Building Services Engineering 5th Edition Handbook

154 Ventilation and air conditioning
Cooling tower on the roof
Outside air
cooling
27°C
30°C
Fresh air
inlet
27°C
12°C
–
Cooling coil
Fan
15°C
30°C
Air temperature
detector
Chilled-water
circuit
8°C 20°C
50°C
T EV
20°C 60°C
Evaporator
Valve controller
5°C
Supply air
Reciprocating
compressors
on an antivibration
base
in the basement
40°C
Condenser
cooling
water
20°C
Pump
Condenser
5.18 Refrigeration plant serving an air-conditioning system, showing typical fluid temperatures.
Figure 5.18 shows the installation of a chilled-water refrigeration plant serving one of the
cooling coils in an air-conditioning system in a large building. Each air-handling system has filters,
heaters, humidifiers and recirculation ducts appropriate to the design. There are several separate
air-handling plants, each serving its own zone. Zones are decided by the similarity of demand
for conditioning. The south-facing orientation has a cyclic requirement for cooling that is distinct
from that of the other sides of a building. Internal areas require cooling throughout the year.
These differing needs are often met by having separate zones for each area.
Absorption refrigeration cycle
An example of a two-drum absorption refrigeration cycle is shown in Fig. 5.19. The input heat
source may be gas, steam or hot water from a district heating scheme, or rejected heat from a
gas-turbine electricity-generating set.
The generator (1) contains a concentrated solution of lithium bromide salt in water. Pure water
is boiled off this solution and condenses on the cooling-water pipes, which are connected to an
external cooling tower. The generator drum pressure is sub-atmospheric at 0.07 bar, with boiling
and condensation taking place at 38 ◦ C. Water leaves the condenser (2), and then passes through
an expansion valve (3), where its pressure and temperature are lowered to 0.01 bar and 7 ◦ C. It
then completely evaporates while being sprayed over water pipes in the evaporator. These pipes
are the chilled-water circuit at 6–10 ◦ C, which supplies the refrigeration for the air-conditioning
cooling coils. Water vapour in the evaporator drum (4) is sucked into a weak lithium bromide
solution by the salt’s affinity for water. Latent heat given up by the water vapour as it condenses
into the solution is removed from the cooling tower by cooling-water pipes. The weak solution (5)