is necessary to keep parts of a distribution network pressurised when other parts
do not require compressed air. In these cases the compressed air system in the idle plant should be isolated from the active plant. For example, an assembly department should be isolated during non-productive hours to prevent wastage due to leakage or misuse. Manually or electronically operated zone isolation valves can be installed in the distribution network to shut off at properly arranged times.
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5. COMPRESSED AIR TREATMENT
••••••••••••••
5.1 • INTRODUCTION
Air treatment is often required in order to provide compressed air of the quality required at the point of use.This avoids product contamination, product spoilage and poor control of, or damage to, air-using equipment. Treatment needs energy in terms of additional generation pressure, and possibly additional compressed air or electrical demand depending on the type of treatment used. Compressing air concentrates the contaminants per unit of volume of air delivered at pressure. Compressed air can be contaminated by water vapour; condensate; particulate matter (either airborne or pipe-scale); oil in vapour or liquid state (either inhaled by the compressor from the atmosphere or added during compression); and microbes. The amount of treatment will depend on the users needs. The desired air quality in terms of dirt water, oil and microbial burden is achieved by treatment after compression.The higher the quality specified, the greater the energy consumed by the treatment system, and the higher the additional generation pressure needed to overcome losses during treatment. Table 9 gives the ISO/DIS recommendations on air quality classes. Treatment systems range from a simple aftercooler, which is neatly always supplied with the compressor package, through to filters and refrigerated, sorption, deliquescent and desiccant dryers.There are many variations of each system; Some are more energy efficient than others. The requirement for high quality compressed air is increasing as production methods become more sophisticated. A general breakdown of recommended standards for different manufacturing applications is included in Table 10. This table is intended for guidance only; in practice there are very many other combinations needed. There is a very wide range of requirements for air quality. It is important to install the right equipment, and equally important to keep the energy requirements within reason. Every effort should be made to avoid unnecessary levels of treatment. The type of compressor used is important. An oilfree machine could save one filtration stage over an oil-injected compressor, so for high quality air requirements an oil-free machine should be purchased wherever possible. In addition to the treatment savings, other benefits include increased efficiency and longevity, and in cases where a desiccant dryer is used there is no chance of contamination of the desiccant by compressor oil. However, when oil-free machines are used in heavily polluted atmospheres, it is still necessary to remove oil by filtration. Many plants need only part of the air treated to very high quality. In these cases, treating all the generated air to the minimum acceptable level and improving the quality to the desired level close to the usage points can achieve excellent savings Ambient air typically contains 12.5g of water for every 1 m3 of free saturated air at 15°C. If a compressor produces air at 500 1/sec (1,000 cfm),
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Table 9: Air quality classifications ISO/DIS 8573.1 QUALITY CLASS DIRT Particle size (micron) 0.1 1 5 15 40 DIRT Concentration (mg/rn3) 0.1 1 5 8 10 WATER OIL Pressure dew point (including vapour) (°C (ppm vol) at (mg/m3) 7bar) -70(0.3) -40(16) -20(128) +3(940) +7(1240) +10(1500) No Spec 0.01 0.1 1 5 25 N/A N/A
1 2 3 4 5 6 7
Table 10 General Recommended Standards for Air Purity Application classes Air agitation Air bearings Air gauging Air motors Brick and glass machines Cleaning of machine parts Construction Conveying, granular products Conveying powder products Fluidics, power circuits Fluidics, sensors Foundry machines Food and beverages Hand operated air tools Machine tools Mining Micro-electronics manufacture Packaging and textile machines Photographic film processing Pneumatic cylinders Pneumatic tools Process control instruments Paint spraying Sand blasting Welding machines General workshop air Oil 3 2 2 4 4 4 4 3 2 4 2 4 2 4 4 4 1 4 1 3 4 2 3 4 4 Typical Quality Classes Dirt 2 3 4-1 4 5 4 3 4 2–1 4 3 5-4 3 5 1 3 1 3 4 2 3 3 4 4 Water 2 3 5 4 5 3 2 4 2 5 1 5-4 5 5 1 3 1 5 4 3 3 3 5 5
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the compressed air produced each hour will contain 22.5 l of water. Provided the free air is maintained at about 15°C, the water will remain as vapour: however, if the air is cooled or compressed the water will be condensed. The temperature at which water condenses is known as the dew point. The rise in air temperature in the compressor generally prevents condensation, but when the air passes though the after-cooler a large amount of water could condense. Typically the air temperature following the after-cooler will be around 35°C and water content will have been reduced. Water will, however, still be present as vapour and if the air temperature falls there will be further water condensation. Figure 8 shows the amount of water removal required for differing end temperatures. To remove significantly more water from the compressed air than can be achieved by aftercooling, a dryer is necessary. Air dryers typically take the air from the compressor after-cooler at a maximum temperature of 35°C.
5.2 DRYERS •
5.2.1 DESICCANT DRYERS
For class 1.1.1 quality air, heated or heatless twin tower desiccant dryers with special desiccant and drying cycles are employed. Oil removal filters, water removal filters, dust removal filters and an activated carbon absorber unit are also needed. This type of system consumes a lot of energy, requiring up to 15% of the compressed air or electrical equivalent for desiccant regeneration, and there is pressure drop across the filters, when in service, of up to 1.5 bar, which will require additional generation pressure. The example 500 l/s, compressor, when supplying 1.1.1 quality air, would cost 21% per annum more due to the treatment devices than if it were simply delivering after-cooled air. As the requirement for air quality becomes less intense than 1.1.1, the energy requirement reduces.
Figure 8:Water removal each week from 500 l/sec of 7 barg air. (Source: ETSU)
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The example compressor, when supplying air of 2.2.1 quality, would only cost 15% more than delivering after-cooled air if it used a lower specification desiccant dryer with pre- and afterfilters performing at 40°C pressure dew-point.
5.2.4 REFRIGERATION DRYERS
Compressed air of 4.4.5 quality can be provided from any type of compressor by use of a refrigerated dryer and, where necessary, oil removal filtration. The extra cost will typically be 5% over delivering standard after-cooled air. This method of drying is very popular as it produces dew points, which are adequate for most duties in an energy efficient and reliable manner. In climates where the dew point produced is above winter ambient temperatures, water taken outside the buildings will condense. It is recommended that a condensate trap be fitted to the system where it enters the next building to prevent problems.
5.2.2 SORPTION DRYERS
Air of 2.3.1 quality can be provided with a sorption dryer, which can only be used with an oilfree compressor. A very small motor slowly rotates a drum, which is impregnated with the drying medium. Compressed air is fed through a sealed segment of the drum and dried within a range of -15°C to -40°C, depending on the compressor load. The drying medium in the part of the drum not being used to dry the air is regenerated by hot air taken from before the machine after-cooler, i.e. by the waste heat of compression. The cost to provide 2.3.1 quality air by this method, with an oil-free compressor and limited filtration, is around 3% more than delivering after-cooled air.
5.3 AFTER FILTERS •
Filters cause pressure drops in compressed air systems. To save energy, it is recommended that only the minimum filtration requirement be met. Filters should be adequately sized for the duty; if the filter connections are considerably smaller than correctly sized pipe-work they will cause pressure drops. It is better to pay more for filters with correctly sized flanges, and avoid pressure drop and energy wastage.
5.2.3 DELIQUESCENT (ABSORPTION) DRYERS
Deliquescent dryers operate by passing the compressed air over soluble material, such as salt, which dissolves as it absorbs moisture. Their main advantages are that they do not consume any energy other than that required to overcome the pressure drop within the dryer (0.1 to 0.4 bar) and they do not lose any air volume.The dryers are not, however, regenerative and deliquescent material needs to be replaced periodically, incurring both labour and material costs. Deliquescent dryers are the least expensive dryer and are very energy efficient, but can only produce dew points about 6ËšC below the inlet temperature.
5.4 AIR INTAKES •
The location of the air compressors on site can have a bearing on the amount of energy used by the compressor. Cool, clean, dry intake air will lead to more efficient compression. Where possible, air should be taken from outside the building because its temperature will be lower. Lower temperature air is denser and the mass of air that can be compressed by the machine is increased. The air inlet should be protected against the entry of rain and wind blown dust, which would clog filters and
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waste energy. Ducting between air intake and compressor should be short, straight and have a generous diameter. The condition of the air entering the compressor is extremely important, since fouling of inlet filters and high ambient air temperatures can result in significant energy wastage. For every 4°C drop in intake temperature, there is a 1% increase in efficiency. For every 25 mbar pressure lost at the inlet, the compressor efficiency is reduced’ by 2%.
the compressor manufacturer, otherwise problems may occur with possible damage to the compressor and overloading of, the main drive motor.
To justify air inlet cooling, high annual utilisation is needed, producing payback on the capital cost of between two and five years depending on local conditions.
5.4.1 AIR INLET COOLING
With air inlet cooling or pre-cooling, the air is cooled prior to the compression process resulting in energy savings and improved air quality. Twin two-stage coolers linked to a conventional refrigeration plant carry out air inlet cooling. Inlet air is cooled to between -20°C and -25°C, which reduces the pressure dew point to about 0°C and, as an added benefit, removes any dust particles (these collect in the water/ice mixture on the cooler tubes). The density of the inlet air is increased by around 15%, thereby improving the volumetric efficiency of compression. Pre-filters and after-coolers are not required and can be removed from the compressors. As there is no after-cooler, the compressed air leaves the machine at 100°C to 120°C and then cools within the distribution system. Some of this air may be utilised at above ambient temperatures, further improving the system efficiency due to its increased volume. In these cases, oil must be filtered from the air to avoid the risk of explosion. Air inlet cooling technology is particularly worthwhile on low-pressure blowers and single stage compressors, but it is not so suitable for multi-stage units. It is not recommended for centrifugal compressors because, although volumetric flow increases, the machine will be functioning well away from its design condition and will therefore, be inefficient. It is important to ensure that the inlet air temperature is not reduced below the minimum recommended by
5.5 INSTALLATION, CONFIGURATION AND SIZING •
Ideally, each compressor should have a dedicated dryer for control and reliability purposes. This is not always practical, due to capital cost and space availability. All configurations are very reliable if well installed and maintained. If a dryer for each compressor is not practical, a unit suitable for 100% duty over the whole plant is recommended to give the best energy and air quality performance.
Dryers should be installed in well-ventilated areas. For continuous processes, all filters should be duplexed with changeover valves for ease of maintenance. A dryer by-pass should also be installed for emergency maintenance; this must be locked off during normal running to prevent accidental operation, which would contaminate the dry air main.
Filters should be adequately sized for the duty. As mentioned previously, if the filter connections are considerably smaller than correctly sized pipe-work, they will cause pressure drop problems. It is better to pay more at the outset and thereby avoid pressure drop and energy wastage.
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5.6 TREATMENT SYSTEMS MAINTENANCE •
Fouling causes an increase in the pressure drop across all filter elements, leading to a higher generation pressure and additional energy use. It is important to keep pressure drops to a minimum. All filters should be fitted with differential pressure gauges, which should be calibrated regularly. With all dryers, particularly desiccant types, the dew point should be checked regularly. Many will be operating well below specification, and yet the energy consumed will be continuing at the same rate as that needed for design dew-point performance.
unnecessary levels of treatment. Many plants only need part of the air treated to very high quality. In these cases, excellent savings are achievable by treating all the generated air to the minimum acceptable level, and improving the quality to the desired level close to the usage point.
A good example of this would be a car production plant where 70% of the requirement is for 4.4.2 quality air, which can be supplied by a refrigeration dryer and oil removal filter. The energy requirement of a refrigeration dryer is much less than that of a desiccant dryer, and the pressure drop across the filter will be 0.2 bar.
5.7 POTENTIAL SAVING AREAS •
It can be seen that there is a very wide range of requirements for air quality. It is most important to install the right equipment for the duty, and equally important to minimise energy requirements. Much energy can be expended on
30% of the air is required at 1.2.1 quality, for the special requirement of the paint and engine assembly areas. The desiccant dryers and special filtration needed for this quality can be installed in the usage areas. Energy cost savings of around R20,000 per annum for every 5001/s (1,000 cfm) delivered can be achieved by only treating to required levels. In addition, savings will be achieved from reduced dryer maintenance and consumables, such as filter elements and desiccant replacement.
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6. COMPRESSED AIR GENERATION
•••••••••
Opportunities for more efficient energy use arise when new compressed air facilities are being designed, particularly when choosing the most appropriate compressor. The features of the main compressor types and their efficiencies are included in this section. Many modern industrial air compressors are sold “packaged” with the compressor, drive motor and many of the accessories mounted on a frame for ease of installation. There are two basic compressor types: positive-displacement and dynamic. In the positive-displacement type, a given quantity of air or gas is trapped in a compression chamber and the volume, which it occupies, is mechanically reduced, causing a corresponding rise in pressure prior to discharge. At constant speed, the airflow remains essentially constant with variations in discharge pressure. Dynamic compressors impart velocity energy to continuously flowing air or gas by means of impellers rotating at very high speeds. The
Compressor
Positive Displacement
Dynamic
Reciprocating
Rotary
Centrifugal
Axial
Single-acting
Double-acting
Helical-screw
Liquid-ring
Scroll
Rotary-vane
Figure 9: Compressor Family Tree
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velocity energy is changed into pressure energy both by the impellers and the discharge volutes or diffusers (these are part of the stationary casing in which the impeller turns). In the centrifugal-type dynamic compressors, the shape of the impeller blades determines the relationship between airflow and the pressure (or head) generated.
Large industrial reciprocating air compressors are double-acting and water-cooled. From 250 to 1,000 1/s (500 to 2,000 cfm) double-acting watercooled piston compressors either in lubricated or non-lubricated forms are available.These machines are the most efficient available in terms of full-load and part-load power consumption. Oil free duties can be served by piston compressors with special carbon or Teflon wearing surfaces. A high number of piston compressors are in use today. Well-maintained piston machines are still the most energy-efficient compressors, although efficiency decreases significantly if they are poorly maintained. Over the last decade, however, the trend has been to purchase rotary vane, rotary screw and centrifugal compressors, because they are quieter and simpler to maintain and install.
6.1 POSITIVE DISPLACEMENT COMPRESSORS •
These may be either reciprocating or rotary. The rotary ones may be double helical screw, single helical screw with vanes, liquid ring, scroll or rotary vane (sometimes called sliding vane). These have very different configurations. In principle, a multistage compressor, with small clearance volumes, is more efficient than a single-stage machine.
6.1.1 RECIPROCATING COMPRESSORS.
Reciprocating compressors have pistons and work like bicycle pumps. A piston, driven through a crankshaft and connecting rod by an electric motor, reduces the volume in the cylinder occupied by the air or gas, compressing it to a higher pressure. Single-acting compressors have a compression stroke in only one direction, while double-acting units provide a compression stroke as the piston moves in each direction. Compressors of the single or two-stage air-cooled piston type can serve capacities from 2.5 to 250 1/s (5 to 500 cfm).These compressors are usually mounted on air receivers. It is possible to get nonlubricated piston compressors for duties such as food, air conditioning and pharmaceutical production. However, in these cases it is more common to use an oil-injected compressor with filtration to remove the oil carried over from the compressor.
6.1.2 ROTARY POSITIVE DISPLACEMENT COMPRESSORS
The most common type of rotary compressor is the helical twin screw-type (also known as rotary screw or helical lobe). Male and female screwrotors mesh trapping air, and reducing the volume of the air along the rotors to the air discharge point Rotary screw compressors have low initial cost, compact size, low weight, and are easy to maintain. Rotary screw compressors are available in sizes from 2.2 – 450 kW (3 - 600 hp) and may be air or water cooled. Less common rotary compressors include the single screw, the rotary vane, the liquid ring and the scroll-type. Single-stage oil-injected rotary vane or screw compressors can serve capacities from 2.5 to 250 1/s (5 to 500 cfm). From 250 1/s up to 1,000 1/s (500 to 2,000 cfm) oil-injected screw machines are used.
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Users that need high quality air choose oilinjected machines with filtration because of the lower capital cost of the machinery. This method of providing high quality air is less energy efficient than using oil-free compressors. For 100 1/s to 2,000 1/s (200 to 4,000 cfm) oilfree applications, the two-stage rotary-toothed rotor compressor or non-lubricated screw compressor can be used. Its efficiency is high because compression takes place in two stages and the rotors have very close operating clearances. They are as efficient as oil-free piston machines and have a long lifetime, but have a high capital cost compared with oil-injected machines. Capacities of 1,000 to 2,000 l/s (2,000 to 4,000 cfm) are met with two stage oil-free rotary screw compressors or multi-stage centrifugal types, both of which are inherently oil-free.
velocity energy, primarily in an axial plane. The stationary vanes then act as a diffuser to convert the residual velocity energy into pressure energy. This type of compressor is restricted to very high flow capacities and generally has relatively high compression efficiency. For efficient operation, it is important to state the site ambient temperatures and pressures, and the design flow and pressure when specifying dynamic compressors.The energy requirements and control ranges of these compressors are seriously affected by operation outside design conditions. Generally for energy efficiency at full and part load, the more stages of compression the better. Centrifugal machines are available from 250 l/s (500 cfm) up to very large capacities, and are popular and most energy efficient for applications over 1,000 1/s (2,000 cfm). Multi-stage oil-free centrifugal compressors can meet capacities over 2,000 l/s (4,000 cfm), until very large mass flow compressors of the axial flow configuration come into consideration. Centrifugal compressors are very reliable and efficient if properly applied, and usually have low maintenance costs.
6.2 DYNAMIC COMPRESSORS •
These compressors raise the pressure of air or gas by imparting velocity energy and converting it to pressure energy. Dynamic compressors include centrifugal and axial types. The centrifugal-type is the most common and is widely used for industrial compressed air. Each impeller, rotating at high speed, imparts primarily radial flow to the air or gas, which then passes through a volute or diffuser to convert the residual velocity energy to pressure energy. Some large manufacturing plants use centrifugal compressors for general plant air, and, in some cases, plants use other compressor types to accommodate demand load swings while the centrifugal compressors handle the base load. Mixed flow compressors have impellers and rotors, which combine the characteristics of both axial and centrifugal compressors. Axial compressors consist of a rotor with multiple rows of blades and a matching stator with rows of stationary vanes. The rotating blades impart
6.3 ENERGY EFFICIENT COMPRESSOR SELECTION •
In general, the choice of compressor and aftertreatment system is dictated by: • • • The capacity and pressure required; The capital available; The specified delivered air quality requirements.
The relative generation efficiencies of each different compressor configuration are summarised in Table 11.
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Table 11: Summary of compressor configurations with relative efficiencies Description Lubricated piston Capacity (l/s) 2-25 25-250 250-1,000 2-25 25-250 250–1,000 2-25 25-250 250-1,000 25- 250 250- 1,000 1,000 - 2,000 250- 1,000 1,000- 2,000 Above 2,000 Specific energy (J/l)* 510 425 361 552 467 404 510 446 404 429 382 382 446 382 361 Part Load efficiency** Good Good Excellent Good Good Excellent Poor Fair Fair Good Good Good Good Excellent Excellent
Non-lubricated piston
Oil-injected vane/screw Non-lubricated toothed rotor/screw Non-lubricated centrifugal
All the compressors in the table are working at 7 barg pressure. * J/l ÷ 21 kW/100 cfm e.g. 510 J/l 24.29 kW/100 cfm ** Efficiencies are measured in specific energy consumption (joules/litre (J/l)).
