Building Services Engineering 5th Edition Handbook

Ventilation and air conditioning 139
where SH is the sensible heat. The specific heat capacity of humid air is 1.012 kJ/kgK. The volume
flow rate of air is normally used for duct design.
Q m3
s
= air mass flow rate kg s × m3
ρ kg
The density of air at 20 ◦ C d.b. and 1013.25 mb is ρ = 1.205 kg/m 3 .
The supply air temperature t s can have any value and as density is inversely proportional to
the absolute temperature, from the general gas laws,
SH kW = Q m3
s
× 1.205 kg (273 + 20)
×
m3 (273 + t s ) × (t r − t s ) K × 1.012 kJ
kgK × kW s
1kJ
For summer cooling, t r is greater than t s during a net heat gain. For winter heating, t r is less
than t s as there is a net heat loss, and so the temperature difference (t s − t r ) must be used in the
equation. It is more convenient to rewrite the equation in the form,
Q =
SH kW
× 273 + t s
m 3 /s
t r − t s 357
EXAMPLE 5.4
An office 20 m × 15 m × 3.2 m high has 30 occupants, 35 m 2 of windows, 25 (2 × 30) W
fluorescent tube light fittings, a photocopier with a power consumption of 1500 W and
conduction heat gains during summer amounting to 2 kW. Solar heat gains are 600 W/m 2
of window area. The sensible heat output from each person is 110 W. The room air temperature
is not to exceed 24 ◦ C when the supply air is at 13 ◦ C. Calculate the supply air
flow rate required and the room air change rate. State whether the answers are likely to
be acceptable.
The sensible heat gains are summarized in Table 5.2. Then:
Q = 29.3 273 + 13
× m 3 /s
24 − 13 357
= 2.13 m 3 /s
Table 5.2 Summary of sensible heat gains in
Example 5.4.
Source Quantity (W/m 2 × m 2 ) SH (W)
Windows 600 × 35 21 000
Occupants 110 × 30 3300
Lights 25 × 2 × 30 1500
Photocopier 1500
Conduction 2000
SH gain = 29 300
= 29.3 kW