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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

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2Air Conditioning Design - Psychrometrics & Coil Load Calculation

The Psychrometric Chart

Dry Bulb Temperature

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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

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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

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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

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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

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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

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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