Psychrometric 2007
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Transcript of Psychrometric 2007
1
1Air Conditioning Design - Psychrometrics & Coil Load Calculation
Air Conditioning DesignPsychrometrics & Coil Load Calculations
Air Conditioning Design
Keith E. Elder, P.E.
Coils
• Coils are the main devices for transferring energyto/from the airstream in an air-handling system.
• Heat Transfer Media
– Hydronic Fluid (Water)
– Steam
– Direct Expansion
• Important Coil Load Concepts
– Apparatus Dew Point
– Bypass Factor
2
2Air Conditioning Design - Psychrometrics & Coil Load Calculation
The Psychrometric Chart
Dry Bulb Temperature
3
3Air Conditioning Design - Psychrometrics & Coil Load Calculation
Terminology
• Sensible Cooling
• Latent Cooling
• Space Sensible Cooling
• Space Sensible Heat Ratio (Factor)
• Grand Sensible Heat Ratio (Capacity)
• Apparatus Dew Point
• Bypass Factor
4
4Air Conditioning Design - Psychrometrics & Coil Load Calculation
SHR
The ratio of space sensible cooling to totalcooling is useful for plotting the slope of thepath that supply air travels after introductioninto the space.
Sensible Heat Ratio (SHR)
Apparatus Dew Point
Apparatus dew point (ADP) is the coil surfacedew point temperature required to accomplisha cooling/dehumidifying process.
CoilEnteringAirAppratus
Dew Point
Dry Bulb Temperature
CoilLeavingAir
5
5Air Conditioning Design - Psychrometrics & Coil Load Calculation
Apparatus Dew Point
An ADP is essential to any cooling ordehumidifying process. The chart belowshows an impossible process because there isno ADP
Coil
Entering
Air
Dry Bulb Temperature
Coil
Leaving
Air
Apparatus Dew Point
Relationship of ADP to Supply Air andChilled Water temperatures
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6Air Conditioning Design - Psychrometrics & Coil Load Calculation
Bypass Factor
Some of the air flowing through the coil impingeson the water tubes or the fins and is cooled to theADP. Other air passes through unchanged.
The percentage of air that passes through the coilunchanged is called the bypass factor.
CoilEnteringAir
Fin
8-Row Coil80 fpf /2 fps/44 EWT / WTR = 10
Coil Bypass
7
7Air Conditioning Design - Psychrometrics & Coil Load Calculation
Coil Selection
• Start With Space Conditions
• Find the intersection of the SHRwith desired room temperature andsupply air temperature
RoomSupplyAir
Dry Bulb Temperature
Room AirTemperatureSHR
SupplyAir
Temperature
Coil Selection
Determine Outside Air Requirements UsingASHRAE Standard 62
Offices 20 CFM per person
Schools 15 CFM per person
Bars 30 CFM per person
Smoking Lounges 60 CFM per person
Corridors 0.05 CFM/ft2
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8Air Conditioning Design - Psychrometrics & Coil Load Calculation
Plot Entering/Leaving Conditions
Coil
Entering
AirAppratus
Dew Point
Dry Bulb Temperature
Outside Air
Room Air
Mixed Air
Coil
Leaving
Air
Define Process in Complete Cycle
Coil
Entering
AirAppratus
Dew Point
Dry Bulb Temperature
Outside Air
Room Air
Mixed Air
Coil
Leaving
Air
SHR
9
9Air Conditioning Design - Psychrometrics & Coil Load Calculation
Determine Required Coil Capacity
Coil Capacity Requirement
Qcoil = 4.5 x CFM x h
Dry Bulb Temperature
EnteringAir Enthalpy
LeavingAir Enthalpy
h
Coil Load Calculation Example
Determine the minimum cooling coil load and requiredchilled water flow for a system serving an office space with asensible cooling load of 30,800 Btu/h and a latent load of8,500 Btu/h?
Indoor design conditions are 75oF/50% RH, and Outdoordesign conditions are 85oF DB/70oF WB.
The minimum ventilation air is 33% of the total supply airwhich is delivered at 55oF. The fan selected is a 70% efficient"draw-through" arrangement, and delivers air at 4.0 inchestotal pressure.
No manufacturer’s coil data is available. Assume 10°Fchilled water temperature rise through the coil.
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10Air Conditioning Design - Psychrometrics & Coil Load Calculation
Coil Load Example
CFM = 30,800 = 1400
1.10 (75 - 55)
30,800 = 0.80
SHR = 30,800 + 8,500
tfan = 0.363 x 4 = 1.95°F
0.7
Coil Load Example
• Plot Space Conditions of 75°F/50% RH
• Find the intersection of the SHR line withspace temperature and supply airtemperature
55°F
Dry Bulb Temperature
Room Air75°F / 50% RHSHR=0.70
SupplyAir
Temperature
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11Air Conditioning Design - Psychrometrics & Coil Load Calculation
Coil Load Example
• Plot Outdoor Conditions of 85°F/ 70°F WB
• Locate Mixed Air Condition (33% OA)
• Account for Fan t
55°F
Dry Bulb Temperature
Room Air Design75°F / 50% RHSHR=0.80
SupplyAir
Temperature
Outside Air85°F / 70°F WB
53°F
tfan
Mixed Air
Coil Load Example
• Determine enthalpy for coil entering andleaving conditions
Dry Bulb Temperature
Room Air Design75°F / 50% RH
Outside Air85°F / 70°F WB
Mixed Air
hLA = 21.5 Btu/lb
hEA = 29.8 Btu/lb
SHR=0.80
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12Air Conditioning Design - Psychrometrics & Coil Load Calculation
Coil Load Example
• qcoil = 4.5 x CFM x h
= 4.5 x 1400 x (29.8 - 21.5)
= 52,290 Btu/hr
Dry Bulb Temperature
Room Air Design75°F / 50% RH
Outside Air85°F / 70°F WB
Mixed Air
hLA = 21.5 Btu/lb
hEA = 29.8 Btu/lb
SHR=0.80
13
13Air Conditioning Design - Psychrometrics & Coil Load Calculation
Determine Chilled Water Flow
qtotal = mCpT
For water at = 62.34 lbm/ft3 and Cp = 1.0
qtotal = 500 x GPM x T
GPM= qcoil = 52,290
(500 x T) (500 x F)
= 10.5 GPM
Homework – Not Required
• Read 05F6.14-6.21
• What is the coil load for the following?– Indoor 76 oF/45% RH - Outdoor 82/66oF
– Load (Btu/h) 100,000 Sensible - 40,000 Latent
– Supply air 55oF through draw-through fan
– Fan Total Pressure = 3 in w.c.
– Fan Efficiency = 60%
– Outside air = 50% of total supply
• Show your work on the psychrometric chart