need to understand
(1) why the refrigerant changes from liquid to vapor,
(2) why it changes from vapor to liquid, and
(3) what happens in terms of heat because of these changes of state. In this section, the refrigerant is traced through its entire cycle, beginning with the thermostatic expansion valve
( TXV Thermostatic Expansion Valve ).
Liquid refrigerant enters the TXV that separates the high side of the system and the low side of the system.
This valve regulates the amount of refrigerant that enters the cooling coil. Because of the pressure differential as the refrigerant passes through the TXV, some of the refrigerant flashes to a vapor.
From the TXV, the refrigerant passes into the cooling coil (or evaporator). The boiling point of the refrigerant under the low pressure in the evaporator is about 20°F lower than the temperature of the space in which the cooling coil is installed.
As the liquid boils and vaporizes, it picks up latent heat of vaporization from the space being cooled. The refrigerant continues to absorb latent heat of vaporization until all the liquid has been vaporized.
By the time the refrigerant leaves the cooling coil, it has not only absorbed this latent heat of vaporization. It has also picked up some additional heat; that is, the vapor has become superheated. As a rule, the amount of superheat is 4° to 12°F.
The refrigerant leaves the evaporator as low-pressure superheated vapor.
The remainder of the cycle is used to dispose of this heat and convert the refrigerant back into a liquid state so that it can again vaporize in the evaporator and absorb the heat again.
The low-pressure superheated vapor is drawn out of the evaporator by the compressor, which also keeps the refrigerant circulating through the system. In the compressor cylinders, the refrigerant is compressed from a low-pressure, low-temperature vapor to a high-pressure vapor, and its temperature rises accordingly.
The high-pressure R-12 vapor is discharged from the compressor into the condenser. Here the refrigerant condenses, giving up its superheat ( sensible heat Heat which causes a change in temperature of a substance. ) and its latent heat of condensation.
The condenser may be air or water cooled. The refrigerant, still at high pressure, is now a liquid again. From the condenser, the refrigerant flows into a receiver, which serves as a storage place for the liquid refrigerant in the system. From the receiver, the refrigerant goes to the TXV and the cycle begins again.
This type of refrigeration system has two pressure sides. The LOW-PRESSURE SIDE extends from the TXV up to and including the intake side of the compressor cylinders. The HIGH-PRESSURE SIDE extends from the discharge valve of the compressor to the TXV.
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