Building Services Engineering 5th Edition Handbook

Ventilation and air conditioning 127
Therefore,
QC r − QC s = P m 3 /s
Hence,
Q(C r − C s ) = P m 3 /s
and,
Q =
=
P
C r − C s
m 3 /s
4.7 × 10 −6
(0.5 − 0.03) × 10 −2 m3 /s
= 0.001 m3
s
= 1 l/s
× 103 l
1m 3
Other factors have a stronger influence on ventilation requirement:
1. bodily heat production, about 100 W per person during sedentary occupation;
2. moisture exhaled and evaporated from the skin, about 40 W per person for sedentary
occupation;
3. body odour;
4. fumes from smoking.
These factors greatly outweigh the CO 2 requirement. The recommended fresh air supply per
person is 10 l/s, which is increased by up to 50% in the event of expected heavy tobacco
smoke. Building Research Establishment Digest 206: 1977 gives design curves for open-plan
and small offices of room height 2.7 m and floor space per person 4.5 m 2 . A small office requires
2.25 air changes/h of outdoor air, and this will normally be provided by natural ventilation. A
single-person workstation occupies around 10 m 2 floor area when half of the walkway is included,
so an outdoor air supply of 10 l/s per person equates to 1 l/s m 2 floor area.
The ventilation system should not produce monotonous draughts but preferably variable air
speed and direction. Facilities for manual control of ventilation terminals allow sedentary workers
some freedom of choice over their environment. Careful location of ventilation grilles and control
of both the temperature and velocity of moving air in mechanical systems can ensure that neither
cool nor hot draughts are caused. The maximum air velocity that can be perceived at neck level
is related to its temperature. If the values given in Table 5.1 are not exceeded, then annoyance
should be avoided.
Table 5.1 DIN criteria for air movement at the neck.
Moving-air temperature (t ◦ ai C) 20 22 24 26
Maximum air speed (v m/s) 0.10 0.20 0.32 0.48
Source: Reproduced from the IHVE Guide (CIBSE, 1986 [IHVE, 1970]) by permission of the
Chartered Institution of Building Services Engineers.