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TM 9-4110-257-14
1-14. REFRIGERATION SYSTEM.
a.
Cooling Cycle. Heat is absorbed from the refrigerator enclosure and released to the outside ambient air using
refrigerant flowing through the refrigeration system. The compressor moves the refrigerant through the refrigeration
system by raising the pressure of the incoming gas from the evaporator coil (evaporator) and discharging it as a high
pressure gas. The high pressure gas passes through the condenser coil (condenser) where heat is released to the
outside ambient air causing the high pressure gas to condense to a high pressure liquid. The high pressure liquid, which
may contain some gas, is collected in the receiver. The receiver contains a dip tube that allows only liquid to flow from it.
The high pressure liquid passes through the heat exchanger where it is cooled by low pressure gas returning to the
compressor. The high pressure liquid passes through the expansion valve (thermostatic expansion valve) which causes
a pressure drop and automatically meters the amount of liquid passing through it. The rapid drop in pressure causes the
liquid to cool. The cool, low pressure liquid passes through the evaporator coil where heat is absorbed from the
refrigerator enclosure air causing the low pressure liquid to evaporate to a low pressure gas. The low pressure gas
passes through the heat exchanger where it absorbs heat from the high pressure liquid going into the expansion valve.
The low pressure gas, which may contain some liquid, is collected in the accumulator (suction accumulator). The
accumulator separates the low pressure gas from any liquid and allows only gas to flow from it. The low pressure gas
returns to the compressor to begin the cycle again.
b.
Defrost Cycle. By lowering the temperature in the refrigerator enclosure to a temperature below freezing, any
moisture that accumulates on the evaporator coil will freeze and prevent proper air circulation through the coil. The
defrost cycle is automatically controlled by a temperature sensor and electronic timer in the electrical system. The
evaporator coil is defrosted when high pressure gas is sent from the compressor directly into the coil. To prevent water
from freezing in the evaporator coil drip pan, the warm high pressure gas tubing is routed through the drain hole in the
drip pan. The high pressure gas tubing then serpentines across the bottom of the drip pan before entering the coil. The
high pressure gas is then returned to the compressor to begin the cycle again. The defrost cycle will continue until either
the temperature sensor or electronic timer terminates it.
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