__Example 3. Summer
Cycle (Cooling Coil contact factor)__

An office is to be maintained
at 22^{o}C 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 28^{o}C, 80% saturation.

The outdoor air and recirculated air ratio is; 20% / 80%.

The Apparatus Dew Point ADP is 8^{o}C

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 8^{0}C.

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 17^{o}C.

__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:

m_{a} = Hs
/ ( Cp ( tr - ts ) )

where:

Hs = Sensible
heat gain to room (kW)

m_{a} = mass
flow rate of air (kg/s)

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

tr = room
temperature (^{o}C)

ts = supply air temperature (^{o}C)

The supply air
temperature is 17^{o}C.

m_{a} = 8
/ ( 1.01 ( 22 - 17 ) )

m_{a} = 1.584 kg/s

__Cooling Coil Output__

The
cooling coil output is as follows:

H _{cooling coil} = m_{a} ( h_{M} - h_{ADP})

where:

H _{cooling
coil} = Cooling coil output (kW)

m_{a} = mass flow rate of air (kg/s)

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

h_{ADP} = 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} = m_{a} ( h_{S}
- h_{W})

where:

H _{heater battery} = Heater
battery output (kW)

m_{a} = mass flow rate of air (kg/s)

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

h_{W} = 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**