Civil Engineering Reference
In-Depth Information
CFCs include R406, R408, R409, R502 (azeotropic mixture of R22 and R115),
R11, R12, and R115.
HCFCs include R22 and other hydrochlorofluorocarbon gases.
HFCs include R134a, R144, and other hydrofluorocarbon compounds.
CFCs and HCFCs are harmful to the ozone layer in the upper atmosphere (they
are called ozone-depleting gases) and therefore their use in the refrigeration indus-
try has been banned in some countries.
HFCs, which are widely used as alternative coolants, are greenhouse gases
(GHGs) (see also Sect.
6.11
).
The main properties of coolants are reported in Table
12.1
. Typical fields of
application are R12 (domestic refrigeration, small plants with
T
c
>
20
C, auto-
motive air conditioning), R22 and R134a (food conservation, small- and medium-
size space air conditioning), R502 (low-temperature food conservation), ammonia
(low-temperature food conservation and freezing), and R11 (large air conditioning
plants).
12.4
The Main Components of Vapor-Compression Systems
As shown in Fig.
12.1
, the main components of a vapor-compression system are
evaporator, compressor, condenser, and expansion valve.
12.4.1 Evaporator
The evaporator is basically an exchanger where the refrigerant, as a liquid or a
liquid-vapor mixture at low temperature and pressure, evaporates by subtraction of
heat from the process or from the space to be cooled.
A liquid-vapor mixture exists when the evaporator is fed immediately down-
stream of the expansion valve.
Two basic types of evaporator are used in the refrigeration industry:
(1) the direct expansion type, in which the fluid evaporates inside tubes, suitable
for direct air cooling, and (2) the flooded type (pool boiling) where the fluid boils in
contact with tubes which contain the warmer liquid (as for the production of chilled
water).
Attention must be paid to the formation of ice around evaporators working in air
at a temperature below 0
C (32
F). The defrosting can be done by recirculating hot
gas or by inverting the cycle or by electric resistors. The energy consumption in this
phase must be taken into account when calculating the COP of the system.
12.4.2 Compressor
The compressor has the task of increasing the pressure of the refrigerant from the
evaporating value to the condensing value, to which corresponds the condensation
