JCCC-HVACR_2012_SAMHUI_01
Content
About the Lecturer Joint Comprehensive Certificate Course on HVAC&R System, 2012 2012年度暖通空調及製冷系統綜合証書課程
Fundamentals of HVAC&R Part 1 Presented by:
Ir Dr. Sam C. M. Hui February 28, 2012
Joint Comprehensive Certificate Course on HVAC&R System, 2012 Feb-Apr 2012
• Dr. Sam C. M. Hui • PhD, BEng(Hons), CEng, CEM, MASHRAE, MCIBSE, MHKIE, MIESNA, LifeMAEE, AssocAIA • ASHRAE Distinguished Lecturer (2009-2011) • CEng = Chartered Engineer • CEM = Certified Energy Manager • LifeMAEE = Life Member, Associatn of Energy Engineers • Worked in 1998 as a visiting researcher in the Asia Pacific Energy Research Centre, Japan • Research interests: energy efficiency in buildings and sustainable building technologies
Contents • Introduction • Psychrometry
Fundamentals of HVAC&R Part 1
• Thermal comfort
Dr. Sam C. M. Hui Department of Mechanical Engineering The University of Hong Kong E-mail: [email protected]
• Load and energy calculations
Introduction
Introduction
• Terminology
• Definition (from ASHRAE*) • Air conditioning is the process of treating air so as to control simultaneously its temperature, humidity, cleanliness, and distribution to meet the requirements of the conditioned space.
• Heating, ventilating, air-conditioning and refrigerating (HVAC&R) 暖通空調及製冷 • Heating, ventilating and air-conditioning (HVAC) 暖通空 調 • Mechanical ventilating and air-conditioning (MVAC or ACMV) 機械通風及空調 • Air conditioning and refrigeration (AC&R) 空調及製冷 • Environmental control systems (ECS) 環境控制系統
• Misused word in HK: • Air cond. “冷氣” (= cold air)
• Basic processes: Cooling and Heating
• Comfort air conditioning • To meet comfort requirements of occupants (*ASHRAE = American Society of Heating, Refrigerating & Air-conditioning Engineers, Inc.)
(Source: www.howstuffworks.com/ac.htm)
See also: “How Air Conditioners Work” (1:07) http://youtu.be/nKZ2DPvvua8
(Source: www.howstuffworks.com/ac.htm)
What are the major components?
Refrigerant cycle A typical air conditioner
Chilled water system
Air conditioning with a chilled water system
Multiple chiller variable flow chilled water system (Source: ASHRAE HVAC Systems and Equipment Handbook 2004)
(Source: www.howstuffworks.com/ac.htm)
Introduction
Introduction
• Air Conditioning and Refrigeration
• The History of Air Conditioning
• No. 10 on the list of the [Greatest Engineering Achievements of the 20th Century] • http://www.greatachievements.org • These cooling technologies have altered some of our most fundamental patterns of living • Buildings are climate-controlled & comfortable • Fresh foods & milk are kept in refrigerators/freezers • Building designs are changed completely • Environment for industrial processes are controlled
• www.air-conditioners-andheaters.com/air_conditioning_history.htm • 1830: Dr. John Gorrie (ice for cooling hospital rooms) • 1881: James Garfield (device w/ melted ice water) • Late 19th century: “manufactured air” (controlling humidity in textile mills) • Early 1900s’: Willis Carrier (designed modern A/C systems for offices, apartments, hotels, hospitals) • 1917-1930: movie theatres were kept cool by A/C
Introduction • Common types of air conditioning systems • Centralised air systems • Constant volume (CV), variable air volume (VAV), Displacement ventilation
• Partially centralised air/water systems • Fan coils, chilled beams, chilled ceilings, room based heat pumps
• Local systems • Split units, variable refrigerant flow (VRF) or variable refrigerant volume (VRV) [??]
Individual room air-conditioning system
Primary air fan coil unit (PA-FCU) system (Source: EnergyWitts newsletter, EMSD)
Variable-air volume (VAV) package system
Psychrometry • Psychrometry • The study of atmospheric air and its associated water vapour • Dry air and moist air
• Dalton’s law of partial pressures • Standard atmospheric pressure = 101.325 kPa • Saturated vapour pressure • Max. pressure of water vapour that can occur at any given temperature
Psychrometry
Can you read them from the chart?