The specific power figures have been obtained from actual field test results according to BS 1511 Part 2: 1984.These take into account electric drive motor inefficiencies and are a true assessment of the actual electrical input, not shaft input power, which is usually stated by manufacturers. There is considerable variation in specific energy consumption between configurations. Another consideration is the ability of a compressor to operate efficiently on part load.
the most important determinants of overall system energy efficiency. Proper control is essential to efficient system operation and high performance.The objective of any control strategy is also to shut off unneeded compressors or delay bringing on additional compressors until needed. All units that are on should be run at full-load, except for one unit for trimming. (This is because the energy costs per unit of compressed air rise markedly when the compressors are running at less than full load.) Compressor systems are typically comprised of multiple compressors delivering air to a common plant air header. The combined capacity of these machines is sized, at a minimum, to meet the maximum plant air demand.This is not necessarily the sum of all the maximum machinery demand, as equipment loading may be at different intervals. System controls are almost always needed to reduce the output of the individual compressor(s)
6.4 ENERGY EFFICIENT COMPRESSOR CONTROL •
Compressed air system controls match the compressed air supply with system demand (although not always in real-time) and are one of
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during times of lower demand. Compressed air systems are usually designed to operate within a fixed pressure range and to deliver a volume of air, which varies with system demand. System pressure is monitored and the control system decreases compressor output when the pressure reaches a predetermined level. Compressor output is then increased again when the pressure drops to a lower predetermined level. The difference between these two pressure levels is called the control range. Depending on air system demand, the control range can be anywhere from 0.14 to 1.4 bar (2 - 20 psi). In the past, individual compressor controls and nonsupervised multiple machine systems were slow and imprecise.This resulted in wide control ranges and large pressure swings. As a result of these large swings, individual compressor pressure control set points were established to maintain pressures higher than needed. This ensured that swings would not go below the minimum requirements for the system. Today, faster and more accurate microprocessor-based system controls with tighter control ranges allow for a drop in the system pressure set points. This advantage is that a precise control system is able to maintain a much lower average pressure without going below the minimum system requirements. Narrower variations in pressure not only use less energy, but also improve production quality control. Caution needs to be taken when lowering average system header pressure because large, sudden changes in demand can cause the pressure to drop below minimum requirements, leading to improper functioning of equipment. With careful matching of system controls and storage capacity, these problems can be avoided. The type of compressor being used largely determines the type of control specified for a given system and the facilities’ demand profile. If a system has a single compressor with a very steady
demand, a simple control system may be appropriate. On the other hand, a complex system with multiple compressors, varying demand, and many types of end-uses will require a more sophisticated strategy. In any case, careful consideration should be given to both compressor and system control selection because they can be the most important factors affecting system performance and efficiency. For energy efficiency, it is important to consider the control of individual machines and the way in which multiple installations meet the demand in terms of flow and pressure requirements. For a relatively low capital outlay, a modern compressor control system can save between 5% and 20% of the total generation costs, and in some cases improved pressure control will also result in productivity gains.
6.4.1 INDIVIDUAL COMPRESSOR CONTROL
Compressor manufacturers have developed a number of different types of control strategies. Controls such as start/stop, which start and stop the compressor motor, and load/unload, which engage and disengage the compressor from the running motor, respond to increases or reductions in air demand. Modulating inlet and multi-step controls allow the compressor to operate at partload and deliver a reduced amount of air during periods of reduced demand. Start/stop is the simplest control available and can be applied to either reciprocating or rotary screw compressors.The motor driving the compressor is turned on or off in response to the discharge pressure of the machine. Typically, a simple pressure switch provides the motor start/stop signal. This type of control should not be used in an application that has frequent cycling because
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starts will cause the motor to overheat and other compressor components to require more frequent maintenance. Most compressors up to 1,000 1/s can be switched to automatic stop/start control. Some machines change to this mode automatically. Automatic stop/start control stops the compressor after a period of no-load running, usually 10 - 15 minutes, and then automatically restarts the machine on a demand for air.The offload running time is essential, unless a soft starter is fitted, to protect the drive motor from too many starts. Soft start controls are available which provide a variable start time with minimised starting currents, and which eliminate current surges thereby preventing motor damage. Load/unload control, also known as constant speed control, allows the motor to run continuously, but unloads the compressor when the discharge pressure is adequate. Compressor manufacturers use different strategies for unloading a compressor, but in most cases, an unloaded rotary screw compressor will consume 15 - 35% of full load power while delivering no useful work. As a result, some load/unload control schemes can be inefficient. Modulating (throttling) inlet control allows the output of a compressor to be varied to meet flow requirements.Throttling is usually accomplished by closing down the inlet valve, thereby restricting inlet air to the compressor.This control scheme is applied to centrifugal compressors. This control method, when applied to displacement compressors, is an inefficient means of varying compressor output. When used on centrifugal compressors, more efficient results are obtained, particularly with the use of inlet guide vanes, which direct the air in the same direction as the impeller inlet. The amount of capacity reduction is limited by the potential for surge and minimum throttling capacity.
Rotary screw machines are often fitted with both two-step unloading and modulating control with a manual or automatic change over switch. Modulation should only be used if the load is over 75% of its design level: below this, two-step unloading is more efficient. Two-step systems, on any machine operate over a pressure differential of around 0.5 bar between full and no load. A correctly sized air receiver should be fitted to avoid control hunting. Piston machines with twoor three-step inlet suction valve unloaders, on- or off-line inlet valves, or five-step clearance pocket unloading, give the best efficiencies at part loads. Piston, vane and screw machines with variable inlet throttle valves that modulate over a close pressure range are not efficient on low loads, because they are positive displacement machines and throttling causes an increase in compression ratio. Some compressors are designed to operate in two or more partially loaded conditions. With such a control scheme, output pressure can be closely controlled without requiring the compressor to start/stop or load/unload. Reciprocating compressors are designed as twostep (start/stop or load/unload), three-step (0%, 50%, l00%) or five-step (0%, 25%, 50%, 75%, 100%) control. These control schemes generally exhibit an almost direct relationship between motor power consumption and loaded capacity. Some rotary screw compressors can vary their compression volumes (ratio) using sliding or turn valves. These are generally applied in conjunction with modulating inlet valves to provide more accurate pressure control with improved partload efficiency. Centrifugal compressors are dynamic machines and behave efficiently on part load. Output is normally reduced by modulation to 70% of the design flow. For installations where the demand is sometimes less than this, machines with automatic dual control systems should be installed to avoid
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wasting energy due to bypassing of pressurised air at part loads. Inlet guide vanes are preferable to inlet throttles, because they improve the part load efficiency and turndown range, particularly at offdesign inlet conditions. Using variable speed drive (VSD) motors to drive piston and screw compressors offers many control and efficiency advantages. In the past costs have been prohibitive; however, new advances in electronics and control gear are making these systems more popular. Care should be taken not to reduce the compressor speed to the extent that it is inadequately lubricated. VSDs are unsuitable for centrifugal compressors unless specifically designed for the purpose.
individual compressor capacity to meet system demand. Sequencers are referred to as single master control units because all compressor, operating decisions are made and directed from the master unit. Sequencers control compressor systems by taking individual compressor capacity on and off-line in response to monitored system pressure (demand). The control system typically offers a higher efficiency because the control range around the system target pressure is tighter. This tighter range allows for a reduction in average system pressure. Again, caution needs to be taken when lowering average system header pressure because large, sudden changes in demand can cause the pressure to drop below minimum requirements, leading to improper functioning of equipment. With careful matching of system controls and storage capacity, these problems can be avoided. Various forms of automatic sequencing control exist for optimising the operation of multiple installations and equalising the wear through rotation of the sequence. Microprocessor-based systems have much more accurate pressure control than pressure switch or air governor controls, and avoid large pressure differentials and energy waste. They can take into account lower pressure requirements during nonproductive hours and adjust accordingly, and can also control system isolation valves. Some multiple machine control systems work with a combination of pressure and demand signals to ensure that only the correct machines are on-line at any one time. Network controls offer the latest in system control. It is important that these controllers be used to shut down any compressors running unnecessarily. They also allow the operating compressors to function in a more efficient mode. Controllers used in networks are combination controllers. They provide individual compressor
6.4.2 MULTIPLE COMPRESSOR CONTROL
By definition, system controls control the actions of the multiple individual compressors that supply air to the system. Prior to the introduction of automatic system controls, compressor systems were set by a method known as cascading set points. Individual compressor operating pressure set points were established to either add or subtract compressor capacity to meet system demand.The additive nature of this strategy results in large control ranges. The objective of an effective, automatic system, control strategy is to match system demand with compressors operated at or near their maximum efficiency levels. This can be accomplished in a number of ways, depending on fluctuations in demand, available storage, and the characteristics of the equipment supplying and treating the compressed air. Sequencers are, as the name implies, devices used to regulate systems by sequencing or staging
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control as well as system control functions. The term multi-master refers to the system control capability within each individual compressor controller. These individual controllers are linked or networked together, thereby sharing all operating information and status. One of the networked controllers is designated as the leader. Because these controllers share information, the compressor operating decisions with respect to changing air demand can be made more quickly and accurately. The effect is a tight pressure control range, which allows a further reduction in the air system target pressure. Although, initial costs for system controls are often high, these controls are becoming more common because of the resulting reductions in operating costs. Control systems can be built into building management systems along with compressor condition monitoring, automatic operation of zone isolation valves, compressor electric motor input
readings and departmental air demand metering from remote outstations.
6.4.3 THE IMPORTANCE OF PRESSURE CONTROL
Energy can be saved in most multi-compressor installations by improving the pressure control system. The majority of systems have two basic shortcomings: • • Pressures are maintained at a higher level than is needed by the end users; and Generation pressures are set too high and not varied according to demand.
The consequence of both of these failings is demonstrated in Figure 10, which represents a typical cascade control system.
Figure 10: Cascade pressure control typical situation (Source: ETSU)
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Usually the minimum pressure needed by the most critical piece of machinery sets the lowest acceptable pressure at the compressor house. To ensure that this pressure is achieved at the critical point, the pressure at the compressor house (i.e. the control pressure) will need to be set to 0.7 bar (10 psi) above the value required, to overcome line pressure losses on the distribution network. This includes occasions when the airflow rate is highest and hence the distribution losses are greatest. For example, if 5.8 barg (85 psig) is required, the control pressure will need to be 6.5 barg (95 psig).This control pressure compensation is shown in Figure 10 by the lower two lines. The left-hand axis in Figure 10 shows the cascade switching of the compressors and it represents a typical cascade pressure control system. At maximum flow rates all four compressors will be operating, as designated by region D. As the air usage falls, the pressure in the lines at the compressor house will rise and one compressor will unload. If usage falls further, the pressure moves into region C where another compressor unloads, and so on up to region A where only one compressor is providing the load. Due to the nature of the pressure switches, the minimum control bands are 0.2 bar for each compressor leading to a very wide overall control band (0.8 bar). The consequence of a wide control band is that, except at maximum air usage, the end user and compressor control pressures will always be up to 10% higher than those actually required. This has two negative effects: firstly, it takes up to 5% more electricity to generate the air at a 10% greater pressure (see Table 12) and secondly the usage of air in most applications is directly proportional to its pressure.The worst case scenario, shown by the upper two lines in Figure 10 which may represent night time and weekend usage, could be costing at least 15% more than is necessary.
Table 12: Generation pressure savings Pressure Barg (psig) 7(100) 6(90) 5.5(80) Energy Savings (%) Single Stage Two Stage 5 10 5 11
To keep generation costs to a minimum: • Pressure control should be based on the pressure at the most sensitive/critical pieces of machinery; Compressor sequencing should be based on as narrow a pressure band as possible to achieve the minimum generation pressure at all times.
•
6.5 SIZING •
Compressors should be sized as closely as possible to the duty. It is not economical to run any machine for long periods at low loads, due to electric motor inefficiencies. The off load power can be 15% - 70% of the on-load power once motor inefficiencies have been taken into account. For new installations with multiple compressors, it is worthwhile considering installation of a selection of unit sizes, so those compressors operating close to full output can meet the demand. Care should be taken to ensure that the overall system efficiency is improved, taking into account the lower generating efficiencies of some smaller compressors.
6.6 MAINTENANCE •
Like all electro-mechanical equipment, industrial compressed air systems require periodic
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maintenance to operate at peak efficiency and minimize unscheduled downtime. Inadequate maintenance can have a significant impact on energy consumption via lower compression efficiency, air leakage, or pressure variability. It can also lead to high operating temperatures, poor moisture control, and excessive contamination. Most problems are minor and can be connected by simple adjustments, cleaning, part replacement, or the elimination of adverse conditions. Compressed air system maintenance is similar to that performed on cars: filters and fluids are replaced; cooling water is inspected, belts are adjusted, and leaks are identified and repaired. All equipment in the compressed air system should be maintained in accordance with manufacturers’ specifications. Manufacturers provide inspection, maintenance and service schedules that should be followed strictly. In many cases, it makes sense from efficiency and economic standpoints to maintain equipment more frequently than the intervals recommended by manufacturers, which are primarily designed to protect equipment. One way to tell if a system is being maintained well and is operating properly is to periodically benchmark the system by tracking power, pressure and flow. If power use at a given pressure and flow rate goes up, the system’s efficiency is degrading. This benchmarking will also let you know if the compressor is operating at full capacity, and if the capacity is decreasing over time. On new systems, specifications should be recorded when the system is first set up and operated properly. Maintenance issues for specific components are discussed below. system
exchanger surfaces, air lubricant separator, lubricant, lubricant filter, and air inlet filter. The compressor and inter-cooling surfaces need to be kept clean and foul free. If they are dirty, compressor efficiency will be adversely affected. Fans and water pumps should also be inspected to ensure that they are operating at peak performance. The air lubricant separator in a lubricant-cooled rotary screw compressor generally starts with a 2 - 3 psi differential pressure drop at full-load when new. Maintenance manuals usually suggest changing them when there is about a l0 psi pressure drop across the separator. In many cases it may make sense to make an earlier separator replacement, especially if electricity prices are high. The compressor lubricant and lubricant filter need to be changed per manufacturer’s specification. Lubricant can become corrosive and degrade both the equipment and system efficiency. For lubricant-injected rotary compressors, the lubricant serves to lubricate bearings, gears, and intermeshing rotor surfaces.The lubricant also acts as a seal and removes most of the heat of compression. Only a lubricant meeting the manufacturer’s specifications should be used. Inlet filters and inlet piping also need to be kept clean. A dirty filter can reduce compressor capacity and efficiency. Filters should be maintained at least per manufacturer’s specifications, taking into account the level of contaminants in the facility’s air.
6.6.2 COMPRESSOR DRIVES 6.6.1 COMPRESSOR PACKAGE
The main areas of the compressor package in
need of maintenance are the compressor, heat
If the electric motor driving a compressor is not properly maintained, it will not only consume more energy, but also be apt to fail before its
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expected lifetime. The two most important aspects of motor maintenance are lubrication and cleaning.
6.6.2.4 GENERAL
Compressors run for many hours, often in appalling conditions. Equating the example 500 1/s compressor to a car, it would cover over 70,000 miles per annum at an average speed of 30 mph. Some compressors run the equivalent of 250,000 miles per annum on this basis. Good maintenance is therefore essential.
6.6.2.1 LUBRICATION
Too much lubrication can be just as harmful as too little and is a major cause of premature motor failure. Motors should be lubricated per the manufacturer’s specification, which can be anywhere from every 2 months to every 18 months, depending on annual hours of operation and motor speed. On motors with bearing grease fittings, the first step in lubrication is to clean the grease fitting and remove the drain plug. High quality new grease should be added, and the motor should be run for about an hour before the drain plug is replaced.This allows excess grease to be purged from the motor without dripping on the windings and damaging them.
Piston compressors, particularly the oil-free type, suffer the most in efficiency terms from lack of maintenance.
6.6.2.2 CLEANING.
Since motors need to dissipate heat, it is important to keep all of the air passages clean and free of obstruction. For enclosed motors, it is vital that cooling fins are kept free of debris. Poor motor cooling can increase motor temperature and winding resistance, which shortens motor life and increases energy consumption.
The efficiency of rotary vane and screw machines does not deteriorate so rapidly; however, there is a finite life for such compressors. As a guide, these types of machine must receive major maintenance after 25,000 hours life to maintain good efficiency. Oil-free toothed rotor and screw machines perform well for periods of up to 40,000 hours, after which there is a slow fall off in efficiency due to gradually increasing internal clearances. These types of machines then need major refurbishment to maintain efficiency.
Centrifugal compressors, having few moving parts and comparatively large ‘as built’ clearances, will maintain their efficiency over longer periods. The inlet air filters, cooling water system and the intercoolers must be rigorously maintained or efficiency will fall off rapidly.
6.6.2.3 BELTS
Motor v-belt drives also require periodic maintenance. Tight belts can lead to excessive bearing wear, and loose belts can slip and waste energy. Under normal operation, belts stretch and wear and, therefore, require adjustment. A good rule-ofthumb is to examine and adjust belts after every 400 hours of operation.
It is a false economy to ignore maintenance on any type of compressor. It is recommended that manufacturers, or their accredited agents, are used for service work and that genuine spare parts, to the original design, are used. An apparently cheaper component, such as an incorrectly designed replacement discharge valve, costs more in the long term due to the detrimental effect that it has on the compressor efficiency.
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6.7 HEAT RECOVERY •
As much as 80-93% of the electrical energy used by an industrial air compressor is converted into heat.The heat is low grade and usually wasted, but in many cases it can be recovered. A properly designed heat recovery unit can recover anywhere from 50-90% of this available thermal energy and put it to useful work heating air or water. Heat is given off by the compressors themselves, by intercoolers on multi-stage compressors (which improve their efficiency) and by aftercoolers. Compressors can be air or water-cooled. On water-cooled compressors heat is normally removed from the compression cylinders by water jackets. Hot water at around 50°C can be collected from a piston compressor and used for a variety of purposes, including increasing the temperature of boiler feed water, process water or domestic hot water. Warm air can be ducted from air-cooled compressors, particularly packaged rotary machines, and can be used for duties such as space heating and air curtains.
the addition of ducting and another fan to handle the duct loading and to eliminate any backpressure on the compressor cooling fan. These heat recovery systems can be modulated with a simple thermostatically controlled hinged vent. When heating is not required - such as in the summer months - the hot air can be ducted outside the building. The vent can also be thermostatically regulated to provide a constant temperature for a heated area. Hot air can be used for space heating, industrial drying, preheating aspirated air for oil burners, or any other application requiring warm air. As a rule of thumb, approximately 15 kWh/hour of energy is available for each 50 l/sec (100 cfm) of capacity (at full-load). Air temperatures of 17 to 22°C (30 to 40°F) above the cooling air inlet temperature can be obtained. Recovery efficiencies of 80-90% are common. If the air supply for the compressor is not from outside, you should be careful not to draw in heated air because this will reduce the compressor efficiency. During the summer months, any hot air should be ducted to atmosphere, otherwise it will be dissipated to the surrounding area and could subsequently be drawn back into the compressor, again reducing efficiency.
6.7.1 HEAT RECOVERY WITH AIR-COOLED COMPRESSORS 6.7.1.1 HEATING AIR
Air-cooled packaged rotary screw compressors are very amenable to heat recovery for space heating or other hot air uses. Ambient atmospheric air is heated by passing it across the system’s after-cooler and lubricant cooler, where it extracts heat from both the compressed air and the lubricant that is used to lubricate and cool the compressor. Since packaged compressors are typically enclosed in cabinets and already include heat exchangers and fans, the only system modifications needed are
6.7.1.2 HEATING WATER
Using a heat exchanger, it is also possible to extract waste heat from the lubricant coolers found in packaged water-cooled reciprocating or rotary screw compressors and produce hot water. Depending on design, heat exchangers can produce non-potable (grey) or potable water. When hot water is not required, the lubricant is muted to the standard lubricant cooler. Hot water can be used in central heating or
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boiler systems, industrial cleaning processes, plating operations, heat pumps, laundries, or any other application where hot water is required. Heat exchangers also offer an opportunity to produce hot air and hot water, and allow the operator some ability to vary the hot air/hot water ratio.
compare heat recovery with the current source of energy for generating thermal energy, which may be a low-price fossil fuel such as coal.
