Building Services Engineering 5th Edition Handbook

138 Ventilation and air conditioning
Latent heat gains result from exhaled and evaporated moisture from people, moisture given
out from industrial processes and humidifiers. These heat gains do not directly affect the temperature
of the surroundings but take the form of transfers of moisture. They can be measured
in weight of water vapour transferred or its latent heat equivalent in watts.
The latent heat of evaporation of water into air at a temperature of 20 ◦ C and a barometric
pressure of 1013.25 mb is 2453.61 kJ/kg. Thus the latent heat (LH) required to evaporate 60 g
of water in this air is:
LH = 60 g × 1kg
kJ
10 3 × 2453.61
g kg
= 147.22 kJ
If this evaporation takes place over, say, 1 h, the rate of latent heat transfer will be:
LH = 147.22 kJ
h × 1h
3600 s × 103 J
× Ws
kJ J
= 40.9 W
This is the moisture output from a sedentary adult.
Removal of sensible heat gains to control room air temperature is carried out by cooling the
ventilation supply air and increasing the air change rate to perhaps 20 changes/h. Figure 5.3
shows this scheme. The temperature and moisture content of the supply air increase as it absorbs
the sensible and latent heat gains until it reaches the desired room condition. The net sensible
heat flow will be into the room in summer and in the outward direction in winter.
Rooms that are isolated from exterior building surfaces have internal heat gains from people
and electrical equipment, producing a net heat gain throughout the year. The heat balance is
as follows:
net sensible heat flow into room = sensible heat absorbed by ventilation air
Therefore:
SH = air mass flow rate × specific heat capacity × temperature rise
Latent heat gain
(or loss)
Sensible heat gain
(or loss)
Supply air
Extract air
Q m 3 /s
t s °C
g s kg/kg
t r °C
g r kg/kg
Room
t r °C
g r kg/kg
5.3 Schematic representation of heating, cooling and humidity control of an air-conditioned room.