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Rating of Air Compressors and Air Equipment

The most common terms rating air flow capacity are ICFM, FAD, ANR, SCFM or nl/min
There is no universal standard for rating air compressors, air equipment and tools. Common terms are:
      



CFM (Cubic Feet per Minute) is the imperial method of describing the volume flow rate of compressed air. It must be defined further to take account of pressure, temperature and relative humidity - see below.


ICFM (Inlet CFM) rating is used to measure air flow in CFM (ft3/min) as it enters the air compressor intake .


ACFM (Actual CFM) rating is used to measure air flow in CFM at some reference point at local conditions. This is the actual volume flow rate in the pipework after the compressor.


FAD (Free Air Delivery) (f.a.d) is the actual quantity of compressed air converted back to the inlet conditions of the compressor. The units for FAD are CFM in the imperial system and l/min in the SI system. The units are in general measured according the ambient inlet standard conditions ISO 1217: Ambient temperature = 20oC, Ambient pressure = 1 bar abs, Relative humidity = 0%, Cooling water/air = 20oC and Effective working pressure at discharge valve = 7 bar abs. 1 m3/min (f.a.d) = 1000 liter/min (f.a.d) = 1000 dm3/min (f.a.d) = 16.7 l/s (f.a.d) = 16.7 dm3/s (f.a.d) = 35.26 ft3/min (f.a.d)


ANR (Atmosphere Normale de Reference) is quantity of air at conditions 1.01325 bar absolute, 20oC and 65% RH (Relative Humidity).


SCFM (Standard CFM) is the flow in CFM measured at some reference point but converted back to standard air conditions (Standard Reference Atmosphere) 14.4 psia, 60oF.


nl/min is the flow in l/min measured at some reference point but converted to standard air conditions 1.01325 bar absolute, 0oC and 0% RH (Relative Humidity).

ISO 1217

standard reference ambient conditions - temperature 20oC, pressure 1 bar abs, relative humidity 0%, cooling air/water 20oC, and working pressure at outlet 7 bar absolute.

Example - Rating a Compressor
A typical rating of a compressor may look like this FAD (CFM) 11.1 cfm @ 7.5 Bar 8.2 cfm @ 10 Bar Description The Free Air Delivery from the compressor is 11.1 CFM at 7.5 Bar The Free Air Delivery from the compressor is 8.2 CFM at 10 Bar

Calculating receivers in compressed air systems
An air receiver is essential to every compressed air system to act as a buffer and a storage medium between the compressor and the consumption system. There are in principal two different air receivers in a compressed air system:
 

PRIMARY receiver - located near the compressor, after the after-cooler but before filtration and drying equipment SECONDARY receivers - located close to points of larger intermittent air consumptions

The maximum capacity of the compressor in a well designed systems always exceed the maximum mean air consumption of the system (maximum mean air consumption is the mean air consumption over some reasonable time). Since the maximum capacity of an air compressor also always exceed the minimum air consumption in the system - the compressor must modulate its capacity during normal work, often by using primitive strategies as on/off modulating or more advanced strategies as frequency drives and inverters. Primitive modulating strategies cause more pressure variations in compressed air systems than more advanced strategies. In addition, the air consumption vary due to the process supported. In shorter periods the demand for compressed air may even exceed the maximum capacity of the compressor. In fact, it is common in well designed systems not to design the compressor for the maximum peek loads. Air receivers in compressed air systems serves the important purposes of
 

equalizing the pressure variation from the start/stop and modulating sequence of the compressor storage of air volume equalizing the variation in consumption and demand from the system

In addition the receiver serve the purpose of

collecting condensate and water in the air after the compressor

Sizing the Air Receiver
The air receiver must in general be sized according
 

the variation in the consumption demand the compressor size and the modulation strategy

In general it is possible to calculate the maximum consumption in the system by summarizing the demand of each consumer. The summarized consumption must be multiplied with a

usage factor ranging 0.1 - 1

depending on the system. In practice it is common that the manufacturer use standardized receivers for specific compressor models based on their know-how. For calculating the receiver, note that it is necessary with a pressure band for the receiver to be effective. If the consumption process requires 100 psig and the compressor is set to 100 psig, there is no storage and no buffer. Any increased demand makes a pressure drop below 100 psig until the compressor controls respond by increasing the volume compressed. If the compressors operates at 110 psig the difference between 110 psig and 100 psig accounts for the air stored in the receiver. If the demand increase, the pressure can drop 10 psig before the minimum requirement is met. Pressure and flow controllers can be used after the receiver for stabilizing downstream pressure to 100 psig and flattening demand peaks. Note that in a compressed air system the pipe work also makes the purpose of a buffered volume. The receiver volume may be calculated with the formula t = V (p1 - p2) / C pa where V = volume of the receiver tank (cu ft) t = time for the receiver to go from upper to lower pressure limits (min) C = free air needed (scfm) pa= atmosphere pressure (14.7 psia) p1 = maximum tank pressure (psia) p2 = minimum tank pressure (psia) It is also common to size receivers
 


to 1 gallon for each ACFM (Actual Cubic Feet per Minute), or 4 gallons per compressor hp (horse power)

Note! Receivers of unsound or questionable constructions may be very dangerous.