Psychrometric Chart Wet-bulb temperature
• Psychrometric Chart: parameters • • • • •
Moisture content (g), or absolute humidity (w) Relative humidity (rh or RH) Percentage saturation (μ) Wet-bulb temperature (twb) Specific volume (v)
Enthalpy
Relative humidity
Dew-point temperature Humidity ratio
• (See the illustration on psychrometric chart) Dry-bulb temperature
Specific volume
Basic psychrometric processes
Psychrometry 3 7
• Common processes:
6
8 4
Process 0-1: Sensible heating Process 0-2: Sensible cooling Process 0-3: Humidifying Process 0-4: Dehumidifying Process 0-5: Heating and humidifying Process 0-6: Cooling and dehumidifying Process 0-7: Cooling and humidifying Process 0-8: Heating and dehumidifying
• Typical devices: • Cooling/heating coils • Humidifiers / dehumifiers
Cooling coil
Psychrometric processes
Cooling and dehumidification
1
2
• Sensible cooling / sensible heating • Cooling and dehumidification / heating and humidification • Humidification / dehumidification • Evaporative cooling / chemical dehydration
Sensible cooling/heating
5
Entering air
Leaving air
Adiabatic dehumidification
Evaporative cooling
Cooling and dehumidification
Simple air conditioning cycle
Can you draw such a cycle for Hong Kong summer conditions? - Outdoor: DBT = 33 ºC; WBT = 28 ºC; flow = 20% of supply air - Indoor: DBT = 25 ºC; %RH = 50% - Air leaving cooling coil: DBT = 13 ºC; %RH = 95%
What is Thermal Comfort? Psychrometry • Further reading & learning: • Air Conditioning: Psychrometrics [www.bsenotes.com] • www.arca53.dsl.pipex.com/index_files/psy1.htm
• CIBSE Journal CPD Programme: The psychrometrics of air conditioning systems (Mar 2010), www.cibsejournal.com/cpd/2010-03/ • Daikin's Free Psychrometrics tool
- That condition of mind which expresses satisfaction with the thermal environment.
• www.daikin.eu/binaries/Psychrometric%20diagram%2 0viewer%20V210_tcm24-133157.zip
Body Temperature 37 oC
ISO 7730
Perception of Thermal Environment
• Normal body core temperature: 37 oC. • We have separate Heat- and Coldsensors.
34 oC
• Heat sensor is located in hypothalamus. Signals when temperature is higher than o 37 C. • Cold sensors are located in the skin. Send signals when skin temperature is o below 34 C.
• Heating mechanism: • Reduced blood flow. • Shivering.
• Cooling mechanism: Hot
Cold
• Increased blood flow. • Sweating (Evaporation).
The Energy Balance
Warm impulses
Cold impulses
Activity
• Heat sensor in Hypothalamus send impulses when temperature exceeds 37 oC. • Cold sensors sends impulses when skin temperature below 34 oC. • The bigger temperature difference, the more impulses. • If impulses are of same magnitude, you feel thermally neutral. • If not, you feel cold or warm.
Thermal comfort • General heat balance
Heat Produced
Heat Lost
• Thermal Comfort can only be maintained when heat produced by metabolism equals the heat lost from body.
S = M - W - E - (R + C) where S = rate of heat storage of human body M = metabolic rate W = mechanical work done by human body E = rate of total evaporation loss R + C = dry heat exchange through radiation & convection
Conditions for Thermal Comfort o
C.
• Two conditions must be fulfilled to maintain Thermal Comfort:
34 33 32 31 30 29
• Heat produced must equal heat lost • Signals from Heat- and Coldsensors must neutralise each other 0
1
2
3 4 Metabolic Rate
W/m2 Sweat prod.
The Comfort Equation
100 80 60
• The sweat production is used instead of body core temperature, as measure of the amount of warm impulses. • Relation between the parameters found empirically in experiments.
40 20 0
1
2
3 4 Metabolic Rate
• No difference between sex, age, race or geographic origin.