6.7.4 USE OF HOT COMPRESSED AIR FOR PROCESS DUTIES 6.7.2 HEAT RECOVERY WITH WATER-COOLED COMPRESSORS
Heat recovery for space heating is not as common with water-cooled compressors because an extra stage of heat exchange is required and the temperature of the available heat is lower. However, since many water-cooled compressors are quite large, heat recovery for space heating can be an attractive opportunity. Recovery efficiencies of 50-60% are typical. Some process applications, such as spool valves in a glass factory or drop forging hammers, benefit from hot compressed air. Hot compressed air is not often used because of safety concerns, since there is a risk of compressor oil carry-over spontaneously igniting if the discharge temperature is too high. If hot compressed air is to be used, all the air pipe work should be lagged to prevent cooling and an over-sized condensate recovery system should be fitted to take care of the additional condensate, which will form. An after-cooler will not be required. Using hot air is especially worth considering if the air is compressed near to the point of usage and the pipe runs (and hence the heat and pressure loss) are therefore small. The volumetric increase achieved by using hot air will save up to 25% of the energy used on the duty, provided the air is kept hot up to the usage point.
6.7.3 CALCULATING COMPRESSOR HEAT ENERGY SAVINGS
When calculating energy savings and payback periods for heat recovery units, it is important to
Table 13: Heat recovery available from air cooled rotary screw compressors at full load (assuming 90% motor efficiency) Capacity (l/s) 40 60 159 314 450 585 725 Nominal Motor Power (kW) 15 22 55 110 160 200 250 Warm Air Flow (l/s) 450 810 1,600 3,700 5,600 8,900 8,900 Heat Available (kWh/h) 12 18 45 89 130 162 203
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6.8
SITE INTEGRATION TO SAVE ENERGY
points of end use, the choice will largely depend on: • • • the physical layout of the factory; the position of off-takes, the compressor operating hours.
It is important that site integration of air compressors is considered at the planning and design stages to get the maximum benefit of using packaged compressors with integrated heat recovery systems. These packaged systems generally use rotary screw compressors, which have the additional benefit of operating with low noise. When there is a choice between using a central installation or smaller compressors nearer to the
Smaller compressors are generally less efficient than larger ones, and this must be taken into account when justifying decentralisation. Where practicable, air compressors should be sited near to points of large air demand and, if possible, near to heat demands, if recovery is an option, to reduce pipe runs and also capital and running costs.
•••••••••
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7. MONITORING OF COMPRESSED AIR SYSTEMS
••••••••••••••
7.1 MONITORING •
Without sufficient instrumentation on compressed air systems it is impossible to determine whether or not they are operating efficiently. Throughout this guide there are many examples of where energy is wasted; without a mechanism for detecting wastage, it is likely to continue and minimum energy costs will not be achieved. This section sets guidelines for the minimum amount of metering to be installed on compressed air systems. the compressor and in the receiver, to help detect any fouling of the after-cooler heat exchanger: • inter-cooler pressure and temperature gauges where applicable: • pressure gauges at selected points along the distribution system, to obtain a pressure profile across the site and hence identify high pressure loss areas; • hours-run meters that differentiate between on-load and off-load running times; • kWh meters (on compressors rated above 50kW (150 1/sec)). Readings from the first four instruments should be recorded daily: hours run and kWh meters should be read weekly to find usage trends. Any sudden increases should be investigated immediately in line with the monitoring and targeting activities. Most modem compressors have excellent electronic monitoring systems, which automatically log their running condition, generate alarms when abnormal running conditions occur and ultimately shut the machines down before any damage occurs. It is possible to purchase similar monitoring units for retrofitting to existing compressors. This, is particularly worthwhile with two-stage watercooled piston compressors, where compressor running condition has a major effect on efficiency. Adequate maintenance skills are not readily available in many factories, making early warning of problems essential.
7.1.1 COMPRESSORS
The running condition of a compressor can be assessed fairly easily. Table 14 summarises the measurements required for different types of compressor. New compressors, particularly those within an integrated package, are now supplied with all the required instrumentation. For existing plant, however, the following list gives the recommended minimum instrumentation: • • pressure gauge on the receiver; water temperature gauges in the compressor cooling jacket and the aftercooler, to detect any blockages that may be occurring; air temperature gauges at the outlet of
•
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Table 14: Compressor Assessment Compressor Type All types Measurement* Air discharge temperature downstream of the after cooler. Inlet filter differential pressure** Indication Over 15°C difference between inlet air and compressed air after the after cooler shows cooling surfaces are becoming fouled and efficiency is falling off. Values outside manufacturer’s specification can indicate fouled inlet filters, leading to efficiency loss. Two-stage piston, toothed rotor or screw types Inter-cooler pressure Measurement outside that in manufacturer’s service manual indicates problem. Greater than 0.3 bar pressure differential indicates problems with oil separator and loss of efficiency. Values outside manufacturer’s specification indicate problems. Machine not in optimum running condition.
Oil-injected vane or screw types
Difference between compressor element discharge pressure and actual delivered pressure.
Centrifugal types
Inter-stage temperatures and pressures, pinion vibration levels and air inlet differential pressure.
* All the pressure and temperature gauges used for this purpose must be regularly calibrated. ** Most compressors have an inlet filter differential pressure gauge fitted: if not, it is recommended that one is fitted.
7.1.2 AIR FLOW METERS
Many different forms of air flow meter are available, each with its strengths and weaknesses. The four common ones are: • pitot tube (or Curnon meter); • turbine meter: • orifice plate: • Vortex shedding meter.
All of the above meters measure the actual air velocity, and pressure and temperature compensation is necessary to get an accurate measure of the standard airflow rate (at 0°C. 760 mm Hg). Details of all these meters are described in greater detail in Appendix I. A chart recording facility attached to any of the meters will give very valuable information about the flow rate demand pattern. Recorders can be
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purchased to give a 24-hour or seven-day circular chart recording. Figure 11 also shows static pressure, which helps detect the critical pressure demand periods. It also gives a clear indication of how the compressors are coping with the demand and whether control pressures can be reduced during certain times of the day or week
with electricity consumption readings i.e. input energy to output air for a given demand can be calculated and used for analysis. Care must be taken when trying to establish the efficiency of an individual compressor, because the meter will be reading plant demand, not compressor output. It is possible to estimate the compressor output if the pressure is carefully monitored. If the system pressure is slowly falling with a compressor on load, then the whole output of the compressor, less any condensate or seal losses, will pass the flow meter and an estimate of the compressor output can be made. It is worth installing a permanent air flow meter on generation systems rated above 200 kW (600 I/s or 1,200 cfm) operating on a single-shift system. For full-time working, systems around half this capacity would warrant a meter.This limit is based on the meter costing around 10% of the annual air costs and assumes that the information given will lead to a 10% saving and hence a 12 month simple payback period.
7.1.3 COMPRESSOR HOUSE AIRFLOW METERING
The installation of an air flow meter on the main airline from the compressor house will give two very valuable pieces of information: • A true usage demand profile, and base leakage or minimum use information of the plant. This is preferable to estimating usage from the on/off-load running times of the compressors, as compressor output capacity must be assumed as design level, although it could be a long way from it. The overall generation efficiency coupled
•
Figure 11: Daily Chart Recording (Source: ETSU)
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7.1.4 DISTRIBUTION LINE AIRFLOW METERING
Permanent metering of airflow around a large distribution network can be very costly. This is mainly because good practice has led to the installation of ring main systems; so many branches have to be metered to account for a reasonable percentage of the total air used. As a guideline, a plant or area using in excesses of 500 1/sec (1,000 cfm) warrants the installation of a permanent meter. An alternative to permanent metering is to fit some means of measuring airflow in all main lines and to use a portable meter. This could mean fitting orifice plates in each line with a portable differential pressure transducer and chart recorder, or the addition of insertion points for turbine flow meters or pitot tubes. A single meter
can then be attached to each measuring point in turn and initiatives taken to reduce air consumption where appropriate.
7.2
MONITORING AND TARGETING
Monitoring and Targeting (M&T) has a much wider application than compressed air systems. When applied to compressed air systems, M&T could be defined as: Comparing the weekly usage of compressed air (as measured by kWh or airflow meters) against a pre-determined target, which reflects good practice. Monitoring and Targeting is dealt with in the booklet: "How to save energy and money: The 3E Strategy".
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8. compressed air audits
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This section outlines a strategy for identifying all the opportunities, including some auditing techniques that can easily be implemented. Ideally, before any actions are taken to improve a compressed air system, an audit should be carried out to determine the annual costs of the current system. If permanent metering is installed, this will provide an enormous amount of information to help find the best solutions. Without permanent meters, the energy consumption of each compressor will have to be estimated from the size of the motor, its average load (or its on/off times) and the number of hours it operates. The total energy costs can be calculated by adding the information for all of the compressors. It is estimated that 30% of the annual costs could be saved, providing good impetus for action. Compressed air system users should consider using an independent auditor to examine their compressed air system. Firms exist that specialize in compressed air system audits. Audits are also performed by electric utilities, equipment distributors and manufacturers, energy service companies, and engineering firms. An informed consumer should be aware that the quality and comprehensiveness of audits could vary. Independent auditors should provide recommendations, which are systems-neutral and commercially impartial. Independent auditors should neither specify nor recommend any particular manufacturer’s products. Having calculated the annual costs and established a baseline against which improvements can be measured, an audit of the complete compressed air system should be carried out. A comprehensive compressed air system audit should include an examination of both air supply and usage and the interaction between the supply and demand. Auditors typically measure the output of a compressed air system, calculate energy consumption in kilowatt-hours, and determine the annual cost of operating the system. The auditor may also measure total air losses due to leaks and locate those that are significant. All components of the compressed air system are inspected individually and problem areas are identified. Losses and poor performance due to system leaks, inappropriate misuse, and total system dynamics are calculated, and a written report with a recommended course of action is provided. It is best to start with the end users, because any improvements here may well have an effect on the air distribution network (i.e. redundant pipe work and reduction in pressure losses) and compressor demand. It is also normally the area where the greatest savings can be achieved.
8.1 END USAGE AUDIT •
8.1.1 LEAKS
The first priority is to assess the site leakage rate, as this is often the greatest wastage. To do this, a no-load leakage test must be carried out (see Section 3.3). From the results, the total percentage air lost and the annual costs of leakage can be calculated. Following this a leakage survey should be carried out and all leaks identified on a site drawing, as well as by tagging. The auditor should recommend a leak management program.
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8.1.2 END USERS
After the leakage appraisal, it is essential to look at each compressed air use in detail.The major tasks that need to be carried out are to: • Estimate the volume of major plant items, from either plant ratings or calculations, and note the number of hours they are worked, to help produce a breakdown of air usage and assist in deciding whether distribution lines are adequately sized; In some cases, recommendations such as specifying equipment that operates at a lower pressure will be made. An auditor may also recommend replacing existing compressed air-powered equipment with equipment that uses a source of energy other than compressed air. Compare the actual operating pressure with the design pressure and, if appropriate, fit a reducing valve (if the overall distribution line pressure cannot be reduced). Local storage and other modifications may be recommended. Investigate other methods of operation not involving the use of compressed air.
8.3 •
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AIR TREATMENT AUDIT
The following program should be carried out: The auditor typically examines the compressed air applications and determines the appropriate level of air treatment required for proper operation of the equipment. Actual air quality levels are then measured. If the air is not being treated enough, alternative treatment strategies are recommended. If the air is being over-treated (an indication of energy being wasted), recommendations are made to modify the system. In some cases, only certain end-use equipment requires highly treated air, and the auditor may recommend a system that, allows for different treatment levels at different points in the system. The total air drying capacity required should be calculated during the end user audit. If more air than necessary is being dried, the possibility of having two distribution systems, a wet and a dry, should be considered. Consideration should also be given to treating the higher quality air at the point of use. All drainage traps should be checked to ensure that they are neither leaking nor air binding. The location of the air intakes into the compressors should be checked to ensure that they are not supplying warm, wet or dusty air.
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8.2 DISTRIBUTION NETWORK AUDIT •
The distribution network should be surveyed and drawings obtained in line with the main things to look out for are: • • Zoning arrangements to isolate areas on different working patterns; Adequate pipe sizing and drainage: a pressure profile across the mains would be useful to identify large pressure losses; Elimination of redundant pipework or shortening of supply lines;
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8.4
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COMPRESSOR HOUSE AUDIT
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Having established the lowest possible demand profile for compressed air, it is necessary to ensure that the demand is serviced in the most efficient way possible.To do this it is necessary to carry out the following steps.
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•
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Record the electricity consumption of the compressors over a week by installing portable recording ammeters or demand recorders on the supply cables. Over the same period also record the hours run and hours ‘on-load’, if meters are available; For systems supplying a demand greater than 500 l/s, which should have an air flow meter installed, record the actual air demand over the week. If this is not possible, estimate, the air demand profile by assuming an output capacity of the compressors, and combining this with the hours ‘on-load’ data (this is not possible if the compressors are on modulation control). If an air flow meter is installed, record the static air pressure over the week to establish times when the control pressure can be reduced to reflect lower air usage. From the electricity and airflow recordings, calculate an air generation efficiency. It is worthwhile running each machine individually to monitor actual output capacity and to determine the J/l (or kWh/100 cfm) performance of each machine. These figures can then be compared to assess whether each
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machine is good, average or poor. A simple calculation will then identify how much energy can be saved by either maintaining the poor machines or preferentially using the more efficient ones to service the demand. Investigate the load profiles of each compressor with a view to deciding whether the optimum size machines are running at any one time. Consider better methods of compressor control, such as predictive switching or rotational sequencing depending on the compressor load profiles.
System audits are designed to identify system inefficiencies. If a system is found poorly designed, in unsatisfactory operating condition or in need of substantial retrofit, a more detailed analysis of the system may be recommended. A comprehensive evaluation may also include extensive measurements and analysis of supply and demand interactions. Some auditors will also prepare a detailed systems flow diagram. A financial evaluation may also be performed, including current and proposed costs after retrofits are taken.
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APPENDIX 1: METER DETAILS
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A1.1 PITOT TUBE
A pitot tube is relatively easy to install. It consists of two pressure lines that measure the static pressure and the static plus dynamic (or total) pressure of the compressed air. By measuring the differential pressure between the two lines, the velocity of the air can be obtained. Pitot tubes can, however, be inaccurate if the dynamic pressure profile across the pipe diameter is not taken into account. Manufacturers’ instructions should be followed to avoid this problem. A typical layout for a pitot tube is shown in Figure 12 below. Pitot tubes are often used for permanent meter installation. They give an excellent indication of flow rate and are relatively cheap.
A1.2 ORIFICE PLATE
Orifice plate meters consist of a drilled and machined disc inserted into the flow stream and clamped between two flanges (Figure 13). The orifice plate continues to be the most commonly used device for measuring flow rate.
Figure 12: Pitot tube operation (Source: ETSU)
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The orifice plate itself is the main element of the meter. The resulting differential pressure is measured via impulse lines connected from the pressure tappings of the orifice plate to a differential pressure transducer. The pressure tappings can be located in the pipeline up and downstream of the orifice plate. Alternatively, orifice plates can be supplied with corner or flange pressure tappings as part of a plate carrier ring assembly. Variations in the performance of the various tapping point locations can be used in formulae used to determine the coefficient of discharge. The location of the orifice plate in the pipe run is also important. The differential pressure measurement is sensitive to swirl and other fluid effects, so the orifice plate should be located a certain distance away, upstream and downstream,
from any pipefitting. British Standard BS1O42 and International Standard ISO 5167 provide details of the dimensions required. Orifice plate manufacturers should, however, advise on standard requirements, and reference to the published Standards is usually unnecessary. A typical requirement would be that there must be at least ten pipe diameters of straight pipe upstream of the meter and five pipe diameters downstream of the meter.
A1.3 TURBINE METERS
Turbine meters consist of a freely rotating propeller or screw located in the air pipe (Figure 14). Provided that bearing drag is minimised and the blades are well designed, the process stream will exert a torque on the turbine causing it to
Figure 13: Orifice Plate (Source: ETSU)
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rotate at a velocity proportional to the airflow rate. If a magnetic coil or optical device is placed in the meter housing, a voltage pulse can be induced each time a turbine blade passes it. The pulse rate will be proportional to the rate of flow and the total number of pulses that can be integrated to give the air volume, which has passed the meter. The response of this type of meter is approximately linear, except at low flow rates when the drag effects of the bearings become significant and may affect the linearity of the response. Any non-linearity can be overcome by incorporating a calibration curve into the system, which is used to convert the signal pulse into a flow rate.
principle that when a fluid stream flows around a bluff body (the vortex ‘shedder’), viscosity-related effects produce vortices downstream. The most common body shapes used in such meters are rectangular or triangular. The vortices are shed sequentially from either side of the bluff body at a frequency proportional to the flow velocity. Several methods of sensing the vortices exist. A common method is to use a piezo-electrical cell located in the bluff body support spindle. Shedding of the vortices creates lift in the bluff body, which in turn causes small movements to the spindle. Each movement compresses the cell, thereby generating a small electric current. Other methods use ultrasonics, which are modulated by the vortices, or involve the detection of the small pressure waves that accompany the shedding of the vortices
A1.4 VORTEX SHEDDING METERS
The vortex shedding meter operates on the
Figure 14:Turbine Meter (Source: ETSU)
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Figure 15:Vortex meter operation (Source: ETSU)
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SOURCES OF FURTHER INFORMATION
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For the latest news in energy efficiency technology: “Energy Management News” is a free newsletter issued by the ERI, which contains information on the latest developments in energy efficiency in Southern Africa and details of forthcoming energy efficiency events. Copies can be obtained from: The Energy Research Institute Department of Mechanical Engineering University of Cape Town Rondebosch 7700 Cape Town South Africa
Tel No: (+27 21) 650 3892 Fax No: (+27 21) 686 4838 Email: [email protected]
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Guide Book 3
COMPRESSED AIR SYSTEMS
STRATEGY
ENERGY EFFICIENCY EARNINGS
3E
STRATEGY
Netherlands Ministery of Economic Affairs
RA
LS
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Technical Services International
TSI
EUROPEAN COMMISSION
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RG
MI
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HOW TO SAVE ENERGY AND MONEY IN COMPRESSED AIR SYSTEMS
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This booklet is part of the 3E strategy series. It provides advice on practical ways of improving energy efficiency in compressed air systems. Prepared for the European Commission DG TREN by: The Energy Research Institute Department of Mechanical Engineering University of Cape Town Rondebosch 7700 Cape Town South Africa This project is funded by the European Commission and co-funded by the Dutch Ministry of Economics, the South African Department of Minerals and Energy and Technology Services International , with the Chief contractor being ETSU. Neither the European Commission, nor any person acting on behalf of the commission, nor NOVEM, ETSU, ERI, nor any of the information sources is responsible for the use of the information contained in this publication The views and judgements given in this publication do not necessarily represent the views of the European Commission
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HOW TO SAVE ENERGY AND MONEY IN COMPRESSED AIR SYSTEMS
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HOW TO SAVE ENERGY AND MONEY IN COMPRESSED AIR SYSTEMS
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Other titles in the 3E strategy series: HOW TO HOW TO HOW TO HOW TO HOW TO HOW TO SAVE SAVE SAVE SAVE SAVE SAVE ENERGY AND ENERGY AND ENERGY AND ENERGY AND ENERGY AND ENERGY AND MONEY:THE 3E STRATEGY MONEY IN ELECTRICITY USE MONEY IN BOILERS AND FURNACES MONEY IN STEAM SYSTEMS MONEY IN REFRIGERATION MONEY IN insulation
Copies of these guides may be obtained from: The Energy Research Institute Department of Mechanical Engineering University of Cape Town Rondebosch 7700 Cape Town South Africa Tel No: (+27 21) 650 3892 Fax No: (+27 21) 686 4838 Email: [email protected] Website: http://www.3e.uct.ac.za/introduction.htm
ACKNOWLEDGEMENTS
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The Energy Research Institute would like to acknowledge the following for their contribution in the production of this guide: • • • Energy Technology Support Unit (ETSU), UK, for permission to use information from the “Energy Efficiency Best Practice” series of handbooks. Wilma Walden of Studio.com for graphic design work ([email protected]). Doug Geddes of South African Breweries for the cover colour photography.
