Example 1

Example 1.
SUMMER CYCLE
A room is to be maintained at 22oC dry-bulb temperature, 50% saturation, when
the sensible heat gain is 10.8 kW in summer. The latent heat gain is 7.2 kW.
Determine the cooling coil and reheater outputs required by using a psychometric
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 Neglect
the cooling coil contact factor.
solution
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.
1
5. finding point m :
VoTo+VrTr
Tm ¿ Vs
Tm=0.2*28+0.8*22
Tm=23.2 C
6. find RRL
Room ratio = Sensible / Total heat

10.8
RRL= 7.2+ 10.8 =0.6
2
10.
find Vs
Qs
Vs= 1.22 ( Te−Ts )
=10.8/(1.22*(22-15)=1.264 m3/s
11. find cooling load and reheater load
Qcc=1.2 Vs(hm-hADP)
=1.2*1.64*(50-25)
= 37.93 KW
Qrh=1.2 Vs(hs-hADP)
=1.2*1.64*(31.5-25)
= 9.932 KW
3
Example 2. 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
Cooling Coil contact factor is 0.8
4
solution
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 point m :
VoT 0+VrTr
Tm ¿ Vs
Tm=0.2*28+0.8*22
Tm=23.2 C
6. find RRL
Room ratio = Sensible / Total heat

8
RRL= 2+ 8 =0.8
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

hm−hw
C.F= hm−hADP
0.8 =(hm-hw)/(50.5-25)
5
hw=30.1 kJ/kg
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
11. find Vs
Qs
Vs= 1.22 ( Te−Ts )
=10.8/(1.22*(22-17)=1.311 m3/s
12. find cooling load and reheater load
Qcc=1.2 Vs(hm-hadp)
=1.2*1.311*(50-25)
= 39.34 KW
Qrh=1.2 Vs(hs-hw)
=1.2*11.311*(36.5-30.1)
= 10 KW
6
Example 3 SUMMER CYCLE (air flows to be
calculated)
A Lecture Theatre measures 15 m x 10 m x 6 m high.
It is to be air conditioned in summer so that the room is maintained at 22 oC

dry-bulb temperature, 50% saturation.
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 Apparatus Dew Point ADP is 7.5oC.
The latent heat gain is 10.0 kW.
The sensible heat gain is 12.0 kW.
Maximum occupancy is 200 people.
The recommended outdoor air rate is 8 l/s/person for non-smoking
The recommended total air supply rate is 6 – 10 air changes per hour
The cooling coil contact factor is unknown at present and should be
calculated.
Determine air flow rates and calculate the mass flow rate of fresh air and
supply air to the room.
The room is a non-smoking area.
7
Solution
Fresh Air Flow Rate
Fresh air rate = 8 l/s/p x 200 people = 1600 l/s = 1.6 m3/s
The specific volume at the outside condition may be determined from a
psychrometric chart. It is 0.88 m3/kg.
Mass flow rate = Volume flow rate / specific volume
Mass flow rate (Fresh Air) = 1.6 / 0.88 = 1.82 kg/s .
Supply Air Flow Rate
If the maximum ventilation supply air rate is taken from table B2.3 to be
10.0 air changes per hour, then the mass flow rate can be calculated.
Volume flow rate (m3/h) = Volume of room (m3) x air change rate
(ac/h)
Volume of room (m3) = 15 x 10 x 6 = 900 m3
Volume flow rate (m3/h) = 900 (m3) x 10 (ac/h)
Volume flow rate (m3/h) = 9000 m3/h
Volume flow rate (m3/s) = 9000 / 3600 = 2.5 m3/s.
Mass flow rate = Volume flow rate / specific volume
The specific volume at the supply condition may be approximated at this
stage from a psychrometric chart. It is 0.834 m 3/kg.
Mass flow rate (Supply Air) = 2.5 / 0.834 = 3.0 kg/s.
Fresh Air And Recirc. Ratio.
8
The ratio by mass is therefore;
Fresh air rate = 1.82 kg/s

Supply air rate = 3.00 kg/s
Recirculation air rate = 3.00 - 1.82 = 1.18 kg/s
The ratio of fresh air to total supply air is;
1.82 / 3.00 = 0.6, i.e. 60% fresh air and therefore 40% recirculated air.
Qs=1.22Vs(Tr-Ts)
Ts=22-(12/1.22)
=18 C
The processes can now be plotted on a psychrometric chart as shown

below
1. Points O, M and R can be shown on the chart.
2. Point ADP can be indicated and lines drawn between these points as
shown.
3. The room ratio line can be drawn.
4. Point S is then shown on the chart, on the room ratio line at 18 oC.
5. A horizontal line is then drawn from point S towards the line O – ADP
6. Point W can then be found where the horizontal line W - S intersects the
line O - ADP.
From the psychrometric chart point W is at approximately 9oC dry

bulb.
9
The heat ratio is 12 kW sensible / 22 kW total = 0.545.
7.
find cooling load and reheater load
Qcc=1.2 Vs(hm-hadp)
=1.2*2.5*(64-24)
= 120 KW
Qrh=1.2 Vs(hs-hw)
=1.2*2.5*(36-26.5)
= 28.5 KW
10

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Example 1.

Procedure (Summer Cycle)

2. Plot room condition R on psychrometric chart.

3. Plot outside condition O on psychrometric chart.

4. Join points O and R.

Room ratio = Sensible / Total heat

11. find cooling load and reheater load

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

Cooling Coil contact factor is 0.8

Procedure (Summer Cycle)

2. Plot room condition R on psychrometric chart.

3. Plot outside condition O on psychrometric chart.

4. Join points O and R.

Room ratio = Sensible / Total heat

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

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

12. find cooling load and reheater load

It is to be air conditioned in summer so that the room is maintained at 22 oC

Determine the cooling coil and reheater outputs required by using a

The recommended outdoor air rate is 8 l/s/person for non-smoking

Fresh Air Flow Rate

Supply Air Flow Rate

Fresh Air And Recirc. Ratio.

Fresh air rate = 1.82 kg/s

The ratio of fresh air to total supply air is;

The processes can now be plotted on a psychrometric chart as shown

1. Points O, M and R can be shown on the chart.

3. The room ratio line can be drawn.

From the psychrometric chart point W is at approximately 9oC dry

find cooling load and reheater load

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