Building Services Engineering 5th Edition Handbook

142 Ventilation and air conditioning
EXAMPLE 5.6
The people in the office in Example 5.4 each produce 40 W of latent heat. Find the supply
air moisture content to be maintained, given that the room air is to be at 50% saturation
and the corresponding moisture content g r is 0.008905 kg H 2 O/kg air.
W
LH = 30 people × 40
person × 1kW
10 3 = 1.2 kW
W
t s = 13 ◦ C Q = 2.13 m 3 /s
air mass flow rate = 2.13 m 3 /s × 1.205 kg/m 3 = 2.57 kg/s
Q =
We obtain,
LH
g r − g s
× 273 + t s
860 000 m3 /s
g r − g s =
Hence,
1.2 × (273 + 13)
2.13 × 860 000 kg H 2O/kg air
g s = 0.008905 − 0.000187 kg H 2 O/kg air
= 0.008718 kg H 2 O/kg air
Psychrometric cycles
Heating and cooling processes in air-conditioning equipment can be represented on the
psychrometric chart in the following manner.
Heating
Heating is performed with a low- or high-pressure hot-water, possibly steam, finned pipe heating
coil, electric resistance heater or fuel-fired heat exchanger as shown in Fig. 5.5. SE is the specific
enthalpy, total heat content, of the air, as read from the chart.
Cooling
Cooling is performed by passing chilled water, brine solution or refrigerant through a finned pipe
coil. When the coolant temperature is below the air dew-point, condensation occurs and the air
will be dehumidified. Figure 5.6 shows the two possible cycles.
Mixing
Mixing of two airstreams occurs when the fresh air intake joins the recirculated room air. The
quantity of each air stream is regulated by multi-leaf dampers operated by electric or pneumatic
motors under the direction of an automatic control system. Varying the intake of fresh