Building Services Engineering 5th Edition Handbook

152 Ventilation and air conditioning
Large air-cooling plant comprises a multi-cylinder in-line or V-formation compressor, a shell
and tube refrigerant to a water evaporator producing chilled water, and a shell and tube
refrigerant to a water condenser where the refrigerant vapour is condensed into liquid and
the heat given out is carried away by a water circuit to a cooling tower on the roof.
2. The centrifugal compressor is used in large chilled-water plants where the noise and vibration
produced by the reciprocating type would be unacceptable. A centrifugal impeller of small
diameter is driven through a step-up gearbox from a three-phase electric motor. The lack of
vibration and compactness of the very high-speed compressor makes siting the plant easier.
3. The screw compressor has two meshed gears, which compress the refrigerant in the spaces
between the helical screws. One gear is driven by an electric motor through a step-up
gearbox. The compressor operates at high speed and has very low noise and vibration levels.
The operation of a vapour-compression refrigeration plant is shown in Fig. 5.16.
Refrigerants commonly used are non-toxic fluids with high latent heat. Refrigeration plant
with a capacity of up to about 175 kW, and motor cars, uses refrigerant HFA134A (replaced
R12, which is CCl 2 F 2 ), which boils at −29.8 ◦ C in the atmosphere. In a typical system it will be
evaporated at 5 ◦ C under a pressure of 3.6 bar and condensed at 40 ◦ C at 9.6 bar. Larger plant
uses fluorinated hydrocarbon R22 (CHClF 2 ), which has a greater refrigerating effect per kilogram
but is more expensive.
The coefficient of performance (COP) is an expression of cycle efficiency and is found from
COP =
heat absorbed by refrigerant in the evaporator W
power consumption by the compressor W
The vapour-compression cycle can be represented on a pressure–enthalpy diagram for the refrigerant
as shown in Fig. 5.17. Referring to Figs 5.16 and 5.17, compression 1–2 raises the temperature
of the refrigerant dry superheated vapour from about 20 ◦ Cto60 ◦ C, where it can then be cooled
and condensed at a sufficiently high temperature to reject the excess heat from the building to
the hot external environment.
It condenses at 40 ◦ C and collects in the liquid receiver. This warm high-pressure liquid passes
through an uninsulated pipe so that it is subcooled to below its saturation temperature (about
20 ◦ C) at the expansion valve located alongside the evaporator.
Low-temperature
and pressure vapour
Suction temperature
sensing phial controls
valve opening
1 2
Electric-motordriven
compressor
Superheated
vapour
Evaporator
Condenser
Refrigerant
absorbs heat
from the
building
Low-temperature
liquid and some
vapour at low
pressure
4 3
Heat contained in
refrigerant is
rejected to the
atmosphere
Liquid receiver
High-pressure warm liquid
Thermostatic expansion valve
5.16 Vapour-compression refrigeration system.