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Example 3. Summer Cycle (Cooling Coil contact factor)

An office is to be maintained at 22oC dry-bulb temperature, 50% saturation in summer.

The sensible heat gain is 8.0 kW.

The latent heat gain is 2.0 kW.

Determine the cooling coil and reheater outputs required by using a psychrometric chart if the plant schematic is as shown below.

DATA:
Outdoor condition is 28oC, 80% saturation.
The outdoor air and recirculated air ratio is; 20% / 80%.
The Apparatus Dew Point ADP is 8oC

The cooling coil contact factor is 0.8. (See explanation of contact factor in page 9)

 

Procedure (Summer Cycle)

1. Draw schematic diagram of air-conditioning plant (see above)

2. Plot room condition R on psychrometric chart.

3. Plot outside condition O on psychrometric chart.

4. Join points O and R.

5. Find the mix point M by measuring the length of the line O-R and multiply this by the mixing ratio.

On a full size CIBSE psychrometric chart this measures 85mm.

The ratio of recirculated air is 0.8. .. therefore; 85mm x 0.8 = 68mm

Measure down the O-R line from point O by 68mm.

This determines point M .

6. Find the room ratio. This is the sensible to total heat gain ratio.

Total heat = 8 kW sensible + 2 kW latent = 10 kW total.

Heat ratio = 8 / 10 = 0.8

Plot this ratio on the protractor, bottom segment, on the psychrometric chart and transfer this line onto the chart so that it passes through point R.

7. Plot the Apparatus Dew Point ADP of the cooling coil.

This is on the 100% saturation curve. The ADP is 80C.

8. Join points M and ADP.

9. Find the off-coil condition W by measuring the length of the line M-ADP and multiply this by the cooling coil contact factor..

On a full size CIBSE psychrometric chart this measures 75mm.

The cooling coil contact factor is 0.8. .. therefore; 75mm x 0.8 = 60mm.

Measure down along the line M-ADP by 60mm.

This determines point W.

10. Plot the supply air condition S.

The reheater process will be a horizontal line from point W to point S.

Point S is on the room ratio line.

The supply air temperature is 17oC.

 

Mass Flow Rate

When the supply air temperature has been found from the psychrometric chart then the mass flow rate of air can be calculated from the following formula:

ma = Hs / ( Cp ( tr - ts ) )

where:

Hs = Sensible heat gain to room (kW)

ma = mass flow rate of air (kg/s)

Cp = Specific heat capacity of humid air (approx.1.01 kJ/kg degC)

tr = room temperature (oC)

ts = supply air temperature (oC)

The supply air temperature is 17oC.

ma = 8 / ( 1.01 ( 22 - 17 ) )

ma = 1.584 kg/s

 

Cooling Coil Output

The cooling coil output is as follows:

H cooling coil = ma ( hM - hADP)

where:

H cooling coil = Cooling coil output (kW)

ma = mass flow rate of air (kg/s)

hM = specific enthalpy at condition M (kJ/kg) determined from psychrometric chart.

hADP = specific enthalpy at condition ADP (kJ/kg) determined from psychrometric chart

The specific enthalpies at points M and ADP are shown on the psychrometric Chart below.

H cooling coil = 1.584 ( 50.5 - 25)

H cooling coil = 40.4 kW

 

Heater Battery Output

The heater battery or reheater output is as follows:

H heater battery = ma ( hS - hW)

where:

H heater battery = Heater battery output (kW)

ma = mass flow rate of air (kg/s)

hS = specific enthalpy at condition S (kJ/kg) determined from psychrometric chart.

hW = specific enthalpy at condition W (kJ/kg) determined from psychrometric chart.

The specific enthalpies at points S and W are shown on the psychrometric Chart below.

H heater battery = 1.584 ( 36.5 - 30.5)

H heater battery = 9.5 kW


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