The Comfort Equation (cont’d) Thermal comfort • Environmental factors: • Dry-bulb temperature (also related to humidity) • Relative humidity (or water vapour pressure) • Influences evap heat loss and skin wettedness • Usually RH between 30% and 70% is comfortable
• Air velocity (increase convective heat loss) • Preferable air velocity
• Mean radiation temperature • Radiation has great effect on thermal sensation
Mean Radiant Temperature
Metabolic Rate 0.8 Met
Actual room
t1
Imaginary room
t4
tr
R’
R Heat exchange by radiation: R=R’
t
8 Met 1 Met
t3
2
• The Mean Radiant Temperature (MRT) is that uniform temperature of an imaginary black enclosure resulting in same heat loss by radiation from the person, as the actual enclosure. • Measuring all surface temperatures and calculation of angle factors is time consuming. Therefore use of Mean Radiant Temperature is avoided when possible.
4 Met
• Energy released by metabolism depends on muscular activity. • Metabolism is measured in Met (1 Met=58.15 W/m2 body surface). • Body surface for normal adult is 1.7 m2. • A sitting person in thermal comfort will have a heat loss of 100 W. • Average activity level for the last hour should be used when evaluating metabolic rate, due to body’s heat capacity.
Calculation of Insulation in Clothing 0.5 Clo
1.2 Clo
Comfort Temperature, tco (typical)
0,15 Clo
1.0 Clo
• 1 Clo = Insulation value of 0,155
m2 oC/W
- +3 Hot
• A complex function of six major comfort parameters • Predict mean value of the subjective ratings of a group of people in a given environment
• Predicted percentage of dissatisfied (PPD) • Determined from PMV as a quantitative measure of thermal comfort • ‘Dissatisfied’ means not voting -1, +1 or 0 in PMV • Normally, PPD < 7.5% at any location and LPPD < 6%
PMV and PPD
0.5 clo 1.2 Met RH=50% tco=24,5oC
Predicted Mean Vote scale
Thermal comfort • Predicted mean vote (PMV)
0.8 clo 2.2 Met RH=50% tco=18oC
1.7 clo 2.5 Met RH=50% tco=6oC
The PMV index is used to quantify the degree of discomfort
- +2 Warm - +1 Slightly warm - +0 Neutral - - 1 Slightly cool - -2 Cool - -3 Cold
Thermal comfort • Comfort zones
• PMV-index (Predicted Mean Vote) predicts the subjective ratings of the environment in a group of people. • 0 = neutral (still 5% people are dissatisfied)
• PPD-index predicts the number of dissatisfied people.
• Defined using isotherms parallel to effective temperature (ET) or standard ET (SET) • ASHRAE comfort zones for summer and winter (for typical indoor and seated person) • Proposed comfort zones • Within 5 to 16 mm Hg water vapour pressure • For summer, 22.8 oC SET 26.1 oC • For winter, 20.0 oC SET 23.9 oC
ASHRAE Comfort Zones (based on 2004 version of ASHRAE Standard 55)
Local Thermal Discomfort • Radiation Asymmetry • Draught
• Vertical Air Temperature Differences.
• Floor temperature
Acclimatisation/Adaptation!