Guide Book Essentials:
QUICK ‘CHECK-LIST’ FOR SAVING ENERGY and MONEY IN COMPRESSED AIR SYSTEMS
This list is a selected summary of energy and cost savings opportunities outlined in the text. Many more are detailed in the body of the booklet.These are intended to be a quick ‘checklist’. EFFICIENT COMPRESSED AIR DISTRIBUTION: • Establish and audit the compressed air needs of the plant. Identify system oversupply in terms of air quality or pressures. • Remove/isolate dead lags and minimise pressure drops. • Ensure the compressor plant is operating according to manufacturers/suppliers specifications. • Minimise the air leakage rate. A planned maintenance program to cover the air distribution system is useful. COMPRESSED AIR TREATMENT: • Keep air treatment to the minimum possible. If only one user in the system requires high quality air, consider treatment of air at that point. COMPRESSED AIR GENERATION: • Make sure the air intake is cool and clean. Use outside air for compression where possible. • Generate compressed air at the lowest possible pressure that will meet site requirements. • Ensure that the design of the plant compressed air distribution system does not produce an excessive pressure drop between generation plant and end user. • Recover heat of compression from inter-coolers and after-coolers wherever possible. • Ensure that the control systems installed result in efficient operation. Investigate the possibility of sequencing a plant with more than one compressor unit. • Control generation to meet demand closely and efficiently. Make sure the control system ensures that energy consumption is minimised during low or no-load conditions for individual machines. MONITORING: • Set up a targeting and monitoring program. This will help ‘benchmark’ consumption and indicate future irregularities. AUDITING: • In order to establish the status quo, and quantify wastage an audit – as described in chapter 8 of this booklet - should be carried out.
Table of Contents
UNITS ............................................................................................................................................................................................................................1 1. INTRODUCTION............................................................................................................................................................................................3 2. ECONOMICS OF COMPRESSED AIR PRODUCTION.........................................................................................................5 2.1 Calculating electricity costs – a simple calculation ...........................................................................................................5 2.1.1 For a compressor running at full load.........................................................................................................................5 2.1.2 A Calculation using Measurements ..............................................................................................................................6 2.2 A Calculation for Part Load Operation ...................................................................................................................................6 2.3 Savings from Performance Improvements...............................................................................................................................6 3. REDUCING COMPRESSED AIR DEMAND TO SAVE ENERGY .....................................................................................7 3.1 Compressed Air Usage........................................................................................................................................................................7 3.2 Assessing Air Needs ..............................................................................................................................................................................7 3.2.1 Air Quality...................................................................................................................................................................................7 3.2.2 Air Quantity - Capacity .......................................................................................................................................................8 3.2.3 Load Profile.................................................................................................................................................................................8 3.3 Compressed Air Misuse ......................................................................................................................................................................9 3.3.1 Unregulated End-Uses..........................................................................................................................................................9 3.3.2 Abandoned Equipment.....................................................................................................................................................10 3.3.3 Periods when Compressed Air is not Required ................................................................................................10 3.4 Leaks .........................................................................................................................................................................................................10 3.4.1 Estimating Amount of Leakage.....................................................................................................................................11 3.4.2 Leak Detection ......................................................................................................................................................................12 3.4.3 How to Fix Leaks .................................................................................................................................................................12 3.4.4 A Leak Prevention Program...........................................................................................................................................13 4. EFFICIENT COMPRESSED AIR DISTRIBUTION .....................................................................................................................14 4.1 Introduction..............................................................................................................................................................................................14 4.2 Distribution Main..................................................................................................................................................................................15 4.3 Water Drainage .....................................................................................................................................................................................18 4.4 Drain Traps................................................................................................................................................................................................18 4.5 Air Receivers ...........................................................................................................................................................................................19 4.6 Regulators..................................................................................................................................................................................................20 4.7 Distribution Network........................................................................................................................................................................20 4.8 Maintenance .............................................................................................................................................................................................21 4.9 System isolation.....................................................................................................................................................................................21
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5. COMPRESSED AIR TREATMENT........................................................................................................................................................22 5.1 Introduction..............................................................................................................................................................................................22 5.2 Dryers ........................................................................................................................................................................................................24 5.2.1 Desiccant Dryers..................................................................................................................................................................24 5.2.2 Sorption Dryers ....................................................................................................................................................................25 5.2.3 Deliquescent (Absorption) Dryers............................................................................................................................25 5.2.4 Refrigeration Dryers ..........................................................................................................................................................25 5.3 After Filters ..............................................................................................................................................................................................25 5.4 Air Intakes ................................................................................................................................................................................................25 5.4.1 Air Inlet Cooling....................................................................................................................................................................26 5.5 Installation, Configuration and Sizing ......................................................................................................................................26 5.6 Treatment Systems Maintenance................................................................................................................................................27 5.7 Potential saving areas ........................................................................................................................................................................27 6. COMPRESSED AIR GENERATION ..................................................................................................................................................28 6.1 Positive Displacement Compressors ......................................................................................................................................29 6.1.1 Reciprocating Compressors...........................................................................................................................................29 6.1.2 Rotary Positive Displacement Compressors........................................................................................................29 6.2 Dynamic Compressors ....................................................................................................................................................................30 6.3 Energy Efficient Compressor Selection..................................................................................................................................30 6.4 Energy Efficient Compressor Control ....................................................................................................................................31 6.4.1 Individual Compressor Control ........................................................................................................................................32 6.4.2 Multiple Compressor Control ......................................................................................................................................34 6.4.3 The Importance of Pressure Control ......................................................................................................................35 6.5 Sizing ........................................................................................................................................................................................................36 6.6 Maintenance ............................................................................................................................................................................................36 6.6.1 Compressor Package..........................................................................................................................................................37 6.6.2 Compressor Drives ............................................................................................................................................................37 6.7 Heat Recovery........................................................................................................................................................................................39 6.7.1 Heat Recovery with Air-cooled Compressors....................................................................................................39 6.7.2 Heat Recovery with Water-cooled Compressors ............................................................................................40 6.7.3 Calculating Compressor Heat Energy Savings ....................................................................................................40 6.7.4 Use of Hot Compressed Air for Process Duties ..............................................................................................40 6.8 Site Integration to Save Energy ..................................................................................................................................................41
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7. MONITORING OF COMPRESSED AIR SYSTEMS..................................................................................................................42 7.1 Monitoring ................................................................................................................................................................................................42 7.1.1 Compressors ..........................................................................................................................................................................42 7.1.2 Air Flow Meters ....................................................................................................................................................................43 7.1.3 Compressor House Airflow Metering......................................................................................................................44 7.1.4 Distribution Line Airflow Metering ............................................................................................................................45 7.2 Monitoring and targeting ................................................................................................................................................................45 8. COMPRESSED AIR AUDITS ..................................................................................................................................................................46 8.1 End Usage Audit ....................................................................................................................................................................................46 8.1.1 Leaks ............................................................................................................................................................................................46 8.1.2 End Users..................................................................................................................................................................................47 8.2 Distribution Network Audit..........................................................................................................................................................47 8.3 Air Treatment Audit ............................................................................................................................................................................47 8.4 Compressor House Audit ..............................................................................................................................................................47 APPENDIX 1: METER DETAILS ................................................................................................................................................................49 A1.1 Pitot Tube ............................................................................................................................................................................................49 A1.2 Orifice Plate........................................................................................................................................................................................49 A1.3 Turbine Meters..................................................................................................................................................................................50 A1.4 Vortex Shedding Meters ............................................................................................................................................................51
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UNITS
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Since South Africa mainly uses metric units, these are the first choice in this guide. However, Imperial units are often given as well in parenthesis. The units used are given in the table below:
Table 1: Unit Conversions Pressure absolute Pressure gauge Flow, volumetric l/sec Power Energy Specific energy Metric bar barg l/sec cfm kW kWh J/l Imperial psi psig cfm 1 l/s = 2 cfm (approx) hp Btu Conversion 1 barg = 14.7 psig 1 barg = 14.7 psi 1 l/s = 2 cfm (approx) J/l 1 kW = 1.34 hp 1 kWh = 3412.4 Btu
ABBREVIATIONS: psi: pounds per square inch psig: pounds per square inch gauge l/sec: litres per second cfm: cubic feet per minute J/l: Joules/litre kW: kilowatt hp: horsepower kWh: kilowatt-hour Btu: British thermal units Pressure absolute = pressure gauge + 1 bar 1 bar = 100 kPa Standard atmospheric pressure = 1.013 bar
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1. INTRODUCTION
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This guide has been written to provide those responsible for energy management and maintenance of compressed air systems with opportunities for optimising system performance in terms of cost. Significant cost savings can be realised as a result of implementation of the energy efficient measures outlined in this booklet. avoidable wastage. This can be done without compromising production at all. Another factor that should be taken into account is the awareness of end users, who often tend to regard compressed air as a free and convenient resource and usually overlook the high cost of compressed air relative to other energy mediums. With typical energy savings of 30%, the annual potential savings are illustrated in the table below for the various compressor capacities. This table has used Eskom's Megaflex tariff rate for energy charges only. The 40 hr week assumes an annual three-week shutdown period and the 168 hr week assumes continuous operation. These figures are approximate, and differences due to electricity tariffs and hours of operation as well as the utilisation of the full compressor capacity should be expected. This guide provides the compressed air user with information on how to improve or optimise the design, performance and operation of compressed air systems from an energy efficiency (and therefore cost) perspective.
Compressed air as an energy-transmitting medium is versatile, flexible and safe, making it a popular choice and ensuring its continued use in industry.
Typically, compressed air accounts for about 10% of the total electrical power consumed by industry. For a time frame of 10 years, the cumulative costs of compressed air comprise of 10% maintenance, 15% capital and 75% energy. Therefore the implementation of a program to reduce energy consumption in compressed air systems (as outlined in this booklet) can translate into significant financial sayings.
Generally 30% of the total electrical energy used to compress air is wasted, and potential savings could be reaped through the introduction of simple cost effective measures that minimize this
Table 2: Potential Savings Compressor Capacity KW 18 90 160 315 litres/sec 55 250 500 1000 Cfm 110 500 1000 2000 Annual Potential Savings (Rands) 40hr/week 168hr/week 1,704 4,118: 8,520 23,590 15,146 41,937 29,818 82,563
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The key areas that savings can be derived from are: • The use/misuse/demand of compressed air (Chapter 3). • The distribution of compressed air (Chapter 4). • The treatment of compressed air (Chapter 5). • Compressed air generation (Chapter 6). • Compressed air control/monitoring (Chapter 7). The best approach to take to obtain these savings would be to tackle the main areas mentioned above sequentially and in a structured manner. For example leakage (coming under compressed air distribution, Chapter 3) and control (Chapter 6) of
compressed air systems are the biggest waste areas and could account for up to 70% of the total wastage. The guide looks at the recovery of the considerable amounts of waste heat generated via the thermodynamic process of air compression. Sections on system management and auditing are incorporated into the latter sections of guide. At the end of the guide, the potential savings that can be derived from the implementation of some of the energy efficient techniques described are illustrated in the presentation of three South African case studies.
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2. ECONOMICS OF COMPRESSED AIR PRODUCTION
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The delivery of compressed air requires costly equipment that consumes large amounts of electricity and needs frequent maintenance. However many users are not aware how much money they could save annually by improving the performance of their compressed air systems. Electricity is the largest cost in producing compressed air. Initial capital costs are usually exceeded by electricity costs after a short period, about one-year (depending on the type of compressor, manufacturer and the electricity charges). Maintenance costs could be ten percent or more of the initial cost of the system depending on the system quality and usage. The following sections of this chapter deal with a simple calculation estimating annual electricity costs.
2.1.1 FOR A COMPRESSOR RUNNING AT FULL LOAD
The following data is needed for this calculation: i. Motor rating ii. Motor efficiency (from the nameplate or manual) iii. Annual hours of operation iv. Costs of electricity i.e. tariff for consumption and maximum demand v. Power factor (PF) of the motor For a motor efficiency of 90% and using Eskom's Standard tariff of 0.0746 R/kWh and R41.33 / kVA, (assuming this motor does not significantly contribute maximum demand): Annual Electricity Energy Cost = kWx (1) x 24hrs x 7 days x 52 weeks x 0.0746 kWh = kWx (0.9) x 24hrs x 7 days x 52 weeks x 0.0746 R/kWh 0.9) x 24hrs x 7 days x 52 weeks x 0.0746 For newer, more efficient motors, this percentage could be higher but for older and rewound motors it could be 80%. The performance of electric motors should be monitored, and they should be serviced or replaced if their efficiency drops below an economically acceptable level. For a demand tariff of R41.33/kVA and assuming that the compressor maximum demand coincides with the peak demand for the whole factory: Monthly demand charge = Max kW / 0.9 / PF x R41.33
2.1 CALCULATING ELECTRICITY COSTS – A SIMPLE CALCULATION •
Electricity consumers may be charged just on the energy (kWh) they consume or they may be charged on the energy and on a maximum demand in kVA. Typically the maximum demand would be highest averaged kVA demand for any half hour in the month.
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2.1.2 A CALCULATION USING MEASUREMENTS
Another more accurate way of determining costs is through a relationship that involves taking electrical measurements of both full load amps and volts. The motor rating and efficiency are not needed, but the full load power factor is required and if not available, could be obtained from the manufacturer. Data needed: i. ii. iii. iv. v. Full load amps Full load volts Full load power factor Annual hours of operation at full load Costs of electricity tariff
loaded as a percentage of the electrical consumption when fully loaded. For a compressor that runs fully loaded for 65% of the time and at 0.3 of full load for 35% of the time: Annual Electricity Energy Costs = 52 weeks Motor rating (kW) x 24 hrs x 7 days x 0.9 x 0.0746 R/kWh x (0.65 + 0.3 x 0.35) Electrical utilities such as Eskom bill industrial customers at different rates and have more complicated rate structures such as Megaflex. These charges include energy (R/kWh) and demand charges (R/kVA) and have different rates depending on the season, time of day and level of consumption. It may be possible to restructure demand to cheaper ‘off-peak’ tariff times, this must be kept in mind when calculating potential savings.
Therefore, Annual Electricity Energy Costs = Full load amps x volts x Power factor 1000
2.3 SAVINGS FROM PERFORMANCE IMPROVEMENTS •
The higher the pressure of the compressed air required, the more expensive it is to deliver. For a system at 7 barg (l00psig), an additional onepercent in operating energy costs is required for every additional 0.14 bar (2psi) in operating pressure. Life cycle costing should be used because the lifetime electricity costs are much greater than the initial costs of the compressors. Overall system efficiency should be considered and not only the motor or compressor efficiency. To achieve this least cost solution for compressed air systems, users should select systems based on appropriate economics to determine compression, treatment, distribution, control and monitoring.
x annual hours of operation x electricity cost per kWh
2.2 A CALCULATION FOR PART LOAD OPERATION •
This calculation is done using the percentage of the time that the compressor is running at full load and the average fraction of the load at which the compressor runs for the rest of the time. Data needed: i. ii. iii. iv. v. vi. vii. Motor rating Motor efficiency - from nameplate Annual hours of operation Cost of electricity Percentage of time running fully loaded Percentage of time running not fully loaded The electrical consumption when not fully
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3. REDUCING COMPRESSED AIR DEMAND TO SAVE ENERGY
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3.1 COMPRESSED AIR USAGE •
Compressed air use should be constantly monitored and re-evaluated. Investigating where and how compressed air is used around site will reveal the areas in which major savings can be achieved. By the time compressed air reaches the end user, its cost as an energy source is very high, around 50c/kWh. It is vitally important that each use is investigated in detail to establish whether: 1. 2. 3. it needs to be operated by compressed air at all; the supply pressure is greater than necessary; there is adequate facility for isolating the supply line when it is not in use. compressed air system. As a general rule, compressed air should only be used if it improves safety, increases productivity or reduces labour. Typical overall efficiency when compressed air is used is around 10%. So the air used should be of minimum quantity, pressure and duration.
3.2 •
ASSESSING AIR NEEDS
Air needs are defined by the air quality, quantity and pressure required by the end uses in your plant. Assessing needs carefully will ensure that a compressed air system is configured properly.
3.2.1 AIR QUALITY
As illustrated in the following table, compressed air quality ranges from plant air to breathing air. Industrial applications typically use one of the first three air quality levels.The quality is determined by
Two commonplace examples of misuse are using compressed air for cleaning or cooling duties: alternatives exist for cleaning benches, and cooling duties can generally be carried out using high pressure fans or a lower pressure
Table 3: Ranges of Air Quality Plant Air Air tools, air-actuated valves, general plant air Instrument Air Process Air Laboraties, paint spraying, Food and pharmaceutical powder coating, process air, electronics climate control Breathing Air Hospital air systems, diving tank refill stations, respirators for cleaning and/or grit blasting
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the dryness and contaminant level required by the end-uses, and is accomplished with filtering and drying equipment.The higher the quality, the more the air costs to produce. Higher quality air usually requires additional equipment, which not only increases initial capital investment, but also makes the overall system more expensive to operate in terms of energy consumption and maintenance costs. One of the main factors in determining air quality is whether or not lubricant-free air is required. Lubricant-free air can be produced with either lubricant-free compressors, or with lubricantinjected compressors that have additional separation and filtration equipment. Lubricant-free rotary screw and reciprocating compressors usually have higher capital costs, lower efficiency and higher maintenance costs than lubricantinjected compressors. However, the additional separation and filtration equipment required by lubricant-injected compressors will cause some reduction in efficiency, especially if systems are not properly maintained. Careful consideration should be given to the specific end-use for the lubricantfree air, including the risk and cost associated with product contamination, before selecting a lubricant-free or lubricant-injected compressor.
effective. In most cases, a thorough evaluation of system demand may result in a control strategy that will meet system demand with reduced overall compressor capacity. Oversized air compressors are extremely inefficient because most systems use more energy per unit volume of air produced when operating at part-load. In many cases it makes sense to use multiple, smaller compressors with sequencing controls to allow for efficient operation at times when demand is less than peak. If a system is properly designed and maintained but is still experiencing capacity problems, an alternative to adding another compressor is to re-examine the use of compressed air for certain applications. For some tasks, blowers or electric tools may be more effective or appropriate.
