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.
THE COMPRESSOR AND
CONDENSER
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
THE EXPANSION VALVE