Load & energy calculations • Thermal load • The amount of heat that must be added or removed from the space to maintain the proper temperature in the space
When the air condition system fails you can adapt by adjusting your CLO value
• When thermal loads push conditions outside of the comfort range, HVAC systems are used to bring the thermal conditions back to comfort conditions
Load & energy calculations • Purpose of HVAC load estimation • • • • •
Calculate peak design loads (cooling/heating) Estimate likely plant/equipment capacity or size Specify the required airflow to individual spaces Provide info for HVAC design e.g. load profiles Form the basis for building energy analysis
• Cooling load is our main target • Important for warm climates & summer design • Affect building performance & its first cost
Cooling load profiles
Load & energy calculations
Cooling Load Components
• Typical HVAC load design process
• External
• 1. Rough estimates of design loads & energy use
• • • •
• Such as by rules of thumb & floor areas • See “Cooling Load Check Figures” • See references for some examples of databooks
• 2. Develop & assess more info (design criteria, building info, system info)
1. Heat gain through exterior walls and roofs 2. Solar heat gain through fenestrations (windows) 3. Conductive heat gain through fenestrations 4. Heat gain through partitions & interior doors
• Internal • 1. People • 2. Electric lights • 3. Equipment and appliances
• Building layouts & plans are developed
• 3. Perform detailed load & energy calculations
Components of building cooling load
Cooling Load Components • Infiltration • Air leakage and moisture migration, e.g. flow of outdoor air into a building through cracks, unintentional openings, normal use of exterior doors for entrance
Internal loads
External loads
• System (HVAC) • Outdoor ventilation air • System heat gain: duct leakage & heat gain, reheat, fan & pump energy, energy recovery + Ventilation load & system heat gains
Load & energy calculations
Load & energy calculations
• Cooling load calculation method
• External
• Example: CLTD/SCL/CLF method • It is a one-step, simple calculation procedure developed by ASHRAE • CLTD = cooling load temperature difference • SCL = solar cooling load • CLF = cooling load factor
• See ASHRAE Handbook Fundamentals for details • Tables for CLTD, SCL and CLF
• Roofs, walls, and glass conduction • q = U A (CLTD)
U = U-value; A = area
• Solar load through glass • q = A (SC) (SCL)
SC = shading coefficient
• For unshaded area and shaded area
• Partitions, ceilings, floors • q = U A (tadjacent - tinside)
Load & energy calculations
Load & energy calculations
• Internal
• Ventilation and infiltration air
• People
• qsensible = 1.23 Q (toutside - tinside) • qlatent = 3010 Q (woutside - winside) • qtotal = 1.2 Q (houtside - hinside)
• qsensible = N (Sensible heat gain) (CLF) • qlatent = N (Latent heat gain)
• Lights
• System heat gain
• q = Watt x Ful x Fsa (CLF)
• Fan heat gain • Duct heat gain and leakage • Ceiling return air plenum
• Ful = lighting use factor; Fsa = special allowance factor
• Appliances • qsensible = qinput x usage factors (CLF) • qlatent = qinput x load factor (CLF)
Load & energy calculations • Definitions • Space heat gain: instantaneous rate of heat gain that enters into or is generated within a space • Space cooling load: the rate at which heat must be removed from the space to maintain a constant space air temperature • Space heat extraction rate: the actual rate of heat removal when the space air temp. may swing • Cooling coil load: the rate at which energy is removed at a cooling coil serving the space
Conversion of heat gain into cooling load (Source: ASHRAE Handbook Fundamentals 2005)
Cooling loads due to windows at different orientations
North
West
East
South Block load and thermal zoning (Source: D.G. Stephenson, 1968)
Load & energy calculations
Load & energy calculations
• From load estimation to energy calculations
• Two categories
• Only determine peak design loads is not enough • Need to evaluate HVAC and building energy consumption • To support design decisions (e.g. evaluate design options) • To enhance system design and operation • To compile with building energy code
• Steady-state methods • Degree-day method • Variable base degree-day method • Bin and modified bin methods
• Dynamic methods
• Energy calculations • More complicated than design load estimation • Form the basis of building energy and economic analysis
• Using computer-based building energy simulation • Try to capture dynamic response of the building • Can be developed based on transfer function, heat balance or other methods
Weather data
Heating degree-day:
Cooling degree-day:
+
Only take the positive values
tbal = base temperature (or balance point temperature) (e.g. 18.3 oC or 65 oF); Qload = Qgain + Qloss = 0 to = outdoor temperature (e.g. average daily max./min.)
Building description
* Degree-hours if summing over 24-hourly intervals Degree-day = Σ(degree-hours)+ / 24
- physical data - design parameters
Simulation tool (computer program)
Simulation outputs
- energy consumption (MWh) - energy demands (kW) - environmental conditions
Building energy simulation process HVAC air systems
HVAC water systems
Energy storage
Load & energy calculations • Further reading & learning:
Thermal Zone
Systems (air-side)
Plant (waterside & refrig.)