3.2.3 LOAD PROFILE
Another key to properly designing and operating a compressed air system is assessing the plant’s compressed air load profile, that is, its requirements over a period of time.The variation in demand for air over time is a major consideration in system design. Plants with wide variations in air demand need a system that operates efficiently under partload. Multiple compressors with sequencing controls may provide more economical operation in such a case. Plants with a flatter load profile can use simpler control strategies.
3.2.2 AIR QUANTITY - CAPACITY
The required capacity for a compressed air system can be determined by summing the requirements of the tools and process operations (taking into account load factors) at the site. The total air requirement is not necessarily the sum of the maximum requirements for each tool and process, but the sum of the average air consumption of each. High short-term demands should be met by air stored in an air receiver. Systems may have more than one air receiver. Strategically locating air receivers near sources of high demand can also be
3.2.3.1 ARTIFICIAL DEMAND
Artificial demand is defined as the excess volume of air that is required by unregulated end uses as a result of supplying higher pressure than necessary for applications.This should be eliminated.
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3.2.3.2 PRESSURE
Different tools and process operations require different pressures. Pneumatic tool manufacturers rate tools for specific pressures, and the process engineers should specify process operation pressure requirements. Required pressure levels must take into account system losses from dryers, separators, filters, and piping. As mentioned before, a rule of thumb is that every 0.14 bar (2psi) increase in operating pressure requires an additional 1% in operating energy costs.
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•
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3.3 COMPRESSED AIR MISUSE •
Compressed air is heavily misused in industry. Careful thought should be given to all uses to see whether another, more cost-effective, method might be used instead of compressed air. For example, at a steelworks, measurements found an average air demand of 5,000 l/s at 7 barg. After investigation, it was established that only 2.000 l/s were actually being used appropriately. 2,000 l/s were used at 3 bar and 1,000 1/s were used for non-compressed air duties such as water-jetting and bearing cooling. Compressed air is probably the most expensive form of energy available in a plant. It is also clean, readily available, and simple-to-use. As a result, it is often chosen for applications in which other energy sources are more economical. Users should always consider more cost-effective forms of power before considering compressed air. Many operations can be accomplished more economically using alternative energy sources. For example, plants should: • Use air conditioning or fans to cool electrical cabinets instead of compressed air vortex tubes.
•
•
Apply a vacuum system instead of making a vacuum using compressed air Venturi methods that flow high-pressure air past an orifice. Use blowers instead of compressed air to provide cooling, aspirating, agitating, mixing, or to inflate packaging. Use brushes, blowers, or vacuum systems instead of compressed air to clean parts or remove debris. Use blowers, electric actuators, or hydraulics instead of compressed air blasts to move parts. Use low-pressure air instead of highpressure compressed air for blowguns and air lances. Use efficient electric motors for tools or actuators where safe and appropriate. (Electric tools can have less precise torque control, shorter lives, and lack the safety of compressed air powered tools). Use dedicated high-pressure blowers rather than compressed air for air knives. On conveyors, these can be automatically switched off if the product stops passing beneath the knife.
Other improper uses of compressed air are unregulated end-uses and supply air to abandoned equipment, both of which are described below.
3.3.1 UNREGULATED END-USES
A pressure regulator is used to limit maximum end-of-use pressure and is placed in the distribution system just prior to the tool. If a tool operates without a regulator, it uses full system pressure. This results in increased system air demand and energy use, since the tool is using air at this higher pressure. High-pressure levels can also increase equipment wear, resulting in higher maintenance costs, and shorter tool life.
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3.3.2 ABANDONED EQUIPMENT
Many plants undergo numerous equipment configuration changes overtime. In some cases, plant equipment is no longer used. Airflow to this unused equipment should be stopped, preferably as far back in the distribution system as possible without affecting operating equipment.
3.4 LEAKS •
This subject, which comes under the general heading of misuse, is treated on its own in this booklet because it is the single, biggest waste area and yet one of the simplest and cheapest to control and obtain savings. A system with 5% of demand as leaks is said to be excellent and one with 10% is good. Systems with leaks as high as 70% of demand have been measured. Leakage is not only a direct source of wasted energy, but is also an indirect contributor to operating costs. As leaks increase, system pressure drops, air tools function less efficiently and production is affected. Often the only solution is to increase generation pressure to compensate for the losses. Increased running time can also lead to additional maintenance requirements and increased unscheduled downtime. Leaks can lead to adding unnecessary compressor capacity.
3.3.3 PERIODS WHEN COMPRESSED AIR IS NOT REQUIRED
Another common malpractice is to generate compressed air at times when it is not needed. In many cases there is no need for compressed air at all during non-production hours, but compressors are often not switched off.
Table 4: Power wastage through leaks Hole Diameter mm 0.4 (pin head) 1.6 (match head) 3.0 Air leakage at 7barg l/s 0.2 3.1 11.0 Cfm 0.4 6.2 22.0 Power required to compress air being wasted kW 0.1 1.0 3.5
Table 5: Cost of Leaks in South Africa Compressor Capacity [kW] Annual Compressed Air Costs [R] 4Ohr/week utilisation [R] 75% 50% 18 90 160 315 4260 21300 7865 74545 2840 14200 25244 49697 l68hr/week utilisation [R] 75% 50% 11795 58975 104843 206408 7864 39317 69895 137605
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While leakage can come from any part of the system, the most common problem areas are: • • • • • • Couplings, hoses, tubes, and fittings, Open condensate traps and shut-off valves, and Pipe joints, disconnects, and threadsealant. Leaking pressure regulators: Air cooling lines left open permanently; Air-using equipment left in operation when not needed.
test should be carried out to establish the percentage leakage from the system.
3.4.1.1 NO-LOAD METHOD 1
This method applies to compressors that are operated in on/off-load i.e. when the compressor is on-load it produces a known amount of air. • • Close down all the air operated equipment. Start the compressor and operate it to full line pressure. After it off-loads air leaks will cause the pressure to fall and the compressor will come on-load again; Over a number of cycles make a note of average on-load time (T) and average offload time (t). Total leakage can then be calculated:
It is estimated that UK industry wastes up to £100m (a billion Rand) each year on air leaks. Table 4 gives an example of how even very small holes can contribute to energy wastage. Leaks can be a significant source of wasted energy in an industrial compressed air system, sometimes wasting 20-30% of a compressor’s output. A typical plant that has not been well maintained will likely have a leak rate equal to 20% of total compressed air production capacity. On the other hand, active leak detection and repair can reduce leaks to less than 10% of compressor output. The first step in tackling leaks is to recognise the costs involved and make a commitment to a plantwide awareness program. Regular, continuous attention to the compressed air system coupled with proper maintenance will lead to effective progress in minimising leaks.
•
•
Leakage (l/sec) = (Q x T)/(T + t) Leakage (%) = [(T x 100)/(T + t)] Where: Q = air capacity of the compressor (litres/sec).
3.4.1.2 NO LOAD METHOD 2
For modulating compressors the test is more difficult as the compressor output is unknown.The following method can be used if you have a pressure gauge down-stream of the receiver. • Calculate the volume V (litres) of air mains downstream of the receiver isolating valve, including all the pipework (25 mm and above) and the receivers. Pump up the system to operating pressure (P1), and then close the isolating valve. Record the time (T) for pressure to drop to P2. Leakage can then be calculated as follows:
3.4.1 ESTIMATING AMOUNT OF LEAKAGE
It is necessary to take measurements to establish the leakage rate, for which several methods exist. The best way to establish the amount of leakage in a system is by measurement. However, in the absence of suitable measuring devices, a no-load
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Leakage (litres/sec) =
{V (litres) x [P1-P2] (barg)} {T (secs)}
Derivation of the equation above: System volume = V (m ) Initial pressure of system = P1 (barg) Final pressure of system = P2 (barg) Time for pressure decline = T (secs) Atmospheric pressure = Pa 1.013 bar Initial quantity of free air = {(P1+Pa) 3 Initial quantity of free air = {(P1+Pa)/ x V (m ) Initial quantity of free air = Pa} Final quantity of free air = {(P2+Pa) 3 Final quantity of free air = {(P2+Pa)/ x V (m ) Final quantity of free air = Pa} Therefore, flow-rate Therefore, flow-rate Therefore, flow-rate =(V P1+Pa - P2+Pa 3 = (m /sec) = T Pa Pa =(V/TP1-P2)/Pa} 3(m /sec) =(V/T) x {(P(m3 /sec) =(T PPa (m /sec) ≈ The above temperature. ≈( V/T) x (P1-P2) since Pa ≈ 1 ≈(V/TP1 – P2 since Pa ≈ 1 T ) x (P P2) since Pa ≈ 1 assume constant
3 3
An Ultrasonic Leak Detector is a more effective leak detection method as it can detect leaks against a background of other equipment noise. The detectors work by picking up the very high frequency sound emitted by a leak, inaudible to the human ear.They are simple to use and do not pick up frequencies emitted by the mechanical actions of machines. These portable units consist of directional microphones, amplifiers, and audio filters, and usually have either visual indicators or earphones to detect leaks
( ) ( ) ( )
3.4.3 HOW TO FIX LEAKS
Once leaks have been detected they should be repaired as soon as possible. Simply tightening components and shutting off valves, which have been left open, can repair most leaks. One method of motivating people to fix leaks is to offer a reward for the person who finds the most leaks. (The motivation chosen should not inspire people to make leaks though !) Leaks occur most often at joints and connections. Stopping leaks can be as simple as tightening a connection or as complex as replacing faulty equipment such as couplings, fittings, pipe sections, hoses, joints, drains, and traps. In many cases leaks are caused by bad or improperly applied thread sealant. Select high quality fittings, disconnects, hose, tubing, and install them properly with appropriate thread sealant. Non-operating equipment can be an additional source of leaks. Equipment no longer in use should be isolated with a valve in the distribution system Another way to reduce leaks is to lower the demand air pressure of the system.The lower the pressure differential across an orifice or leak, the lower the rate of flow, so reduced system pressure
equations
Having established the size of the problem, a realistic target for leakage rate (say 10%) should be set. No-load tests should then be carried out regularly, approximately every two to three months, with inspections when necessary during shutdown conditions.
3.4.2 LEAK DETECTION
During shutdown of the whole factory, the detection of larger leaks within the compressed air system is simple, as they are audible. Once the compressor is started the exact leak positions should be marked with tags. In addition, checking joints and unions with soapy water is recommended to identify the smaller leaks, which invariably develop with time. An organised approach is required to obtain good results.
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will result in reduced leakage rates. Stabilising the system header pressure at its lowest practical range will minimize the leakage (as well as minimize compressor load and thus electrical demand). Where a system cannot be shut down to test for leaks this method is recommended. Once leaks have been repaired, the compressor control system should be re-evaluated to realize the total savings potential.
A leak prevention program should be part of an overall program aimed at improving the performance of compressed air systems. Once the leaks are found and repaired, the system should be re-evaluated. Having established the size of the leakage problem, an aggressive campaign of leak reduction should take place. Targets should be set and careful monitoring of results conducted. This work will involve inspections during silent hours checking for pipe work and tool leaks, and checking hoses and couplings for air tightness. Following this program the leakage rate should be re-measured and the work continued until the target is met. This is an ongoing exercise, which must be repeated every six months at least, otherwise problems, and energy and money losses will return.
3.4.4 A LEAK PREVENTION PROGRAM
A good leak prevention program will include the following components: identification (including tagging), tracking, repair, verification, and employee involvement. All facilities with compressed air systems should establish an aggressive leak program. A crosscutting energy management team involving decision-making representatives from production should be formed.
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4. EFFECIENT COMPRESSED AIR DISTRIBUTION
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4.1 INTRODUCTION •
Having investigated possibilities for reducing the end usage of air and having determined the extent of leakage, the next step is to examine the distribution network. The main factors that affect energy consumption at this stage are: • • Pressure losses due to inadequate pipe sizing; Water condensing in the lines, causing • damage to components and also reducing pipe cross-sectional areas which leads to additional pressure losses; Air leakage from pipework and end-using equipment, due to poor maintenance and in many cases permanently open condensate drain valves.
A typical compressed air system is outlined in Figure 1 below.The following sections refer to this diagram, describing the main design criteria and components.
Figure 1: Compressed air system components (Source: ETSU)
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4.2 DISTRIBUTION MAIN •
Generally, a ring main is the most effective method of supplying compressed air to a point of use.The main advantages of this type of network are that: • Velocity to any one point is reduced, since air can converge from two directions, thereby reducing pressure drop over the system; Automatic zone valves can be fitted to
•
isolate areas operating different working patterns; Alteration or extension of the distribution system is made easy.
Ideally a ring main should be placed around each building and a single branch point should feed the main, enabling each area of the network to be metered by a single meter. An example of such a network is given below.
•
Figure 2: Preferred distribution line layout (Source: ETSU)
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Airline diameters are usually based on calculations of throughput velocities. A figure of approximately 6.0 m/s is the accepted target value, because this is sufficiently low to prevent an excessive pressure drop.Table 6 utilises the maximum recommended flow in various line sizes based on a 7 barg output pressure. For a quick reference the nomographs outlined in Figures 3 and 4 are a useful guide to sizing distribution lines. The distribution system should be designed to cause no more than 0.2 bar pressure drop at full demand at the point of use.
Table 6: Maximum recommended flow rates Pipe Bore mm 10 25 50 65 80 100 150 Max Flow Cfm 10 55 220 375 500 875 1,900
l/s 5 25 100 180 240 410 900
Figure 3: Pipe Carrying capacities at varying velocities (Source: ETSU)
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Figure 4: Pressure Drop in Steel Pipes (Source: ETSU)
When sizing ring mains it is advisable to err on the high side as future demand may increase. The consequence of under-sizing is very significant in energy efficiency terms, because air velocity will be excessive leading to high-pressure drops and nonseparation of condensed water. The table below shows the power wasted for different pipe diameters with a flow rate of 500 l/s (1,000cfm) at 7barg pressure. Compressor house to ring main feeds should be
oversized to avoid pressure drop problems. As a guide it is recommended that a pipe of twice the cross-sectional area of that used for the ring main be selected. Higher air velocities (up to 20 m/s) are acceptable where the distribution pipe-work does not exceed 8 meters in length.This would be the case where dedicated compressors are installed near to an associated large end user.
Table 7: Distribution line power wastage (500 l/s, 7barg)
Pipe Nominal Bore (mm) 50 65 80 100
Pressure Drop per 100 m (bar) 2.6 0.9 0.2 0.1
Equivalent Power Lost (kW) 18 5 0.8 0.4
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4.3 WATER DRAINAGE •
For undried compressed air, system problems may occur through water condensation in the distribution network. It is good practice to remove as much of this water as possible. Condensation typically occurs where the air main travels outside the buildings and is therefore subject to temperatures below those in the compressor house. The accumulation of condensed water and scale in the system can lead to pressure drop. To prevent this occurring, the air mains should slope to strategically located drain legs equipped with automatically operated drain valves (avoiding air wastage, which frequently occurs with manually operated drain valves). Building layout will dictate the best position for drain points, but in general the main should be installed with a fall of not less than 1 meter in 100 meters of ring main pipe-work in the direction of air flow. The recommended distance between drain points is approximately 30 meters. Any branch line should be taken off the top of the main to prevent any water in the main pipe from running into it. In addition, the bottom of a falling branch line pipe should be drained.
4.4 DRAIN TRAPS •
For the sake of energy efficiency, automatic drain traps could be fitted to the bottom of the drain line within a compressed air system. Reliable electronic condensate traps are available that ensure water is regularly drained away. Manual traps left open account for a substantial percentage of total wastage. The ball float type of drain trap is the most common because it gives a positive shut off, opening only if water is present and closing immediately the water has cleared. If, however, there is a possibility of large quantities of water at the trap, then air binding can take place and a balance pipe must be fitted.With this arrangement, the water will flow freely into the trap, displacing air, which then passes through the balance pipe and into the main system.
All drain traps require occasional maintenance, to remove any build-up of oils or emulsions, which may be present in the condensate. If oil or emulsion build-up is heavy, consideration should be given to using drain trays, which have a blast action discharge.
Figure 5: Air binding of water traps (Source: ETSU)
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4.5 •
AIR RECEIVERS
Air receivers are designed to provide a supply buffer to meet short-term demand spikes, which can exceed the compressor capacity. They also serve to dampen compressor pulsation, separate out particles and liquids, and make the compressed air system easier to control. Installing a larger receiver tank to meet occasional peak demands can even allow for the use of a smaller compressor. In most systems, the receiver will be located just after the dryer. In some cases it makes sense to use multiple receivers, one prior to the dryer and one closer to points of intermittent use. Storage can be used to control demand events (peak demand periods) in the system by controlling both the amount of pressure drop and the rate of decay. Storage can be used to protect critical pressure applications from other events in the system. Storage can also be used to control the rate of pressure drop in demand while supporting the speed of transmission response from supply. Many
systems have a compressor operating in modulation to support demand events, and sometimes strategic storage solutions can allow for this compressor to be turned off. Storage can also help systems ride through compressor failure or short energy outages. Receivers have three main functions:
Providing storage capacity; Acting as a secondary cooler; Creating more stable pressure conditions, effectively acting as pulsation dampers. On most installations, the receiver is fed from the after-cooler and further cooling will take place in the receiver. On installations where the compressor plant is small, an after-cooler may not be fitted, making the receiver the point at which most condensed liquid will be found. If liquid is allowed to build up in the receiver, carry-over into the ring main is likely, with resulting efficiency implications. The receiver, therefore, needs to be fitted with both automatic and manual drain traps, in order to remove the condensate and any carryover solids such as dust, scale, carbon and so on.
• • •
Figure 6: Air binding of water traps (Source: ETSU)
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4.6 •
REGULATORS
Some applications, such as control and instrumentation, require air at a pressure lower than the main supply and in these circumstances, if a separate pressure system cannot be installed, then regulators should be fitted. Figure 6 illustrates a direct-acting regulator. Excellent savings can be achieved by reducing pressure at the usage point as opposed to ‘over supplying’ the pressure. The air consumption of most air-using devices, such as air tools, spray guns and air knives, increases in proportion to the operating pressure ratio. Reducing pressure to the minimum acceptable saves energy.
and other air line component sizing which cause high flow velocities and pressure drops. All systems should be designed to minimise pressure drops. It is worthwhile obtaining a flow/pressure profile of the distribution network to establish where the bottlenecks occur and how they can be overcome. Air velocities should not exceed 6 meters per second in the main components of a distribution system. The distribution system should be designed to cause no more than 0.1-0.2 bar pressure drop at full demand at the usage points. The nomograph shown in Figure 7 is a useful method of arriving at pipe sizes. In conjunction with establishing the correct pipe diameters for the flow, a check on the pressure drops caused by the major valves and fittings should be made.Table 8 gives the equivalent pipe length for these components at the relative nominal pipe diameter. The equivalent lengths should be added to the actual pipe length to be used, and the total length should then be used with the nomograph shown in figure 7 for finalising the pipe diameter.
