Handbook of air conditioning and refrigeration / Shan K

11.4 CHAPTER ELEVEN
refrigerants. An additional refrigerant pump is required to circulate the refrigerant to each evaporator
at a flow rate several times greater than the actual evaporating rate. After the expansion valve,
the refrigerant is throttled to the evaporating pressure. Although liquid refrigerant directly expands
into vapor inside the evaporator’s tubes and accumulates in the low-pressure receiver, it is not a dryexpansion
evaporator. In a liquid overfeed system, a water-cooled condenser is often used.
Liquid overfeed refrigeration systems have the advantages of a higher heat-transfer coefficient
of the wetted inner surface and simpler refrigerant flow control. However, they are large and heavy
and are generally used for industrial refrigeration, food storage, and sometimes in ice storage
systems.
Multistage Reciprocating Refrigeration Systems. As described in Chap. 9, a multistage reciprocating
system could be a compound system or a cascade system. Multistage reciprocating refrigeration
systems are used for industrial applications to provide refrigeration at low temperature, such as
below �10°F.
Real Cycle of a Single-Stage Reciprocating Refrigeration System
In a reciprocating refrigeration system, the real refrigeration cycle differs from the ideal cycle
because of the pressure drop of the refrigerant when it flows through the pipelines, valves, compressor
passages, evaporator, and condenser. Also, the compression and expansion processes are not
isentropic.
Figure 11.4 shows the real cycle of a single-stage reciprocating vapor compression refrigeration
system on a p-h diagram. In a single-stage reciprocating refrigeration system, the pipeline
from the outlet of the evaporator to the inlet of the reciprocating compressor is called the suction
line. The suction line is shown by 1 s1 v in Figs. 11.1 and 11.4. The pipeline from the outlet of
the reciprocating compressor to the inlet of the air-cooled condenser is called the discharge line,
or hot-gas line. This is shown as 2 v2� in both figures. The pipeline from the outlet of the condenser
to the inlet of the thermostatic expansion valve is called the liquid line. This is shown as 3�3 x in
both figures.
In Fig. 11.4, the thick lines represent the real refrigeration cycle, and the thin lines the ideal
cycle. Various line segments indicate the following refrigeration processes:
FIGURE 11.4 Real reciprocating vapor compression refrigeration cycle on
p-h diagram.