• Comfort [www.bsenotes.com] • www.arca53.dsl.pipex.com/index_files/science1.htm • Thermal comfort – Wikipedia • http://en.wikipedia.org/wiki/Thermal_comfort • ASHRAE Handbook Fundamentals 2009, Chps. 14-19 (on load and energy calculations)
Energy input by appliance
Energy input by HVAC air/water systems
Energy input by HVAC plant
Fundamentals of HVAC&R Part 1 Presented by:
Ir Dr. Sam C. M. Hui February 28, 2012
Joint Comprehensive Certificate Course on HVAC&R System, 2012 Feb-Apr 2012
• Dr. Sam C. M. Hui • PhD, BEng(Hons), CEng, CEM, MASHRAE, MCIBSE, MHKIE, MIESNA, LifeMAEE, AssocAIA • ASHRAE Distinguished Lecturer (2009-2011) • CEng = Chartered Engineer • CEM = Certified Energy Manager • LifeMAEE = Life Member, Associatn of Energy Engineers • Worked in 1998 as a visiting researcher in the Asia Pacific Energy Research Centre, Japan • Research interests: energy efficiency in buildings and sustainable building technologies
Contents • Introduction • Psychrometry
Fundamentals of HVAC&R Part 1
• Thermal comfort
Dr. Sam C. M. Hui Department of Mechanical Engineering The University of Hong Kong E-mail: [email protected]
• Load and energy calculations
Introduction
Introduction
• Terminology
• Definition (from ASHRAE*) • Air conditioning is the process of treating air so as to control simultaneously its temperature, humidity, cleanliness, and distribution to meet the requirements of the conditioned space.
• Heating, ventilating, air-conditioning and refrigerating (HVAC&R) 暖通空調及製冷 • Heating, ventilating and air-conditioning (HVAC) 暖通空 調 • Mechanical ventilating and air-conditioning (MVAC or ACMV) 機械通風及空調 • Air conditioning and refrigeration (AC&R) 空調及製冷 • Environmental control systems (ECS) 環境控制系統
• Misused word in HK: • Air cond. “冷氣” (= cold air)
• Basic processes: Cooling and Heating
• Comfort air conditioning • To meet comfort requirements of occupants (*ASHRAE = American Society of Heating, Refrigerating & Air-conditioning Engineers, Inc.)
(Source: www.howstuffworks.com/ac.htm)
See also: “How Air Conditioners Work” (1:07) http://youtu.be/nKZ2DPvvua8
(Source: www.howstuffworks.com/ac.htm)
What are the major components?
Refrigerant cycle A typical air conditioner
Chilled water system
Air conditioning with a chilled water system
Multiple chiller variable flow chilled water system (Source: ASHRAE HVAC Systems and Equipment Handbook 2004)
(Source: www.howstuffworks.com/ac.htm)
Introduction
Introduction
• Air Conditioning and Refrigeration
• The History of Air Conditioning
• No. 10 on the list of the [Greatest Engineering Achievements of the 20th Century] • http://www.greatachievements.org • These cooling technologies have altered some of our most fundamental patterns of living • Buildings are climate-controlled & comfortable • Fresh foods & milk are kept in refrigerators/freezers • Building designs are changed completely • Environment for industrial processes are controlled
• www.air-conditioners-andheaters.com/air_conditioning_history.htm • 1830: Dr. John Gorrie (ice for cooling hospital rooms) • 1881: James Garfield (device w/ melted ice water) • Late 19th century: “manufactured air” (controlling humidity in textile mills) • Early 1900s’: Willis Carrier (designed modern A/C systems for offices, apartments, hotels, hospitals) • 1917-1930: movie theatres were kept cool by A/C
Introduction • Common types of air conditioning systems • Centralised air systems • Constant volume (CV), variable air volume (VAV), Displacement ventilation
• Partially centralised air/water systems • Fan coils, chilled beams, chilled ceilings, room based heat pumps
• Local systems • Split units, variable refrigerant flow (VRF) or variable refrigerant volume (VRV) [??]
Individual room air-conditioning system
Primary air fan coil unit (PA-FCU) system (Source: EnergyWitts newsletter, EMSD)
Variable-air volume (VAV) package system
Psychrometry • Psychrometry • The study of atmospheric air and its associated water vapour • Dry air and moist air
• Dalton’s law of partial pressures • Standard atmospheric pressure = 101.325 kPa • Saturated vapour pressure • Max. pressure of water vapour that can occur at any given temperature
Psychrometry
Can you read them from the chart?