4.7 DISTRIBUTION NETWORK •
Following compression and treatment, compressed air is distributed to the usage points by a piping network. Much energy can be wasted during distribution by having to generate overpressure to overcome incorrect pipe, valve
Figure 7: Nomograph (Source: ETSU)
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Table 8: Pressure loss through steel fittings
Equivalent Pipe lengths in meters Inner Pipe Diameter (mm) Item Gate valve Fully open Gate valve Half closed Diaphragm valve Fully open Angle valve Fully open Globe valve Fully open Ball valve (full bore) Fully open Ball valve (red. bore) Fully open Swing check valve Fully open Bend R=2d Bend R=d Mitre bend 90° Run of tee Side outlet tee Reducer 15 0.1 20 0.2 32 0.6 1.5 2.7 0.5 3.4 1.0 2.6 4.8 0.2 4.9 1.3 0.1 0.2 0.6 0.6 0.2 0.3 1.0 0.3 1.0 0.3 25 0.3 5 1.3 4 7.5 0.2 2.4 2.0 0.3 0.4 1.5 0.5 1.5 0.5 40 0.5 8 2.5 6 12 0.4 2.2 3.2 Ø.5 0.6 2.4 0.8 2.4 0.7 50 0.6 10 3 7 15 0.3 5 4 0.6 0.8 3 1.0 3 1.0 80 1.0 16 4.5 12 24 0.4 2.6 6.4 1.0 1.3 4.8 1.6 4.8 2 100 1.3 20 6 13 30 0.3 4.1 8 1.2 1.6 6 2 6 2.5 125 1.6 30 8 18 38 0.5 3.3 10 1.5 2 7.5 2.5 7.5 3.1 150 1.9 40 10 22 45 0.6 12.1 12 1.8 2.4 9 3 9 3.6 30 60 0.6 22.3 16 2.4 3.2 12 4 12 4.8 200 2.6
4.8 MAINTENANCE •
Piping systems need regular inspection and maintenance. Inspections for leaks, checking drains and blowing down contamination are all worthwhile measures to avoid energy losses.
4.9 SYSTEM ISOLATION •
Sometimes it is necessary to keep parts of a distribution network pressurised when other parts
do not require compressed air. In these cases the compressed air system in the idle plant should be isolated from the active plant. For example, an assembly department should be isolated during non-productive hours to prevent wastage due to leakage or misuse. Manually or electronically operated zone isolation valves can be installed in the distribution network to shut off at properly arranged times.
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5. COMPRESSED AIR TREATMENT
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5.1 • INTRODUCTION
Air treatment is often required in order to provide compressed air of the quality required at the point of use.This avoids product contamination, product spoilage and poor control of, or damage to, air-using equipment. Treatment needs energy in terms of additional generation pressure, and possibly additional compressed air or electrical demand depending on the type of treatment used. Compressing air concentrates the contaminants per unit of volume of air delivered at pressure. Compressed air can be contaminated by water vapour; condensate; particulate matter (either airborne or pipe-scale); oil in vapour or liquid state (either inhaled by the compressor from the atmosphere or added during compression); and microbes. The amount of treatment will depend on the users needs. The desired air quality in terms of dirt water, oil and microbial burden is achieved by treatment after compression.The higher the quality specified, the greater the energy consumed by the treatment system, and the higher the additional generation pressure needed to overcome losses during treatment. Table 9 gives the ISO/DIS recommendations on air quality classes. Treatment systems range from a simple aftercooler, which is neatly always supplied with the compressor package, through to filters and refrigerated, sorption, deliquescent and desiccant dryers.There are many variations of each system; Some are more energy efficient than others. The requirement for high quality compressed air is increasing as production methods become more sophisticated. A general breakdown of recommended standards for different manufacturing applications is included in Table 10. This table is intended for guidance only; in practice there are very many other combinations needed. There is a very wide range of requirements for air quality. It is important to install the right equipment, and equally important to keep the energy requirements within reason. Every effort should be made to avoid unnecessary levels of treatment. The type of compressor used is important. An oilfree machine could save one filtration stage over an oil-injected compressor, so for high quality air requirements an oil-free machine should be purchased wherever possible. In addition to the treatment savings, other benefits include increased efficiency and longevity, and in cases where a desiccant dryer is used there is no chance of contamination of the desiccant by compressor oil. However, when oil-free machines are used in heavily polluted atmospheres, it is still necessary to remove oil by filtration. Many plants need only part of the air treated to very high quality. In these cases, treating all the generated air to the minimum acceptable level and improving the quality to the desired level close to the usage points can achieve excellent savings Ambient air typically contains 12.5g of water for every 1 m3 of free saturated air at 15°C. If a compressor produces air at 500 1/sec (1,000 cfm),
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Table 9: Air quality classifications ISO/DIS 8573.1 QUALITY CLASS DIRT Particle size (micron) 0.1 1 5 15 40 DIRT Concentration (mg/rn3) 0.1 1 5 8 10 WATER OIL Pressure dew point (including vapour) (°C (ppm vol) at (mg/m3) 7bar) -70(0.3) -40(16) -20(128) +3(940) +7(1240) +10(1500) No Spec 0.01 0.1 1 5 25 N/A N/A
1 2 3 4 5 6 7
Table 10 General Recommended Standards for Air Purity Application classes Air agitation Air bearings Air gauging Air motors Brick and glass machines Cleaning of machine parts Construction Conveying, granular products Conveying powder products Fluidics, power circuits Fluidics, sensors Foundry machines Food and beverages Hand operated air tools Machine tools Mining Micro-electronics manufacture Packaging and textile machines Photographic film processing Pneumatic cylinders Pneumatic tools Process control instruments Paint spraying Sand blasting Welding machines General workshop air Oil 3 2 2 4 4 4 4 3 2 4 2 4 2 4 4 4 1 4 1 3 4 2 3 4 4 Typical Quality Classes Dirt 2 3 4-1 4 5 4 3 4 2–1 4 3 5-4 3 5 1 3 1 3 4 2 3 3 4 4 Water 2 3 5 4 5 3 2 4 2 5 1 5-4 5 5 1 3 1 5 4 3 3 3 5 5
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the compressed air produced each hour will contain 22.5 l of water. Provided the free air is maintained at about 15°C, the water will remain as vapour: however, if the air is cooled or compressed the water will be condensed. The temperature at which water condenses is known as the dew point. The rise in air temperature in the compressor generally prevents condensation, but when the air passes though the after-cooler a large amount of water could condense. Typically the air temperature following the after-cooler will be around 35°C and water content will have been reduced. Water will, however, still be present as vapour and if the air temperature falls there will be further water condensation. Figure 8 shows the amount of water removal required for differing end temperatures. To remove significantly more water from the compressed air than can be achieved by aftercooling, a dryer is necessary. Air dryers typically take the air from the compressor after-cooler at a maximum temperature of 35°C.
5.2 DRYERS •
5.2.1 DESICCANT DRYERS
For class 1.1.1 quality air, heated or heatless twin tower desiccant dryers with special desiccant and drying cycles are employed. Oil removal filters, water removal filters, dust removal filters and an activated carbon absorber unit are also needed. This type of system consumes a lot of energy, requiring up to 15% of the compressed air or electrical equivalent for desiccant regeneration, and there is pressure drop across the filters, when in service, of up to 1.5 bar, which will require additional generation pressure. The example 500 l/s, compressor, when supplying 1.1.1 quality air, would cost 21% per annum more due to the treatment devices than if it were simply delivering after-cooled air. As the requirement for air quality becomes less intense than 1.1.1, the energy requirement reduces.
Figure 8:Water removal each week from 500 l/sec of 7 barg air. (Source: ETSU)
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The example compressor, when supplying air of 2.2.1 quality, would only cost 15% more than delivering after-cooled air if it used a lower specification desiccant dryer with pre- and afterfilters performing at 40°C pressure dew-point.
5.2.4 REFRIGERATION DRYERS
Compressed air of 4.4.5 quality can be provided from any type of compressor by use of a refrigerated dryer and, where necessary, oil removal filtration. The extra cost will typically be 5% over delivering standard after-cooled air. This method of drying is very popular as it produces dew points, which are adequate for most duties in an energy efficient and reliable manner. In climates where the dew point produced is above winter ambient temperatures, water taken outside the buildings will condense. It is recommended that a condensate trap be fitted to the system where it enters the next building to prevent problems.
5.2.2 SORPTION DRYERS
Air of 2.3.1 quality can be provided with a sorption dryer, which can only be used with an oilfree compressor. A very small motor slowly rotates a drum, which is impregnated with the drying medium. Compressed air is fed through a sealed segment of the drum and dried within a range of -15°C to -40°C, depending on the compressor load. The drying medium in the part of the drum not being used to dry the air is regenerated by hot air taken from before the machine after-cooler, i.e. by the waste heat of compression. The cost to provide 2.3.1 quality air by this method, with an oil-free compressor and limited filtration, is around 3% more than delivering after-cooled air.
5.3 AFTER FILTERS •
Filters cause pressure drops in compressed air systems. To save energy, it is recommended that only the minimum filtration requirement be met. Filters should be adequately sized for the duty; if the filter connections are considerably smaller than correctly sized pipe-work they will cause pressure drops. It is better to pay more for filters with correctly sized flanges, and avoid pressure drop and energy wastage.
5.2.3 DELIQUESCENT (ABSORPTION) DRYERS
Deliquescent dryers operate by passing the compressed air over soluble material, such as salt, which dissolves as it absorbs moisture. Their main advantages are that they do not consume any energy other than that required to overcome the pressure drop within the dryer (0.1 to 0.4 bar) and they do not lose any air volume.The dryers are not, however, regenerative and deliquescent material needs to be replaced periodically, incurring both labour and material costs. Deliquescent dryers are the least expensive dryer and are very energy efficient, but can only produce dew points about 6ËšC below the inlet temperature.
5.4 AIR INTAKES •
The location of the air compressors on site can have a bearing on the amount of energy used by the compressor. Cool, clean, dry intake air will lead to more efficient compression. Where possible, air should be taken from outside the building because its temperature will be lower. Lower temperature air is denser and the mass of air that can be compressed by the machine is increased. The air inlet should be protected against the entry of rain and wind blown dust, which would clog filters and
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waste energy. Ducting between air intake and compressor should be short, straight and have a generous diameter. The condition of the air entering the compressor is extremely important, since fouling of inlet filters and high ambient air temperatures can result in significant energy wastage. For every 4°C drop in intake temperature, there is a 1% increase in efficiency. For every 25 mbar pressure lost at the inlet, the compressor efficiency is reduced’ by 2%.
the compressor manufacturer, otherwise problems may occur with possible damage to the compressor and overloading of, the main drive motor.
To justify air inlet cooling, high annual utilisation is needed, producing payback on the capital cost of between two and five years depending on local conditions.
5.4.1 AIR INLET COOLING
With air inlet cooling or pre-cooling, the air is cooled prior to the compression process resulting in energy savings and improved air quality. Twin two-stage coolers linked to a conventional refrigeration plant carry out air inlet cooling. Inlet air is cooled to between -20°C and -25°C, which reduces the pressure dew point to about 0°C and, as an added benefit, removes any dust particles (these collect in the water/ice mixture on the cooler tubes). The density of the inlet air is increased by around 15%, thereby improving the volumetric efficiency of compression. Pre-filters and after-coolers are not required and can be removed from the compressors. As there is no after-cooler, the compressed air leaves the machine at 100°C to 120°C and then cools within the distribution system. Some of this air may be utilised at above ambient temperatures, further improving the system efficiency due to its increased volume. In these cases, oil must be filtered from the air to avoid the risk of explosion. Air inlet cooling technology is particularly worthwhile on low-pressure blowers and single stage compressors, but it is not so suitable for multi-stage units. It is not recommended for centrifugal compressors because, although volumetric flow increases, the machine will be functioning well away from its design condition and will therefore, be inefficient. It is important to ensure that the inlet air temperature is not reduced below the minimum recommended by
5.5 INSTALLATION, CONFIGURATION AND SIZING •
Ideally, each compressor should have a dedicated dryer for control and reliability purposes. This is not always practical, due to capital cost and space availability. All configurations are very reliable if well installed and maintained. If a dryer for each compressor is not practical, a unit suitable for 100% duty over the whole plant is recommended to give the best energy and air quality performance.
Dryers should be installed in well-ventilated areas. For continuous processes, all filters should be duplexed with changeover valves for ease of maintenance. A dryer by-pass should also be installed for emergency maintenance; this must be locked off during normal running to prevent accidental operation, which would contaminate the dry air main.
Filters should be adequately sized for the duty. As mentioned previously, if the filter connections are considerably smaller than correctly sized pipe-work, they will cause pressure drop problems. It is better to pay more at the outset and thereby avoid pressure drop and energy wastage.
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5.6 TREATMENT SYSTEMS MAINTENANCE •
Fouling causes an increase in the pressure drop across all filter elements, leading to a higher generation pressure and additional energy use. It is important to keep pressure drops to a minimum. All filters should be fitted with differential pressure gauges, which should be calibrated regularly. With all dryers, particularly desiccant types, the dew point should be checked regularly. Many will be operating well below specification, and yet the energy consumed will be continuing at the same rate as that needed for design dew-point performance.
unnecessary levels of treatment. Many plants only need part of the air treated to very high quality. In these cases, excellent savings are achievable by treating all the generated air to the minimum acceptable level, and improving the quality to the desired level close to the usage point.
A good example of this would be a car production plant where 70% of the requirement is for 4.4.2 quality air, which can be supplied by a refrigeration dryer and oil removal filter. The energy requirement of a refrigeration dryer is much less than that of a desiccant dryer, and the pressure drop across the filter will be 0.2 bar.
5.7 POTENTIAL SAVING AREAS •
It can be seen that there is a very wide range of requirements for air quality. It is most important to install the right equipment for the duty, and equally important to minimise energy requirements. Much energy can be expended on
30% of the air is required at 1.2.1 quality, for the special requirement of the paint and engine assembly areas. The desiccant dryers and special filtration needed for this quality can be installed in the usage areas. Energy cost savings of around R20,000 per annum for every 5001/s (1,000 cfm) delivered can be achieved by only treating to required levels. In addition, savings will be achieved from reduced dryer maintenance and consumables, such as filter elements and desiccant replacement.
•••••••••
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6. COMPRESSED AIR GENERATION
•••••••••
Opportunities for more efficient energy use arise when new compressed air facilities are being designed, particularly when choosing the most appropriate compressor. The features of the main compressor types and their efficiencies are included in this section. Many modern industrial air compressors are sold “packaged” with the compressor, drive motor and many of the accessories mounted on a frame for ease of installation. There are two basic compressor types: positive-displacement and dynamic. In the positive-displacement type, a given quantity of air or gas is trapped in a compression chamber and the volume, which it occupies, is mechanically reduced, causing a corresponding rise in pressure prior to discharge. At constant speed, the airflow remains essentially constant with variations in discharge pressure. Dynamic compressors impart velocity energy to continuously flowing air or gas by means of impellers rotating at very high speeds. The
Compressor
Positive Displacement
Dynamic
Reciprocating
Rotary
Centrifugal
Axial
Single-acting
Double-acting
Helical-screw
Liquid-ring
Scroll
Rotary-vane
Figure 9: Compressor Family Tree
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velocity energy is changed into pressure energy both by the impellers and the discharge volutes or diffusers (these are part of the stationary casing in which the impeller turns). In the centrifugal-type dynamic compressors, the shape of the impeller blades determines the relationship between airflow and the pressure (or head) generated.
Large industrial reciprocating air compressors are double-acting and water-cooled. From 250 to 1,000 1/s (500 to 2,000 cfm) double-acting watercooled piston compressors either in lubricated or non-lubricated forms are available.These machines are the most efficient available in terms of full-load and part-load power consumption. Oil free duties can be served by piston compressors with special carbon or Teflon wearing surfaces. A high number of piston compressors are in use today. Well-maintained piston machines are still the most energy-efficient compressors, although efficiency decreases significantly if they are poorly maintained. Over the last decade, however, the trend has been to purchase rotary vane, rotary screw and centrifugal compressors, because they are quieter and simpler to maintain and install.
6.1 POSITIVE DISPLACEMENT COMPRESSORS •
These may be either reciprocating or rotary. The rotary ones may be double helical screw, single helical screw with vanes, liquid ring, scroll or rotary vane (sometimes called sliding vane). These have very different configurations. In principle, a multistage compressor, with small clearance volumes, is more efficient than a single-stage machine.
6.1.1 RECIPROCATING COMPRESSORS.
Reciprocating compressors have pistons and work like bicycle pumps. A piston, driven through a crankshaft and connecting rod by an electric motor, reduces the volume in the cylinder occupied by the air or gas, compressing it to a higher pressure. Single-acting compressors have a compression stroke in only one direction, while double-acting units provide a compression stroke as the piston moves in each direction. Compressors of the single or two-stage air-cooled piston type can serve capacities from 2.5 to 250 1/s (5 to 500 cfm).These compressors are usually mounted on air receivers. It is possible to get nonlubricated piston compressors for duties such as food, air conditioning and pharmaceutical production. However, in these cases it is more common to use an oil-injected compressor with filtration to remove the oil carried over from the compressor.
6.1.2 ROTARY POSITIVE DISPLACEMENT COMPRESSORS
The most common type of rotary compressor is the helical twin screw-type (also known as rotary screw or helical lobe). Male and female screwrotors mesh trapping air, and reducing the volume of the air along the rotors to the air discharge point Rotary screw compressors have low initial cost, compact size, low weight, and are easy to maintain. Rotary screw compressors are available in sizes from 2.2 – 450 kW (3 - 600 hp) and may be air or water cooled. Less common rotary compressors include the single screw, the rotary vane, the liquid ring and the scroll-type. Single-stage oil-injected rotary vane or screw compressors can serve capacities from 2.5 to 250 1/s (5 to 500 cfm). From 250 1/s up to 1,000 1/s (500 to 2,000 cfm) oil-injected screw machines are used.
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Users that need high quality air choose oilinjected machines with filtration because of the lower capital cost of the machinery. This method of providing high quality air is less energy efficient than using oil-free compressors. For 100 1/s to 2,000 1/s (200 to 4,000 cfm) oilfree applications, the two-stage rotary-toothed rotor compressor or non-lubricated screw compressor can be used. Its efficiency is high because compression takes place in two stages and the rotors have very close operating clearances. They are as efficient as oil-free piston machines and have a long lifetime, but have a high capital cost compared with oil-injected machines. Capacities of 1,000 to 2,000 l/s (2,000 to 4,000 cfm) are met with two stage oil-free rotary screw compressors or multi-stage centrifugal types, both of which are inherently oil-free.
velocity energy, primarily in an axial plane. The stationary vanes then act as a diffuser to convert the residual velocity energy into pressure energy. This type of compressor is restricted to very high flow capacities and generally has relatively high compression efficiency. For efficient operation, it is important to state the site ambient temperatures and pressures, and the design flow and pressure when specifying dynamic compressors.The energy requirements and control ranges of these compressors are seriously affected by operation outside design conditions. Generally for energy efficiency at full and part load, the more stages of compression the better. Centrifugal machines are available from 250 l/s (500 cfm) up to very large capacities, and are popular and most energy efficient for applications over 1,000 1/s (2,000 cfm). Multi-stage oil-free centrifugal compressors can meet capacities over 2,000 l/s (4,000 cfm), until very large mass flow compressors of the axial flow configuration come into consideration. Centrifugal compressors are very reliable and efficient if properly applied, and usually have low maintenance costs.
6.2 DYNAMIC COMPRESSORS •
These compressors raise the pressure of air or gas by imparting velocity energy and converting it to pressure energy. Dynamic compressors include centrifugal and axial types. The centrifugal-type is the most common and is widely used for industrial compressed air. Each impeller, rotating at high speed, imparts primarily radial flow to the air or gas, which then passes through a volute or diffuser to convert the residual velocity energy to pressure energy. Some large manufacturing plants use centrifugal compressors for general plant air, and, in some cases, plants use other compressor types to accommodate demand load swings while the centrifugal compressors handle the base load. Mixed flow compressors have impellers and rotors, which combine the characteristics of both axial and centrifugal compressors. Axial compressors consist of a rotor with multiple rows of blades and a matching stator with rows of stationary vanes. The rotating blades impart
6.3 ENERGY EFFICIENT COMPRESSOR SELECTION •
In general, the choice of compressor and aftertreatment system is dictated by: • • • The capacity and pressure required; The capital available; The specified delivered air quality requirements.
