Civil Engineering Reference
In-Depth Information
Fig. 12.3
Reverse Carnot vapor refrigeration cycle
top and bottom limits of the reverse Carnot cycle, as shown in Fig.
12.3
. Of course,
if the
T
H
or the
T
C
temperature varies, the coefficient of performance also varies.
The maximum theoretical coefficient of performance for a refrigeration
cycle operating between a cold region at a temperature
T
C
(K) and a
warm region at higher temperature
T
H
(K) (the reverse Carnot cycle as
shown in Fig.
12.3
)is
T
C
T
H
T
C
COP
max
¼
The main differences between the reverse Carnot cycle and any other cycles may
be summarized as follows:
• To achieve a significant rate of heat transfer, the difference between the refrig-
erant temperature in the evaporator and the cold-region temperature
T
C
must be
equal to 5-10
C (9-18
F). Also, the temperature of the refrigerant in the
condenser must be several degrees above the
T
H
temperature. Consequently,
the coefficient of performance is lower than the reverse Carnot coefficient.
• The reverse Carnot cycle has a compression process with a two-phase liquid-
vapor mixture or wet compression. In practice this is avoided because liquid
droplets can damage the compressor, so dry compression with only saturated or
superheated vapor is required. In consequence, the coefficient of performance is
even further reduced below the reverse Carnot coefficient.
• The expansion process, which in a reverse Carnot cycle occurs in a turbine
performing mechanical work, takes place in a throttling valve with a saving in
capital and maintenance costs, but with a further reduction of the performance
coefficient.
