Building Services Engineering 5th Edition Handbook

Ventilation and air conditioning 129
Exhaust air
Q EX
Extract fan
Q E
10% of
supply air leakage
Q L
Recirculated
air
Control
Room
Temperature
detector
Q R
Electrically
operated
flow control
valve
Q
Q F
Fresh air inlet Filter
Mixed air
5.1 Single-duct air-conditioning system.
+ – +
Cooling
coil
Heater
battery
Humidifier
Heater
battery
Supply air fan
The designers of new buildings bear a heavy responsibility for the future environmental
condition of the planet. A building that is a large user of primary energy, that is, fossil fuel,
to power its mechanical systems, usually air conditioning, is a charge on society for 50 or more
years. Such buildings are unlikely to have their mechanical heating, cooling, lifts, lighting and
computer power supply networks removed for reasons of economy by future tenants. Once the
pattern of energy use is set for a building by the architect and original client, it remains that way
until it is demolished.
The increasing need for comfortably habitable buildings in the temperate climate of the British
Isles since the 1960s led to demands for air conditioning. Warm weather data for the UK shows
that an external air temperature of 25 ◦ C d.b. is normally only exceeded during 1% of the summer
period June to September (CIBSE, 1986, figure A2.11). (Compare this to those areas of the world
within 45 ◦ C of latitude from the equator, where the outside air temperature exceeds 30 ◦ C d.b.;
people living and working in such areas would probably consider the use of air conditioning in
the UK a waste of energy resources.) These 4 months total 120 days and include weekends, a
public holiday and many people’s annual holidays from their places of work. Therefore, only one
day per summer is likely to have an outdoor air temperature that exceeds 25 ◦ C d.b. When a
long, hot summer is experienced in southern England, there can be several days which exceed
25 ◦ C d.b., but this may not be repeated for a few years. In the rest of the UK, lower outdoor air
temperatures are experienced most of the time. The UK has a basically maritime climate, with
the Atlantic Ocean, English Channel and North Sea providing humid air flows at air temperatures
that are modified by the evaporative cooling effect of the seas. Long, hot summers are produced
by strong winds from eastern Europe. These winds have travelled across the hot and dry land
mass of northern Europe and then been reduced in temperature due to the evaporation of water
from the North Sea. Such evaporation of sea water is aided by high wind speed. Sea water is
evaporated into the wind and a change of phase occurs in the water. In order to evaporate
water into moisture that is carried by the air stream, the water is boiled into steam, a change
of phase. This phase change can only take place with the removal of sensible heat from the air
and the water, as the latent heat of evaporation has to be provided, just as in a steam boiler.