The relative generation efficiencies of each different compressor configuration are summarised in Table 11.
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Table 11: Summary of compressor configurations with relative efficiencies Description Lubricated piston Capacity (l/s) 2-25 25-250 250-1,000 2-25 25-250 250–1,000 2-25 25-250 250-1,000 25- 250 250- 1,000 1,000 - 2,000 250- 1,000 1,000- 2,000 Above 2,000 Specific energy (J/l)* 510 425 361 552 467 404 510 446 404 429 382 382 446 382 361 Part Load efficiency** Good Good Excellent Good Good Excellent Poor Fair Fair Good Good Good Good Excellent Excellent
Non-lubricated piston
Oil-injected vane/screw Non-lubricated toothed rotor/screw Non-lubricated centrifugal
All the compressors in the table are working at 7 barg pressure. * J/l ÷ 21 kW/100 cfm e.g. 510 J/l 24.29 kW/100 cfm ** Efficiencies are measured in specific energy consumption (joules/litre (J/l)).
The specific power figures have been obtained from actual field test results according to BS 1511 Part 2: 1984.These take into account electric drive motor inefficiencies and are a true assessment of the actual electrical input, not shaft input power, which is usually stated by manufacturers. There is considerable variation in specific energy consumption between configurations. Another consideration is the ability of a compressor to operate efficiently on part load.
the most important determinants of overall system energy efficiency. Proper control is essential to efficient system operation and high performance.The objective of any control strategy is also to shut off unneeded compressors or delay bringing on additional compressors until needed. All units that are on should be run at full-load, except for one unit for trimming. (This is because the energy costs per unit of compressed air rise markedly when the compressors are running at less than full load.) Compressor systems are typically comprised of multiple compressors delivering air to a common plant air header. The combined capacity of these machines is sized, at a minimum, to meet the maximum plant air demand.This is not necessarily the sum of all the maximum machinery demand, as equipment loading may be at different intervals. System controls are almost always needed to reduce the output of the individual compressor(s)
6.4 ENERGY EFFICIENT COMPRESSOR CONTROL •
Compressed air system controls match the compressed air supply with system demand (although not always in real-time) and are one of
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during times of lower demand. Compressed air systems are usually designed to operate within a fixed pressure range and to deliver a volume of air, which varies with system demand. System pressure is monitored and the control system decreases compressor output when the pressure reaches a predetermined level. Compressor output is then increased again when the pressure drops to a lower predetermined level. The difference between these two pressure levels is called the control range. Depending on air system demand, the control range can be anywhere from 0.14 to 1.4 bar (2 - 20 psi). In the past, individual compressor controls and nonsupervised multiple machine systems were slow and imprecise.This resulted in wide control ranges and large pressure swings. As a result of these large swings, individual compressor pressure control set points were established to maintain pressures higher than needed. This ensured that swings would not go below the minimum requirements for the system. Today, faster and more accurate microprocessor-based system controls with tighter control ranges allow for a drop in the system pressure set points. This advantage is that a precise control system is able to maintain a much lower average pressure without going below the minimum system requirements. Narrower variations in pressure not only use less energy, but also improve production quality control. Caution needs to be taken when lowering average system header pressure because large, sudden changes in demand can cause the pressure to drop below minimum requirements, leading to improper functioning of equipment. With careful matching of system controls and storage capacity, these problems can be avoided. The type of compressor being used largely determines the type of control specified for a given system and the facilities’ demand profile. If a system has a single compressor with a very steady
demand, a simple control system may be appropriate. On the other hand, a complex system with multiple compressors, varying demand, and many types of end-uses will require a more sophisticated strategy. In any case, careful consideration should be given to both compressor and system control selection because they can be the most important factors affecting system performance and efficiency. For energy efficiency, it is important to consider the control of individual machines and the way in which multiple installations meet the demand in terms of flow and pressure requirements. For a relatively low capital outlay, a modern compressor control system can save between 5% and 20% of the total generation costs, and in some cases improved pressure control will also result in productivity gains.
6.4.1 INDIVIDUAL COMPRESSOR CONTROL
Compressor manufacturers have developed a number of different types of control strategies. Controls such as start/stop, which start and stop the compressor motor, and load/unload, which engage and disengage the compressor from the running motor, respond to increases or reductions in air demand. Modulating inlet and multi-step controls allow the compressor to operate at partload and deliver a reduced amount of air during periods of reduced demand. Start/stop is the simplest control available and can be applied to either reciprocating or rotary screw compressors.The motor driving the compressor is turned on or off in response to the discharge pressure of the machine. Typically, a simple pressure switch provides the motor start/stop signal. This type of control should not be used in an application that has frequent cycling because
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starts will cause the motor to overheat and other compressor components to require more frequent maintenance. Most compressors up to 1,000 1/s can be switched to automatic stop/start control. Some machines change to this mode automatically. Automatic stop/start control stops the compressor after a period of no-load running, usually 10 - 15 minutes, and then automatically restarts the machine on a demand for air.The offload running time is essential, unless a soft starter is fitted, to protect the drive motor from too many starts. Soft start controls are available which provide a variable start time with minimised starting currents, and which eliminate current surges thereby preventing motor damage. Load/unload control, also known as constant speed control, allows the motor to run continuously, but unloads the compressor when the discharge pressure is adequate. Compressor manufacturers use different strategies for unloading a compressor, but in most cases, an unloaded rotary screw compressor will consume 15 - 35% of full load power while delivering no useful work. As a result, some load/unload control schemes can be inefficient. Modulating (throttling) inlet control allows the output of a compressor to be varied to meet flow requirements.Throttling is usually accomplished by closing down the inlet valve, thereby restricting inlet air to the compressor.This control scheme is applied to centrifugal compressors. This control method, when applied to displacement compressors, is an inefficient means of varying compressor output. When used on centrifugal compressors, more efficient results are obtained, particularly with the use of inlet guide vanes, which direct the air in the same direction as the impeller inlet. The amount of capacity reduction is limited by the potential for surge and minimum throttling capacity.
Rotary screw machines are often fitted with both two-step unloading and modulating control with a manual or automatic change over switch. Modulation should only be used if the load is over 75% of its design level: below this, two-step unloading is more efficient. Two-step systems, on any machine operate over a pressure differential of around 0.5 bar between full and no load. A correctly sized air receiver should be fitted to avoid control hunting. Piston machines with twoor three-step inlet suction valve unloaders, on- or off-line inlet valves, or five-step clearance pocket unloading, give the best efficiencies at part loads. Piston, vane and screw machines with variable inlet throttle valves that modulate over a close pressure range are not efficient on low loads, because they are positive displacement machines and throttling causes an increase in compression ratio. Some compressors are designed to operate in two or more partially loaded conditions. With such a control scheme, output pressure can be closely controlled without requiring the compressor to start/stop or load/unload. Reciprocating compressors are designed as twostep (start/stop or load/unload), three-step (0%, 50%, l00%) or five-step (0%, 25%, 50%, 75%, 100%) control. These control schemes generally exhibit an almost direct relationship between motor power consumption and loaded capacity. Some rotary screw compressors can vary their compression volumes (ratio) using sliding or turn valves. These are generally applied in conjunction with modulating inlet valves to provide more accurate pressure control with improved partload efficiency. Centrifugal compressors are dynamic machines and behave efficiently on part load. Output is normally reduced by modulation to 70% of the design flow. For installations where the demand is sometimes less than this, machines with automatic dual control systems should be installed to avoid
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wasting energy due to bypassing of pressurised air at part loads. Inlet guide vanes are preferable to inlet throttles, because they improve the part load efficiency and turndown range, particularly at offdesign inlet conditions. Using variable speed drive (VSD) motors to drive piston and screw compressors offers many control and efficiency advantages. In the past costs have been prohibitive; however, new advances in electronics and control gear are making these systems more popular. Care should be taken not to reduce the compressor speed to the extent that it is inadequately lubricated. VSDs are unsuitable for centrifugal compressors unless specifically designed for the purpose.
individual compressor capacity to meet system demand. Sequencers are referred to as single master control units because all compressor, operating decisions are made and directed from the master unit. Sequencers control compressor systems by taking individual compressor capacity on and off-line in response to monitored system pressure (demand). The control system typically offers a higher efficiency because the control range around the system target pressure is tighter. This tighter range allows for a reduction in average system pressure. Again, caution needs to be taken when lowering average system header pressure because large, sudden changes in demand can cause the pressure to drop below minimum requirements, leading to improper functioning of equipment. With careful matching of system controls and storage capacity, these problems can be avoided. Various forms of automatic sequencing control exist for optimising the operation of multiple installations and equalising the wear through rotation of the sequence. Microprocessor-based systems have much more accurate pressure control than pressure switch or air governor controls, and avoid large pressure differentials and energy waste. They can take into account lower pressure requirements during nonproductive hours and adjust accordingly, and can also control system isolation valves. Some multiple machine control systems work with a combination of pressure and demand signals to ensure that only the correct machines are on-line at any one time. Network controls offer the latest in system control. It is important that these controllers be used to shut down any compressors running unnecessarily. They also allow the operating compressors to function in a more efficient mode. Controllers used in networks are combination controllers. They provide individual compressor
6.4.2 MULTIPLE COMPRESSOR CONTROL
By definition, system controls control the actions of the multiple individual compressors that supply air to the system. Prior to the introduction of automatic system controls, compressor systems were set by a method known as cascading set points. Individual compressor operating pressure set points were established to either add or subtract compressor capacity to meet system demand.The additive nature of this strategy results in large control ranges. The objective of an effective, automatic system, control strategy is to match system demand with compressors operated at or near their maximum efficiency levels. This can be accomplished in a number of ways, depending on fluctuations in demand, available storage, and the characteristics of the equipment supplying and treating the compressed air. Sequencers are, as the name implies, devices used to regulate systems by sequencing or staging
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control as well as system control functions. The term multi-master refers to the system control capability within each individual compressor controller. These individual controllers are linked or networked together, thereby sharing all operating information and status. One of the networked controllers is designated as the leader. Because these controllers share information, the compressor operating decisions with respect to changing air demand can be made more quickly and accurately. The effect is a tight pressure control range, which allows a further reduction in the air system target pressure. Although, initial costs for system controls are often high, these controls are becoming more common because of the resulting reductions in operating costs. Control systems can be built into building management systems along with compressor condition monitoring, automatic operation of zone isolation valves, compressor electric motor input
readings and departmental air demand metering from remote outstations.
6.4.3 THE IMPORTANCE OF PRESSURE CONTROL
Energy can be saved in most multi-compressor installations by improving the pressure control system. The majority of systems have two basic shortcomings: • • Pressures are maintained at a higher level than is needed by the end users; and Generation pressures are set too high and not varied according to demand.
The consequence of both of these failings is demonstrated in Figure 10, which represents a typical cascade control system.
Figure 10: Cascade pressure control typical situation (Source: ETSU)
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Usually the minimum pressure needed by the most critical piece of machinery sets the lowest acceptable pressure at the compressor house. To ensure that this pressure is achieved at the critical point, the pressure at the compressor house (i.e. the control pressure) will need to be set to 0.7 bar (10 psi) above the value required, to overcome line pressure losses on the distribution network. This includes occasions when the airflow rate is highest and hence the distribution losses are greatest. For example, if 5.8 barg (85 psig) is required, the control pressure will need to be 6.5 barg (95 psig).This control pressure compensation is shown in Figure 10 by the lower two lines. The left-hand axis in Figure 10 shows the cascade switching of the compressors and it represents a typical cascade pressure control system. At maximum flow rates all four compressors will be operating, as designated by region D. As the air usage falls, the pressure in the lines at the compressor house will rise and one compressor will unload. If usage falls further, the pressure moves into region C where another compressor unloads, and so on up to region A where only one compressor is providing the load. Due to the nature of the pressure switches, the minimum control bands are 0.2 bar for each compressor leading to a very wide overall control band (0.8 bar). The consequence of a wide control band is that, except at maximum air usage, the end user and compressor control pressures will always be up to 10% higher than those actually required. This has two negative effects: firstly, it takes up to 5% more electricity to generate the air at a 10% greater pressure (see Table 12) and secondly the usage of air in most applications is directly proportional to its pressure.The worst case scenario, shown by the upper two lines in Figure 10 which may represent night time and weekend usage, could be costing at least 15% more than is necessary.
Table 12: Generation pressure savings Pressure Barg (psig) 7(100) 6(90) 5.5(80) Energy Savings (%) Single Stage Two Stage 5 10 5 11
To keep generation costs to a minimum: • Pressure control should be based on the pressure at the most sensitive/critical pieces of machinery; Compressor sequencing should be based on as narrow a pressure band as possible to achieve the minimum generation pressure at all times.
•
6.5 SIZING •
Compressors should be sized as closely as possible to the duty. It is not economical to run any machine for long periods at low loads, due to electric motor inefficiencies. The off load power can be 15% - 70% of the on-load power once motor inefficiencies have been taken into account. For new installations with multiple compressors, it is worthwhile considering installation of a selection of unit sizes, so those compressors operating close to full output can meet the demand. Care should be taken to ensure that the overall system efficiency is improved, taking into account the lower generating efficiencies of some smaller compressors.
6.6 MAINTENANCE •
Like all electro-mechanical equipment, industrial compressed air systems require periodic
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maintenance to operate at peak efficiency and minimize unscheduled downtime. Inadequate maintenance can have a significant impact on energy consumption via lower compression efficiency, air leakage, or pressure variability. It can also lead to high operating temperatures, poor moisture control, and excessive contamination. Most problems are minor and can be connected by simple adjustments, cleaning, part replacement, or the elimination of adverse conditions. Compressed air system maintenance is similar to that performed on cars: filters and fluids are replaced; cooling water is inspected, belts are adjusted, and leaks are identified and repaired. All equipment in the compressed air system should be maintained in accordance with manufacturers’ specifications. Manufacturers provide inspection, maintenance and service schedules that should be followed strictly. In many cases, it makes sense from efficiency and economic standpoints to maintain equipment more frequently than the intervals recommended by manufacturers, which are primarily designed to protect equipment. One way to tell if a system is being maintained well and is operating properly is to periodically benchmark the system by tracking power, pressure and flow. If power use at a given pressure and flow rate goes up, the system’s efficiency is degrading. This benchmarking will also let you know if the compressor is operating at full capacity, and if the capacity is decreasing over time. On new systems, specifications should be recorded when the system is first set up and operated properly. Maintenance issues for specific components are discussed below. system
exchanger surfaces, air lubricant separator, lubricant, lubricant filter, and air inlet filter. The compressor and inter-cooling surfaces need to be kept clean and foul free. If they are dirty, compressor efficiency will be adversely affected. Fans and water pumps should also be inspected to ensure that they are operating at peak performance. The air lubricant separator in a lubricant-cooled rotary screw compressor generally starts with a 2 - 3 psi differential pressure drop at full-load when new. Maintenance manuals usually suggest changing them when there is about a l0 psi pressure drop across the separator. In many cases it may make sense to make an earlier separator replacement, especially if electricity prices are high. The compressor lubricant and lubricant filter need to be changed per manufacturer’s specification. Lubricant can become corrosive and degrade both the equipment and system efficiency. For lubricant-injected rotary compressors, the lubricant serves to lubricate bearings, gears, and intermeshing rotor surfaces.The lubricant also acts as a seal and removes most of the heat of compression. Only a lubricant meeting the manufacturer’s specifications should be used. Inlet filters and inlet piping also need to be kept clean. A dirty filter can reduce compressor capacity and efficiency. Filters should be maintained at least per manufacturer’s specifications, taking into account the level of contaminants in the facility’s air.
6.6.2 COMPRESSOR DRIVES 6.6.1 COMPRESSOR PACKAGE
The main areas of the compressor package in
need of maintenance are the compressor, heat
If the electric motor driving a compressor is not properly maintained, it will not only consume more energy, but also be apt to fail before its
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expected lifetime. The two most important aspects of motor maintenance are lubrication and cleaning.
6.6.2.4 GENERAL
Compressors run for many hours, often in appalling conditions. Equating the example 500 1/s compressor to a car, it would cover over 70,000 miles per annum at an average speed of 30 mph. Some compressors run the equivalent of 250,000 miles per annum on this basis. Good maintenance is therefore essential.
6.6.2.1 LUBRICATION
Too much lubrication can be just as harmful as too little and is a major cause of premature motor failure. Motors should be lubricated per the manufacturer’s specification, which can be anywhere from every 2 months to every 18 months, depending on annual hours of operation and motor speed. On motors with bearing grease fittings, the first step in lubrication is to clean the grease fitting and remove the drain plug. High quality new grease should be added, and the motor should be run for about an hour before the drain plug is replaced.This allows excess grease to be purged from the motor without dripping on the windings and damaging them.
Piston compressors, particularly the oil-free type, suffer the most in efficiency terms from lack of maintenance.
6.6.2.2 CLEANING.
Since motors need to dissipate heat, it is important to keep all of the air passages clean and free of obstruction. For enclosed motors, it is vital that cooling fins are kept free of debris. Poor motor cooling can increase motor temperature and winding resistance, which shortens motor life and increases energy consumption.
The efficiency of rotary vane and screw machines does not deteriorate so rapidly; however, there is a finite life for such compressors. As a guide, these types of machine must receive major maintenance after 25,000 hours life to maintain good efficiency. Oil-free toothed rotor and screw machines perform well for periods of up to 40,000 hours, after which there is a slow fall off in efficiency due to gradually increasing internal clearances. These types of machines then need major refurbishment to maintain efficiency.
Centrifugal compressors, having few moving parts and comparatively large ‘as built’ clearances, will maintain their efficiency over longer periods. The inlet air filters, cooling water system and the intercoolers must be rigorously maintained or efficiency will fall off rapidly.
6.6.2.3 BELTS
Motor v-belt drives also require periodic maintenance. Tight belts can lead to excessive bearing wear, and loose belts can slip and waste energy. Under normal operation, belts stretch and wear and, therefore, require adjustment. A good rule-ofthumb is to examine and adjust belts after every 400 hours of operation.
It is a false economy to ignore maintenance on any type of compressor. It is recommended that manufacturers, or their accredited agents, are used for service work and that genuine spare parts, to the original design, are used. An apparently cheaper component, such as an incorrectly designed replacement discharge valve, costs more in the long term due to the detrimental effect that it has on the compressor efficiency.
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6.7 HEAT RECOVERY •
As much as 80-93% of the electrical energy used by an industrial air compressor is converted into heat.The heat is low grade and usually wasted, but in many cases it can be recovered. A properly designed heat recovery unit can recover anywhere from 50-90% of this available thermal energy and put it to useful work heating air or water. Heat is given off by the compressors themselves, by intercoolers on multi-stage compressors (which improve their efficiency) and by aftercoolers. Compressors can be air or water-cooled. On water-cooled compressors heat is normally removed from the compression cylinders by water jackets. Hot water at around 50°C can be collected from a piston compressor and used for a variety of purposes, including increasing the temperature of boiler feed water, process water or domestic hot water. Warm air can be ducted from air-cooled compressors, particularly packaged rotary machines, and can be used for duties such as space heating and air curtains.
the addition of ducting and another fan to handle the duct loading and to eliminate any backpressure on the compressor cooling fan. These heat recovery systems can be modulated with a simple thermostatically controlled hinged vent. When heating is not required - such as in the summer months - the hot air can be ducted outside the building. The vent can also be thermostatically regulated to provide a constant temperature for a heated area. Hot air can be used for space heating, industrial drying, preheating aspirated air for oil burners, or any other application requiring warm air. As a rule of thumb, approximately 15 kWh/hour of energy is available for each 50 l/sec (100 cfm) of capacity (at full-load). Air temperatures of 17 to 22°C (30 to 40°F) above the cooling air inlet temperature can be obtained. Recovery efficiencies of 80-90% are common. If the air supply for the compressor is not from outside, you should be careful not to draw in heated air because this will reduce the compressor efficiency. During the summer months, any hot air should be ducted to atmosphere, otherwise it will be dissipated to the surrounding area and could subsequently be drawn back into the compressor, again reducing efficiency.
