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Appendix
I.
The Basic Refrigeration Cycle
Mechanical refrigeration is accomplished by contin-
uously circulating, evaporating, and condensing a
fixed supply of refrigerant in a closed system. Evapo-
ration occurs at a low temperature and low pressure
while condensation occurs at a high temperature
and high pressure. Thus, it is possible to transfer
heat from an area of low temperature (i.e., refrigera-
tor cabinet) to an area of high temperature (i.e.,
kitchen).
Beginning the cycle at the evaporator inlet (1) the
low pressure liquid expands, absorbs heat, and evap-
orates, changing to a low pressure gas at the evapora-
tor outlet (2).
The compressor (4) pumps this gas from the evapo-
rator through the accumulator (3), increases its pres-
sure, and discharges the high pressure gas to the
condenser (5). The accumulator is designed to pro-
tect the compressor by preventing slugs of liquid
refrigerant from passing directly into the compres-
sor. An accumulator should be included on all sys-
tems subjected to varying load conditions or
frequent compressor cycling. In the condenser, heat
is removed from the gas, which then condenses and
becomes a high pressure liquid. In some systems,
this high pressure liquid drains from the condenser
into a liquid storage or receiver tank (6). On other
systems, both the receiver and the liquid line valve
(7) are omitted.
A heat exchanger (8) between the liquid line and the
suction line is also an optional item, which may or
may not be included in a given system design.
Between the condenser and the evaporator an
expansion device (10) is located. Immediately pre-
ceding this device is a liquid line strainer/drier (9),
which prevents plugging of the valve or tube by
retaining scale, dirt, and moisture. The flow of
refrigerant into the evaporator is controlled by the
pressure differential across the expansion device or,
in the case of a thermal expansion valve, by the
degree of superheat of the suction gas. Thus, the
thermal expansion valve shown requires a sensor
bulb located at the evaporator outlet. In any case,
the flow of refrigerant into the evaporator normally
increases as the evaporator load increases.
As the high pressure liquid refrigerant enters the
evaporator, it is subjected to a much lower pressure
due to the suction of the compressor and the pres-
sure drop across the expansion device. Thus, the
refrigerant tends to expand and evaporate. In order
to evaporate, the liquid must absorb heat from the
air passing over the evaporator.
Eventually, the desired air temperature is reached
and the thermostat or cold control (11) will break
the electrical circuit to the compressor motor and
stop the compressor.
As the temperature of the air through the evaporator
rises, the thermostat or cold control remakes the
electrical circuit. The compressor starts, and the
cycle continues.
In addition to the accumulator, a compressor crank-
case heater (12) is included on many systems. This
heater prevents accumulation of refrigerant in the
compressor crankcase during the non-operating
periods and prevents liquid slugging or oil pump-
out on startup.
Additional protection to the compressor and system
is afforded by a high and low pressure cutout (13).
This control is set to stop the compressor in the
event that the system pressures rise above or fall
below the design operating range.
Other controls not indicated on the basic cycle
which may be part of a system include: evaporator
pressure regulators, hot gas by-pass regulators, elec-
tric solenoid valves, suction pressure regulators, con-
denser pressure regulators, low side or high side float
refrigerant controllers, oil separators, etc.
It is extremely important to analyze completely
every system and understand the intended function
of each component before attempting to determine
the cause of a malfunction or failure.