psychrometric 2007

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An easy psychrometric chart

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  • 11Air Conditioning Design - Psychrometrics & Coil Load Calculation

    Air Conditioning DesignPsychrometrics & Coil Load Calculations

    Air Conditioning DesignKeith 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

  • 22Air Conditioning Design - Psychrometrics & Coil Load Calculation

    The Psychrometric Chart

    Dry Bulb Temperature

  • 33Air 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

  • 44Air 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

  • 55Air 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

    CoilEnteringAir

    Dry Bulb Temperature

    CoilLeavingAir

    Apparatus Dew Point

    Relationship of ADP to Supply Air andChilled Water temperatures

  • 66Air Conditioning Design - Psychrometrics & Coil Load Calculation

    Bypass FactorSome 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 = 10Coil Bypass

  • 77Air Conditioning Design - Psychrometrics & Coil Load Calculation

    Coil Selection

    Start With Space Conditions Find the intersection of the SHR

    with 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 personSchools 15 CFM per personBars 30 CFM per personSmoking Lounges 60 CFM per personCorridors 0.05 CFM/ft2

  • 88Air 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

  • 99Air Conditioning Design - Psychrometrics & Coil Load Calculation

    Determine Required Coil Capacity

    Coil Capacity RequirementQcoil = 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 manufacturers coil data is available. Assume 10Fchilled water temperature rise through the coil.

  • 10

    10Air Conditioning Design - Psychrometrics & Coil Load Calculation

    Coil Load ExampleCFM = 30,800 = 1400

    1.10 (75 - 55)30,800 = 0.80

    SHR = 30,800 + 8,500

    tfan = 0.363 x 4 = 1.95F0.7

    Coil Load Example

    Plot Space Conditions of 75F/50% RH Find the intersection of the SHR line with

    space temperature and supply airtemperature

    55F

    Dry Bulb Temperature

    Room Air75F / 50% RHSHR=0.70

    SupplyAir

    Temperature

  • 11

    11Air Conditioning Design - Psychrometrics & Coil Load Calculation

    Coil Load Example

    Plot Outdoor Conditions of 85F/ 70F WB Locate Mixed Air Condition (33% OA) Account for Fan t

    55F

    Dry Bulb Temperature

    Room Air Design75F / 50% RHSHR=0.80

    SupplyAir

    Temperature

    Outside Air85F / 70F WB

    53F

    tfan

    Mixed Air

    Coil Load Example

    Determine enthalpy for coil entering andleaving conditions

    Dry Bulb Temperature

    Room Air Design75F / 50% RH

    Outside Air85F / 70F WB

    Mixed Air

    hLA = 21.5 Btu/lb

    hEA = 29.8 Btu/lb

    SHR=0.80

  • 12

    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 Design75F / 50% RH

    Outside Air85F / 70F 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 = 1.0qtotal = 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

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