Psychrometric Chart Wet-bulb temperature
• Psychrometric Chart: parameters • • • • •
Moisture content (g), or absolute humidity (w) Relative humidity (rh or RH) Percentage saturation (μ) Wet-bulb temperature (twb) Specific volume (v)
Enthalpy
Relative humidity
Dew-point temperature Humidity ratio
• (See the illustration on psychrometric chart) Dry-bulb temperature
Specific volume
Basic psychrometric processes
Psychrometry 3 7
• Common processes:
6
8 4
Process 0-1: Sensible heating Process 0-2: Sensible cooling Process 0-3: Humidifying Process 0-4: Dehumidifying Process 0-5: Heating and humidifying Process 0-6: Cooling and dehumidifying Process 0-7: Cooling and humidifying Process 0-8: Heating and dehumidifying
• Typical devices: • Cooling/heating coils • Humidifiers / dehumifiers
Cooling coil
Psychrometric processes
Cooling and dehumidification
1
2
• Sensible cooling / sensible heating • Cooling and dehumidification / heating and humidification • Humidification / dehumidification • Evaporative cooling / chemical dehydration
Sensible cooling/heating
5
Entering air
Leaving air
Adiabatic dehumidification
Evaporative cooling
Cooling and dehumidification
Simple air conditioning cycle
Can you draw such a cycle for Hong Kong summer conditions? - Outdoor: DBT = 33 ºC; WBT = 28 ºC; flow = 20% of supply air - Indoor: DBT = 25 ºC; %RH = 50% - Air leaving cooling coil: DBT = 13 ºC; %RH = 95%
What is Thermal Comfort? Psychrometry • Further reading & learning: • Air Conditioning: Psychrometrics [www.bsenotes.com] • www.arca53.dsl.pipex.com/index_files/psy1.htm
• CIBSE Journal CPD Programme: The psychrometrics of air conditioning systems (Mar 2010), www.cibsejournal.com/cpd/2010-03/ • Daikin's Free Psychrometrics tool
- That condition of mind which expresses satisfaction with the thermal environment.
• www.daikin.eu/binaries/Psychrometric%20diagram%2 0viewer%20V210_tcm24-133157.zip
Body Temperature 37 oC
ISO 7730
Perception of Thermal Environment
• Normal body core temperature: 37 oC. • We have separate Heat- and Coldsensors.
34 oC
• Heat sensor is located in hypothalamus. Signals when temperature is higher than o 37 C. • Cold sensors are located in the skin. Send signals when skin temperature is o below 34 C.
• Heating mechanism: • Reduced blood flow. • Shivering.
• Cooling mechanism: Hot
Cold
• Increased blood flow. • Sweating (Evaporation).
The Energy Balance
Warm impulses
Cold impulses
Activity
• Heat sensor in Hypothalamus send impulses when temperature exceeds 37 oC. • Cold sensors sends impulses when skin temperature below 34 oC. • The bigger temperature difference, the more impulses. • If impulses are of same magnitude, you feel thermally neutral. • If not, you feel cold or warm.
Thermal comfort • General heat balance
Heat Produced
Heat Lost
• Thermal Comfort can only be maintained when heat produced by metabolism equals the heat lost from body.
S = M - W - E - (R + C) where S = rate of heat storage of human body M = metabolic rate W = mechanical work done by human body E = rate of total evaporation loss R + C = dry heat exchange through radiation & convection
Conditions for Thermal Comfort o
C.
• Two conditions must be fulfilled to maintain Thermal Comfort:
34 33 32 31 30 29
• Heat produced must equal heat lost • Signals from Heat- and Coldsensors must neutralise each other 0
1
2
3 4 Metabolic Rate
W/m2 Sweat prod.
The Comfort Equation
100 80 60
• The sweat production is used instead of body core temperature, as measure of the amount of warm impulses. • Relation between the parameters found empirically in experiments.
40 20 0
1
2
3 4 Metabolic Rate
• No difference between sex, age, race or geographic origin.
