Civil Engineering Reference
In-Depth Information
If air is used, forced circulation of air must be employed to facilitate heat transfer
in refrigerant fluid/air exchangers (air condenser). This system, which requires
electric power for fans, has the disadvantage that working temperatures vary with
air temperature. If the temperatures are too high, excessively high condensation
temperatures may be required. As a result, the COP is reduced.
Practical values of energy consumption in air-condenser fans are
0.05-0.1 kWh/kWh of input energy, generally electric energy consumed
by the compressor prime mover.
12.4.4 Expansion Valve
The expansion valve is a typical reduction valve that lowers the pressure to a value
suitable for the downstream evaporator's operation. The pressure is controlled by
the liquid fluid level in flooded evaporators and by the superheated vapor tempera-
ture in direct expansion evaporators (thus ensuring roughly 5-8
C; 9-14.4
Fof
superheating temperature increase at the suction line).
12.5
Cascade and Multistage Systems
In order to obtain a good coefficient of performance with very low evaporating
pressure and relatively high condenser pressure, cascade and multistage cycles are
generally used.
A cascade cycle, in which two vapor-compression cycles are located in series, is
illustrated in Fig.
12.4
. A counterflow heat exchanger is situated between the two
cycles: the heat rejected during the condensation of the low-temperature cycle
(cycle A) is used to evaporate the refrigerant in the second cycle which works in
a higher temperature range. If necessary, two refrigerants with different saturation
pressure temperature relationships can be selected to ensure the best temperature
and pressure for the two temperature ranges: low evaporation temperature without
excessively low pressure for the first cycle, and relatively high condensation
temperature without excessively high condensation pressure for the second cycle.
Multistage compression with intercooling between the stages, achieved by
means of the refrigerant itself, can also be used if high temperature differences
between cold and warm regions are required.
12.6 Absorption Refrigerating Systems
Absorption refrigeration cycles requiring heat instead of mechanical energy as the
main input power can be used in particular applications and in trigeneration plants
(see Chap.
9
). The basic scheme is shown in Fig.
12.5
. Notice that this system must
always operate at temperature above 0
C (32
F).
