MODEL PCA090480WH
CHAPTER 2
SERVICE
Section 3: Refrigeration Basics and Service
March 2013
2-3
Proprietary/Confidential
Page 37
© 2013 Twist, Inc.
1. Theory
The unit employs a vapor-cycle refrigeration system using a scroll type
compressor and refrigerant.
A.
Basic cycle explanation
The following is a simplified description of the basic cycle starting at the
compressor:
The compressor receives low-pressure refrigerant vapor and the
compressing process increases both the pressure and the temperature.
The hot, high-pressure gas is forced from the compressor into the
condenser. As the high-pressure gas passes through the finned
condenser coil, it is cooled by outside air moved by motor-driven fans.
As the temperature of the refrigerant vapor reaches the saturation point
corresponding to the pressure in the condenser, the vapor condenses
into a liquid and flows through the thermostatic expansion valve, which
controls the feed of liquid refrigerant to the evaporator. It reduces the
pressure of the refrigerant to the evaporator by means of an orifice.
The reduction of pressure on the liquid refrigerant causes it to boil or
vaporize until the refrigerant is at the saturation point corresponding to
its pressure. As the low temperature refrigerant passes through the
evaporator coil, heat from the air being conditioned flows through the
walls of the evaporator tubing to the refrigerant, causing the boiling
action to continue until the refrigerant is completely vaporized into a
gas.
The expansion valve regulates the flow through the evaporator coil as
necessary to maintain a preset temperature difference (or superheat)
between the evaporating refrigerant and the vapor leaving the
evaporator. As the temperature of the gas leaving the evaporator coil
varies, the expansion valve power element bulb senses its temperature
and acts to modulate the feed through the expansion valve as required.
The refrigerant vapor leaving the evaporator coil travels through the
suction line to the compressor inlet. The refrigeration process starts
over again and continues as long as the compressor is in operation.
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