Building Services Engineering 5th Edition Handbook

140 Ventilation and air conditioning
Some engineers prefer to calculate the mass flow rate of air flow through the air-conditioned
space. This is easily found by multiplying the volume flow rate by the density of air at that
location, thus,
supply air mass flow rate = 2.13 m3
s
× 1.205 kg = 2.57 kg/s
m3 Air mass flow rate does not change with temperature, volume flow does.
Q = NV
3600
N = 3600 Q
V
3600 × 2.13
= air changes/h
20 × 15 × 3.2
= 8 air changes/h
Between 4 and 20 air changes/h are likely to create fresh air circulation through an office without
causing draughts and should be suitable for the application.
EXAMPLE 5.5
A room has a heat loss in winter of 32 kW and a supply air flow rate of 3.5 m 3 /s. The
room air temperature is to be maintained at 22 ◦ C. Calculate the supply air temperature to
be used.
For winter, the equation is:
Q =
SH × 273 + t s
t s − t r 357
This can be rearranged to find the supply air temperature t s required:
Q × 357(t s − t r ) = SH(273 + t s )
Q × 357t s − Q × 357t r = 273SH + SH × t s
Q × 357t s − SH × t s = 273SH + Q × 357t s
Thus, for winter:
t s (Q × 357 − SH) = 273SH + Q × 357t r
In this example:
t s =
t s = 273SH + 357Qt r ◦ C
357Q − SH
273 × 32 + 357 × 3.5 × 22
357 × 3.5 − 32
= 29.75 ◦ C