The Basic Refrigeration Cycle - Tecumseh AH5540E Service Handbook

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SERVICE HANDBOOK

The Basic Refrigeration Cycle

Mechanical refrigeration is accomplished by continuously circulating, evaporating, and condens-
ing a fixed supply of refrigerant in a closed system. Evaporation occurs at a low temperature and
low pressure while condensation occurs at a high temperature and high pressure. Thus, it is pos-
sible to transfer heat from an area of low temperature (i.e., refrigerator 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
evaporates, changing to a low pressure gas at the evaporator outlet (2).
The compressor (4) pumps this gas from the evaporator through the accumulator (3), increases
its pressure, and discharges the high pressure gas to the condenser (5). The accumulator is de-
signed to protect the compressor by preventing slugs of liquid refrigerant from passing directly
into the compressor. An accumulator should be included on all systems 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
preceding this device is a liquid line filter-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 thermostatic expansion valve
(TEV), by the amount of superheat of the suction gas. Thus, the TEV shown requires its sensing
bulb located at the evaporator outlet. The TEV allows the flow of refrigerant into the evaporator to
increase as the evaporator load increases.
As the high pressure liquid refrigerant enters the evaporator, it is subjected to a much lower pres-
sure due to the suction of the compressor and the pressure 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 crankcase 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: evapora-
tor pressure regulators, hot gas bypass regulators, electric solenoid valves, crank case pressure
regulators, condenser pressure regulators, 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.