The Comfort Equation (cont’d) Thermal comfort • Environmental factors: • Dry-bulb temperature (also related to humidity) • Relative humidity (or water vapour pressure) • Influences evap heat loss and skin wettedness • Usually RH between 30% and 70% is comfortable
• Air velocity (increase convective heat loss) • Preferable air velocity
• Mean radiation temperature • Radiation has great effect on thermal sensation
Mean Radiant Temperature
Metabolic Rate 0.8 Met
Actual room
t1
Imaginary room
t4
tr
R’
R Heat exchange by radiation: R=R’
t
8 Met 1 Met
t3
2
• The Mean Radiant Temperature (MRT) is that uniform temperature of an imaginary black enclosure resulting in same heat loss by radiation from the person, as the actual enclosure. • Measuring all surface temperatures and calculation of angle factors is time consuming. Therefore use of Mean Radiant Temperature is avoided when possible.
4 Met
• Energy released by metabolism depends on muscular activity. • Metabolism is measured in Met (1 Met=58.15 W/m2 body surface). • Body surface for normal adult is 1.7 m2. • A sitting person in thermal comfort will have a heat loss of 100 W. • Average activity level for the last hour should be used when evaluating metabolic rate, due to body’s heat capacity.
Calculation of Insulation in Clothing 0.5 Clo
1.2 Clo
Comfort Temperature, tco (typical)
0,15 Clo
1.0 Clo
• 1 Clo = Insulation value of 0,155
m2 oC/W
- +3 Hot
• A complex function of six major comfort parameters • Predict mean value of the subjective ratings of a group of people in a given environment
• Predicted percentage of dissatisfied (PPD) • Determined from PMV as a quantitative measure of thermal comfort • ‘Dissatisfied’ means not voting -1, +1 or 0 in PMV • Normally, PPD < 7.5% at any location and LPPD < 6%
PMV and PPD
0.5 clo 1.2 Met RH=50% tco=24,5oC
Predicted Mean Vote scale
Thermal comfort • Predicted mean vote (PMV)
0.8 clo 2.2 Met RH=50% tco=18oC
1.7 clo 2.5 Met RH=50% tco=6oC
The PMV index is used to quantify the degree of discomfort
- +2 Warm - +1 Slightly warm - +0 Neutral - - 1 Slightly cool - -2 Cool - -3 Cold
Thermal comfort • Comfort zones
• PMV-index (Predicted Mean Vote) predicts the subjective ratings of the environment in a group of people. • 0 = neutral (still 5% people are dissatisfied)
• PPD-index predicts the number of dissatisfied people.
• Defined using isotherms parallel to effective temperature (ET) or standard ET (SET) • ASHRAE comfort zones for summer and winter (for typical indoor and seated person) • Proposed comfort zones • Within 5 to 16 mm Hg water vapour pressure • For summer, 22.8 oC SET 26.1 oC • For winter, 20.0 oC SET 23.9 oC
ASHRAE Comfort Zones (based on 2004 version of ASHRAE Standard 55)
Local Thermal Discomfort • Radiation Asymmetry • Draught
• Vertical Air Temperature Differences.
• Floor temperature
Acclimatisation/Adaptation!