Air Receivers Capacities
Air Receiver Capacities (cubic feet) Tank Size Tank Size Gauge Pressure on Tank (psig) (inches) 12 x 24 14 x 36 16 x 36 20 x 48 20 x 63 24 x 68 30 x 84 (gallons) 0 10 20 30 60 80 120 240 1.3 2.7 4.0 8.0 11 16 32 100 11 21 31 62 83 125 250 150 15 30 45 90 120 180 360 200 19 39 59 117 156 234 467

   

1 ft3 = 0.02832 m3 1 inch = 25.4 mm 1 psig = 6.9 kPa = 0.069 bar 1 Gallon (U.S.) = 3.785x10-3 m3 = 3.785 dm3 (liter) = 231 in3

Actual air compressor capacity (ACFM) versus standard air capacity (SCFM) and inlet air capacity (ICFM)

SCFM - Standard Cubic Feet per Minute
It is common to rate the compressed air consumption in Standard Cubic Feet per Minute SCFM. The SCFM - Standard Cubic Feet per Minute - determines the weight of air to fixed or "Standard" conditions. There are several definitions of SCFM. The most common used in the United States is with "sea-level" properties:

  

14.696 Pounds per Square Inch (psia) 60 Degrees Fahrenheit (oF) (520oR) 0% Relative Humidity (RH)

Europeans normally use one ata and 0 oC as SCFM.

ACFM - Actual Cubic Feet per Minute
Unfortunately, real life "actual conditions" are seldom "standard conditions". When
 

pressure is applied a volume of air - it gets smaller vacuum is applied to a volume of air - it expand

Actual air volume flow is often termed ACFM - Actual Cubic Feet per Minute. Actual Cubic Feet per Minute - ACFM, depends on the
  

pressure temperature humidity

of the actual air. The conversion from SCFM to ACFM can be expressed as ACFM = SCFM [Pstd / (Pact - Psat Φ)](Tact / Tstd) (1)

where ACFM = Actual Cubic Feet per Minute SCFM = Standard Cubic Feet per Minute Pstd = Standard absolute air pressure (psia) Pact = absolute pressure at the actual level (psia) Psat = Saturation pressure at the actual temperature (psi) Φ = Actual relative humidity Tact = Actual ambient air temperature (oR) Tstd = Standard temperature (oR)

Related Mobile Apps from The Engineering ToolBox


- free apps for offline use on mobile devices.

Online SCFM - ACFM Calculator
The calculator below can used to calculate ACFM:

SCFM Standard absolute air pressure (psia) Actual absolute air pressure (psia) Saturation pressure at the actual temperature (psia) Actual relative humidity Actual ambient air temperature (oR) Standard air temperature (oR)







 

temperature converter pressure converter

Example - SCFM to ACFM

The actual CFM of a compressor operating at "non-standard" conditions like
    

elevation 5000 feet (1500 m) - atmospheric pressure Pact = 12.23 psia temperature 80oF - absolute temperature Tact = 540oR saturation pressure Psat = 0.5069 psia relative humidity Φ = 80% demand 100 SCFM

can be calculated as ACFM = (100 SCFM) [(14.7 psia) / ((12.23 psia) - (0.5069 psia) (80 / 100))]((540 oR) / (520 oR)) = 129.1

ICFM - Inlet Cubic Feet per Minute
Inlet Cubic Feet per Minute - ICFM - is used by compressor vendors to establish conditions in front of additional equipment like inlet filter, blower or booster. When air passes through the filter there will be a pressure drop. The conversion from ICFM to ACFM can be expressed as ACFM = ICFM (Pact / Pf) (Tf / Tact) where ICFM = Inlet Cubic Feet per Minute Pf = Pressure after filter or inlet equipment (psia) Tf = Temperature after filter or inlet equipment (oR) (2)

The Ideal Gas Law is accurate only at relatively low pressures and high temperatures. To account for the deviation from the ideal situation, another factor is included. It is called the Gas Compressibility Factor, or Z-factor. This correction factor is dependent on pressure and temperature for each gas considered. The True Gas Law, or the Non-Ideal Gas Law, becomes:

PV=ZnRT where


Z = Gas Compressibility Factor n = number of moles of gas present

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