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Liquids vaporize more rapidly under a low
pressure. To follow this principle in a
refrigeration system, it is necessary to reduce
the pressure in the evaporator to less than the
liquid pressure in the supply line. This is
accomplished by placing a restricting device,
called an expansion valve, in the liquid line at
the evaporator.
The amount of restriction in the expansion
valve is varied automatically by a temperature
control tube.
If the temperature at the evaporator outlet
starts to rise, the substance in the temperature
tube expands. This places a pressure on a
diaphragm in the expansion valve which in turn
moves a needle away from its seat. This
action reduces the restriction and thus permits
more liquid refrigerant to flow into the
evaporator to increase the refrigerating action.
In order to maintain a flow of liquid refrigerant
to the evaporator, it is necessary to:
1. Pump the vapor from the evaporator 2.
Compress the vapor
3. Condense the vapor to a liquid
4. Return the liquid refrigerant to the
evaporator. See figure 4.
If the temperature at the evaporator outlet falls
below a predetermined value, the substance in
the temperature tube contracts. This reduces
the pressure
Fig. 4. The Refrigerant Cycle
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