6.7.1 HEAT RECOVERY WITH AIR-COOLED COMPRESSORS 6.7.1.1 HEATING AIR
Air-cooled packaged rotary screw compressors are very amenable to heat recovery for space heating or other hot air uses. Ambient atmospheric air is heated by passing it across the system’s after-cooler and lubricant cooler, where it extracts heat from both the compressed air and the lubricant that is used to lubricate and cool the compressor. Since packaged compressors are typically enclosed in cabinets and already include heat exchangers and fans, the only system modifications needed are
6.7.1.2 HEATING WATER
Using a heat exchanger, it is also possible to extract waste heat from the lubricant coolers found in packaged water-cooled reciprocating or rotary screw compressors and produce hot water. Depending on design, heat exchangers can produce non-potable (grey) or potable water. When hot water is not required, the lubricant is muted to the standard lubricant cooler. Hot water can be used in central heating or
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boiler systems, industrial cleaning processes, plating operations, heat pumps, laundries, or any other application where hot water is required. Heat exchangers also offer an opportunity to produce hot air and hot water, and allow the operator some ability to vary the hot air/hot water ratio.
compare heat recovery with the current source of energy for generating thermal energy, which may be a low-price fossil fuel such as coal.
6.7.4 USE OF HOT COMPRESSED AIR FOR PROCESS DUTIES 6.7.2 HEAT RECOVERY WITH WATER-COOLED COMPRESSORS
Heat recovery for space heating is not as common with water-cooled compressors because an extra stage of heat exchange is required and the temperature of the available heat is lower. However, since many water-cooled compressors are quite large, heat recovery for space heating can be an attractive opportunity. Recovery efficiencies of 50-60% are typical. Some process applications, such as spool valves in a glass factory or drop forging hammers, benefit from hot compressed air. Hot compressed air is not often used because of safety concerns, since there is a risk of compressor oil carry-over spontaneously igniting if the discharge temperature is too high. If hot compressed air is to be used, all the air pipe work should be lagged to prevent cooling and an over-sized condensate recovery system should be fitted to take care of the additional condensate, which will form. An after-cooler will not be required. Using hot air is especially worth considering if the air is compressed near to the point of usage and the pipe runs (and hence the heat and pressure loss) are therefore small. The volumetric increase achieved by using hot air will save up to 25% of the energy used on the duty, provided the air is kept hot up to the usage point.
6.7.3 CALCULATING COMPRESSOR HEAT ENERGY SAVINGS
When calculating energy savings and payback periods for heat recovery units, it is important to
Table 13: Heat recovery available from air cooled rotary screw compressors at full load (assuming 90% motor efficiency) Capacity (l/s) 40 60 159 314 450 585 725 Nominal Motor Power (kW) 15 22 55 110 160 200 250 Warm Air Flow (l/s) 450 810 1,600 3,700 5,600 8,900 8,900 Heat Available (kWh/h) 12 18 45 89 130 162 203
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6.8
SITE INTEGRATION TO SAVE ENERGY
points of end use, the choice will largely depend on: • • • the physical layout of the factory; the position of off-takes, the compressor operating hours.
It is important that site integration of air compressors is considered at the planning and design stages to get the maximum benefit of using packaged compressors with integrated heat recovery systems. These packaged systems generally use rotary screw compressors, which have the additional benefit of operating with low noise. When there is a choice between using a central installation or smaller compressors nearer to the
Smaller compressors are generally less efficient than larger ones, and this must be taken into account when justifying decentralisation. Where practicable, air compressors should be sited near to points of large air demand and, if possible, near to heat demands, if recovery is an option, to reduce pipe runs and also capital and running costs.
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7. MONITORING OF COMPRESSED AIR SYSTEMS
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7.1 MONITORING •
Without sufficient instrumentation on compressed air systems it is impossible to determine whether or not they are operating efficiently. Throughout this guide there are many examples of where energy is wasted; without a mechanism for detecting wastage, it is likely to continue and minimum energy costs will not be achieved. This section sets guidelines for the minimum amount of metering to be installed on compressed air systems. the compressor and in the receiver, to help detect any fouling of the after-cooler heat exchanger: • inter-cooler pressure and temperature gauges where applicable: • pressure gauges at selected points along the distribution system, to obtain a pressure profile across the site and hence identify high pressure loss areas; • hours-run meters that differentiate between on-load and off-load running times; • kWh meters (on compressors rated above 50kW (150 1/sec)). Readings from the first four instruments should be recorded daily: hours run and kWh meters should be read weekly to find usage trends. Any sudden increases should be investigated immediately in line with the monitoring and targeting activities. Most modem compressors have excellent electronic monitoring systems, which automatically log their running condition, generate alarms when abnormal running conditions occur and ultimately shut the machines down before any damage occurs. It is possible to purchase similar monitoring units for retrofitting to existing compressors. This, is particularly worthwhile with two-stage watercooled piston compressors, where compressor running condition has a major effect on efficiency. Adequate maintenance skills are not readily available in many factories, making early warning of problems essential.
7.1.1 COMPRESSORS
The running condition of a compressor can be assessed fairly easily. Table 14 summarises the measurements required for different types of compressor. New compressors, particularly those within an integrated package, are now supplied with all the required instrumentation. For existing plant, however, the following list gives the recommended minimum instrumentation: • • pressure gauge on the receiver; water temperature gauges in the compressor cooling jacket and the aftercooler, to detect any blockages that may be occurring; air temperature gauges at the outlet of
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Table 14: Compressor Assessment Compressor Type All types Measurement* Air discharge temperature downstream of the after cooler. Inlet filter differential pressure** Indication Over 15°C difference between inlet air and compressed air after the after cooler shows cooling surfaces are becoming fouled and efficiency is falling off. Values outside manufacturer’s specification can indicate fouled inlet filters, leading to efficiency loss. Two-stage piston, toothed rotor or screw types Inter-cooler pressure Measurement outside that in manufacturer’s service manual indicates problem. Greater than 0.3 bar pressure differential indicates problems with oil separator and loss of efficiency. Values outside manufacturer’s specification indicate problems. Machine not in optimum running condition.
Oil-injected vane or screw types
Difference between compressor element discharge pressure and actual delivered pressure.
Centrifugal types
Inter-stage temperatures and pressures, pinion vibration levels and air inlet differential pressure.
* All the pressure and temperature gauges used for this purpose must be regularly calibrated. ** Most compressors have an inlet filter differential pressure gauge fitted: if not, it is recommended that one is fitted.
7.1.2 AIR FLOW METERS
Many different forms of air flow meter are available, each with its strengths and weaknesses. The four common ones are: • pitot tube (or Curnon meter); • turbine meter: • orifice plate: • Vortex shedding meter.
All of the above meters measure the actual air velocity, and pressure and temperature compensation is necessary to get an accurate measure of the standard airflow rate (at 0°C. 760 mm Hg). Details of all these meters are described in greater detail in Appendix I. A chart recording facility attached to any of the meters will give very valuable information about the flow rate demand pattern. Recorders can be
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purchased to give a 24-hour or seven-day circular chart recording. Figure 11 also shows static pressure, which helps detect the critical pressure demand periods. It also gives a clear indication of how the compressors are coping with the demand and whether control pressures can be reduced during certain times of the day or week
with electricity consumption readings i.e. input energy to output air for a given demand can be calculated and used for analysis. Care must be taken when trying to establish the efficiency of an individual compressor, because the meter will be reading plant demand, not compressor output. It is possible to estimate the compressor output if the pressure is carefully monitored. If the system pressure is slowly falling with a compressor on load, then the whole output of the compressor, less any condensate or seal losses, will pass the flow meter and an estimate of the compressor output can be made. It is worth installing a permanent air flow meter on generation systems rated above 200 kW (600 I/s or 1,200 cfm) operating on a single-shift system. For full-time working, systems around half this capacity would warrant a meter.This limit is based on the meter costing around 10% of the annual air costs and assumes that the information given will lead to a 10% saving and hence a 12 month simple payback period.
7.1.3 COMPRESSOR HOUSE AIRFLOW METERING
The installation of an air flow meter on the main airline from the compressor house will give two very valuable pieces of information: • A true usage demand profile, and base leakage or minimum use information of the plant. This is preferable to estimating usage from the on/off-load running times of the compressors, as compressor output capacity must be assumed as design level, although it could be a long way from it. The overall generation efficiency coupled
•
Figure 11: Daily Chart Recording (Source: ETSU)
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7.1.4 DISTRIBUTION LINE AIRFLOW METERING
Permanent metering of airflow around a large distribution network can be very costly. This is mainly because good practice has led to the installation of ring main systems; so many branches have to be metered to account for a reasonable percentage of the total air used. As a guideline, a plant or area using in excesses of 500 1/sec (1,000 cfm) warrants the installation of a permanent meter. An alternative to permanent metering is to fit some means of measuring airflow in all main lines and to use a portable meter. This could mean fitting orifice plates in each line with a portable differential pressure transducer and chart recorder, or the addition of insertion points for turbine flow meters or pitot tubes. A single meter
can then be attached to each measuring point in turn and initiatives taken to reduce air consumption where appropriate.
7.2
MONITORING AND TARGETING
Monitoring and Targeting (M&T) has a much wider application than compressed air systems. When applied to compressed air systems, M&T could be defined as: Comparing the weekly usage of compressed air (as measured by kWh or airflow meters) against a pre-determined target, which reflects good practice. Monitoring and Targeting is dealt with in the booklet: "How to save energy and money: The 3E Strategy".
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8. compressed air audits
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This section outlines a strategy for identifying all the opportunities, including some auditing techniques that can easily be implemented. Ideally, before any actions are taken to improve a compressed air system, an audit should be carried out to determine the annual costs of the current system. If permanent metering is installed, this will provide an enormous amount of information to help find the best solutions. Without permanent meters, the energy consumption of each compressor will have to be estimated from the size of the motor, its average load (or its on/off times) and the number of hours it operates. The total energy costs can be calculated by adding the information for all of the compressors. It is estimated that 30% of the annual costs could be saved, providing good impetus for action. Compressed air system users should consider using an independent auditor to examine their compressed air system. Firms exist that specialize in compressed air system audits. Audits are also performed by electric utilities, equipment distributors and manufacturers, energy service companies, and engineering firms. An informed consumer should be aware that the quality and comprehensiveness of audits could vary. Independent auditors should provide recommendations, which are systems-neutral and commercially impartial. Independent auditors should neither specify nor recommend any particular manufacturer’s products. Having calculated the annual costs and established a baseline against which improvements can be measured, an audit of the complete compressed air system should be carried out. A comprehensive compressed air system audit should include an examination of both air supply and usage and the interaction between the supply and demand. Auditors typically measure the output of a compressed air system, calculate energy consumption in kilowatt-hours, and determine the annual cost of operating the system. The auditor may also measure total air losses due to leaks and locate those that are significant. All components of the compressed air system are inspected individually and problem areas are identified. Losses and poor performance due to system leaks, inappropriate misuse, and total system dynamics are calculated, and a written report with a recommended course of action is provided. It is best to start with the end users, because any improvements here may well have an effect on the air distribution network (i.e. redundant pipe work and reduction in pressure losses) and compressor demand. It is also normally the area where the greatest savings can be achieved.
8.1 END USAGE AUDIT •
8.1.1 LEAKS
The first priority is to assess the site leakage rate, as this is often the greatest wastage. To do this, a no-load leakage test must be carried out (see Section 3.3). From the results, the total percentage air lost and the annual costs of leakage can be calculated. Following this a leakage survey should be carried out and all leaks identified on a site drawing, as well as by tagging. The auditor should recommend a leak management program.
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8.1.2 END USERS
After the leakage appraisal, it is essential to look at each compressed air use in detail.The major tasks that need to be carried out are to: • Estimate the volume of major plant items, from either plant ratings or calculations, and note the number of hours they are worked, to help produce a breakdown of air usage and assist in deciding whether distribution lines are adequately sized; In some cases, recommendations such as specifying equipment that operates at a lower pressure will be made. An auditor may also recommend replacing existing compressed air-powered equipment with equipment that uses a source of energy other than compressed air. Compare the actual operating pressure with the design pressure and, if appropriate, fit a reducing valve (if the overall distribution line pressure cannot be reduced). Local storage and other modifications may be recommended. Investigate other methods of operation not involving the use of compressed air.
8.3 •
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AIR TREATMENT AUDIT
The following program should be carried out: The auditor typically examines the compressed air applications and determines the appropriate level of air treatment required for proper operation of the equipment. Actual air quality levels are then measured. If the air is not being treated enough, alternative treatment strategies are recommended. If the air is being over-treated (an indication of energy being wasted), recommendations are made to modify the system. In some cases, only certain end-use equipment requires highly treated air, and the auditor may recommend a system that, allows for different treatment levels at different points in the system. The total air drying capacity required should be calculated during the end user audit. If more air than necessary is being dried, the possibility of having two distribution systems, a wet and a dry, should be considered. Consideration should also be given to treating the higher quality air at the point of use. All drainage traps should be checked to ensure that they are neither leaking nor air binding. The location of the air intakes into the compressors should be checked to ensure that they are not supplying warm, wet or dusty air.
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8.2 DISTRIBUTION NETWORK AUDIT •
The distribution network should be surveyed and drawings obtained in line with the main things to look out for are: • • Zoning arrangements to isolate areas on different working patterns; Adequate pipe sizing and drainage: a pressure profile across the mains would be useful to identify large pressure losses; Elimination of redundant pipework or shortening of supply lines;
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8.4
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COMPRESSOR HOUSE AUDIT
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Having established the lowest possible demand profile for compressed air, it is necessary to ensure that the demand is serviced in the most efficient way possible.To do this it is necessary to carry out the following steps.
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•
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Record the electricity consumption of the compressors over a week by installing portable recording ammeters or demand recorders on the supply cables. Over the same period also record the hours run and hours ‘on-load’, if meters are available; For systems supplying a demand greater than 500 l/s, which should have an air flow meter installed, record the actual air demand over the week. If this is not possible, estimate, the air demand profile by assuming an output capacity of the compressors, and combining this with the hours ‘on-load’ data (this is not possible if the compressors are on modulation control). If an air flow meter is installed, record the static air pressure over the week to establish times when the control pressure can be reduced to reflect lower air usage. From the electricity and airflow recordings, calculate an air generation efficiency. It is worthwhile running each machine individually to monitor actual output capacity and to determine the J/l (or kWh/100 cfm) performance of each machine. These figures can then be compared to assess whether each
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machine is good, average or poor. A simple calculation will then identify how much energy can be saved by either maintaining the poor machines or preferentially using the more efficient ones to service the demand. Investigate the load profiles of each compressor with a view to deciding whether the optimum size machines are running at any one time. Consider better methods of compressor control, such as predictive switching or rotational sequencing depending on the compressor load profiles.
System audits are designed to identify system inefficiencies. If a system is found poorly designed, in unsatisfactory operating condition or in need of substantial retrofit, a more detailed analysis of the system may be recommended. A comprehensive evaluation may also include extensive measurements and analysis of supply and demand interactions. Some auditors will also prepare a detailed systems flow diagram. A financial evaluation may also be performed, including current and proposed costs after retrofits are taken.
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APPENDIX 1: METER DETAILS
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A1.1 PITOT TUBE
A pitot tube is relatively easy to install. It consists of two pressure lines that measure the static pressure and the static plus dynamic (or total) pressure of the compressed air. By measuring the differential pressure between the two lines, the velocity of the air can be obtained. Pitot tubes can, however, be inaccurate if the dynamic pressure profile across the pipe diameter is not taken into account. Manufacturers’ instructions should be followed to avoid this problem. A typical layout for a pitot tube is shown in Figure 12 below. Pitot tubes are often used for permanent meter installation. They give an excellent indication of flow rate and are relatively cheap.
A1.2 ORIFICE PLATE
Orifice plate meters consist of a drilled and machined disc inserted into the flow stream and clamped between two flanges (Figure 13). The orifice plate continues to be the most commonly used device for measuring flow rate.
Figure 12: Pitot tube operation (Source: ETSU)
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The orifice plate itself is the main element of the meter. The resulting differential pressure is measured via impulse lines connected from the pressure tappings of the orifice plate to a differential pressure transducer. The pressure tappings can be located in the pipeline up and downstream of the orifice plate. Alternatively, orifice plates can be supplied with corner or flange pressure tappings as part of a plate carrier ring assembly. Variations in the performance of the various tapping point locations can be used in formulae used to determine the coefficient of discharge. The location of the orifice plate in the pipe run is also important. The differential pressure measurement is sensitive to swirl and other fluid effects, so the orifice plate should be located a certain distance away, upstream and downstream,
from any pipefitting. British Standard BS1O42 and International Standard ISO 5167 provide details of the dimensions required. Orifice plate manufacturers should, however, advise on standard requirements, and reference to the published Standards is usually unnecessary. A typical requirement would be that there must be at least ten pipe diameters of straight pipe upstream of the meter and five pipe diameters downstream of the meter.
A1.3 TURBINE METERS
Turbine meters consist of a freely rotating propeller or screw located in the air pipe (Figure 14). Provided that bearing drag is minimised and the blades are well designed, the process stream will exert a torque on the turbine causing it to
Figure 13: Orifice Plate (Source: ETSU)
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rotate at a velocity proportional to the airflow rate. If a magnetic coil or optical device is placed in the meter housing, a voltage pulse can be induced each time a turbine blade passes it. The pulse rate will be proportional to the rate of flow and the total number of pulses that can be integrated to give the air volume, which has passed the meter. The response of this type of meter is approximately linear, except at low flow rates when the drag effects of the bearings become significant and may affect the linearity of the response. Any non-linearity can be overcome by incorporating a calibration curve into the system, which is used to convert the signal pulse into a flow rate.
principle that when a fluid stream flows around a bluff body (the vortex ‘shedder’), viscosity-related effects produce vortices downstream. The most common body shapes used in such meters are rectangular or triangular. The vortices are shed sequentially from either side of the bluff body at a frequency proportional to the flow velocity. Several methods of sensing the vortices exist. A common method is to use a piezo-electrical cell located in the bluff body support spindle. Shedding of the vortices creates lift in the bluff body, which in turn causes small movements to the spindle. Each movement compresses the cell, thereby generating a small electric current. Other methods use ultrasonics, which are modulated by the vortices, or involve the detection of the small pressure waves that accompany the shedding of the vortices
A1.4 VORTEX SHEDDING METERS
The vortex shedding meter operates on the
Figure 14:Turbine Meter (Source: ETSU)
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Figure 15:Vortex meter operation (Source: ETSU)
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SOURCES OF FURTHER INFORMATION
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For the latest news in energy efficiency technology: “Energy Management News” is a free newsletter issued by the ERI, which contains information on the latest developments in energy efficiency in Southern Africa and details of forthcoming energy efficiency events. Copies can be obtained from: The Energy Research Institute Department of Mechanical Engineering University of Cape Town Rondebosch 7700 Cape Town South Africa
Tel No: (+27 21) 650 3892 Fax No: (+27 21) 686 4838 Email: [email protected]
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