Civil Engineering Reference
In-Depth Information
Evaporation temperature must be always higher than 3-5
C (37-41
F).
As a general indication, 2-1 kW of heat at temperatures between 80 and
120
C (176-250
F) or higher are required to produce 1 kW of refriger-
ation capacity. The coefficient of performance varies between 0.5 and
1, which is much lower than the corresponding coefficient for vapor-
compression cycles. Lower values correspond to low-temperature hot
water as heat source and higher ones to the direct use of fuel. The heat
power to be discarded by means of condensing equipment (cooling tower,
air condenser, etc.) is roughly 2.5 kW for a refrigeration capacity of 1 kW
(
Q
out
¼Q
in
+ heat source).
Higher COP up to 1.2 can be obtained in cycles using a direct fired system fed by
combustibles, with a two-stage absorption system.
Additional equipment is needed to improve the coefficient of performance, for
example heat exchangers that allow the strong solution entering the generator to be
preheated by the weak solution leaving it, and rectifiers to remove any traces of
water from the refrigerant before it enters the condenser.
In most cases absorption systems use recovering heat in trigeneration plants (see
also Chap.
9
).
12.7
Brayton Refrigeration Cycle
The Brayton refrigeration cycle is one of the gas refrigerating systems in which the
process involves no change in phase. The basic scheme of this cycle is represented
in Fig.
12.6
. A modified scheme incorporating an intermediate heat exchanger
between the fluids leaving the two basic exchangers is the form most often used.
Fig. 12.6
Basic scheme of a Brayton refrigeration cycle