Load & energy calculations • Thermal load • The amount of heat that must be added or removed from the space to maintain the proper temperature in the space
When the air condition system fails you can adapt by adjusting your CLO value
• When thermal loads push conditions outside of the comfort range, HVAC systems are used to bring the thermal conditions back to comfort conditions
Load & energy calculations • Purpose of HVAC load estimation • • • • •
Calculate peak design loads (cooling/heating) Estimate likely plant/equipment capacity or size Specify the required airflow to individual spaces Provide info for HVAC design e.g. load profiles Form the basis for building energy analysis
• Cooling load is our main target • Important for warm climates & summer design • Affect building performance & its first cost
Cooling load profiles
Load & energy calculations
Cooling Load Components
• Typical HVAC load design process
• External
• 1. Rough estimates of design loads & energy use
• • • •
• Such as by rules of thumb & floor areas • See “Cooling Load Check Figures” • See references for some examples of databooks
• 2. Develop & assess more info (design criteria, building info, system info)
1. Heat gain through exterior walls and roofs 2. Solar heat gain through fenestrations (windows) 3. Conductive heat gain through fenestrations 4. Heat gain through partitions & interior doors
• Internal • 1. People • 2. Electric lights • 3. Equipment and appliances
• Building layouts & plans are developed
• 3. Perform detailed load & energy calculations
Components of building cooling load
Cooling Load Components • Infiltration • Air leakage and moisture migration, e.g. flow of outdoor air into a building through cracks, unintentional openings, normal use of exterior doors for entrance
Internal loads
External loads
• System (HVAC) • Outdoor ventilation air • System heat gain: duct leakage & heat gain, reheat, fan & pump energy, energy recovery + Ventilation load & system heat gains
Load & energy calculations
Load & energy calculations
• Cooling load calculation method
• External
• Example: CLTD/SCL/CLF method • It is a one-step, simple calculation procedure developed by ASHRAE • CLTD = cooling load temperature difference • SCL = solar cooling load • CLF = cooling load factor
• See ASHRAE Handbook Fundamentals for details • Tables for CLTD, SCL and CLF
• Roofs, walls, and glass conduction • q = U A (CLTD)
U = U-value; A = area
• Solar load through glass • q = A (SC) (SCL)
SC = shading coefficient
• For unshaded area and shaded area
• Partitions, ceilings, floors • q = U A (tadjacent - tinside)
Load & energy calculations
Load & energy calculations
• Internal
• Ventilation and infiltration air
• People
• qsensible = 1.23 Q (toutside - tinside) • qlatent = 3010 Q (woutside - winside) • qtotal = 1.2 Q (houtside - hinside)
• qsensible = N (Sensible heat gain) (CLF) • qlatent = N (Latent heat gain)
• Lights
• System heat gain
• q = Watt x Ful x Fsa (CLF)
• Fan heat gain • Duct heat gain and leakage • Ceiling return air plenum
• Ful = lighting use factor; Fsa = special allowance factor
• Appliances • qsensible = qinput x usage factors (CLF) • qlatent = qinput x load factor (CLF)
Load & energy calculations • Definitions • Space heat gain: instantaneous rate of heat gain that enters into or is generated within a space • Space cooling load: the rate at which heat must be removed from the space to maintain a constant space air temperature • Space heat extraction rate: the actual rate of heat removal when the space air temp. may swing • Cooling coil load: the rate at which energy is removed at a cooling coil serving the space
Conversion of heat gain into cooling load (Source: ASHRAE Handbook Fundamentals 2005)
Cooling loads due to windows at different orientations
North
West
East
South Block load and thermal zoning (Source: D.G. Stephenson, 1968)
Load & energy calculations
Load & energy calculations
• From load estimation to energy calculations
• Two categories
• Only determine peak design loads is not enough • Need to evaluate HVAC and building energy consumption • To support design decisions (e.g. evaluate design options) • To enhance system design and operation • To compile with building energy code
• Steady-state methods • Degree-day method • Variable base degree-day method • Bin and modified bin methods
• Dynamic methods
• Energy calculations • More complicated than design load estimation • Form the basis of building energy and economic analysis
• Using computer-based building energy simulation • Try to capture dynamic response of the building • Can be developed based on transfer function, heat balance or other methods
Weather data
Heating degree-day:
Cooling degree-day:
+
Only take the positive values
tbal = base temperature (or balance point temperature) (e.g. 18.3 oC or 65 oF); Qload = Qgain + Qloss = 0 to = outdoor temperature (e.g. average daily max./min.)
Building description
* Degree-hours if summing over 24-hourly intervals Degree-day = Σ(degree-hours)+ / 24
- physical data - design parameters
Simulation tool (computer program)
Simulation outputs
- energy consumption (MWh) - energy demands (kW) - environmental conditions
Building energy simulation process HVAC air systems
HVAC water systems
Energy storage
Load & energy calculations • Further reading & learning:
Thermal Zone
Systems (air-side)
Plant (waterside & refrig.)
• Comfort [www.bsenotes.com] • www.arca53.dsl.pipex.com/index_files/science1.htm • Thermal comfort – Wikipedia • http://en.wikipedia.org/wiki/Thermal_comfort • ASHRAE Handbook Fundamentals 2009, Chps. 14-19 (on load and energy calculations)
Energy input by appliance
Energy input by HVAC air/water systems
Energy input by HVAC plant
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