Figure 10-11.-Exploded view of a typical evaporator pressure
regulating valve.
It is located on the suction side of the compressor and is
actuated by pressure changes in the suction line.
When the solenoid valves in the lines to the various
evaporators are closed, the flow of refrigerant to the
evaporators is stopped. This action causes the pressure
of the vapor in the compressor suction line to drop
quickly. When the suction pressure has dropped to the
desired pressure, the low-pressure cutout switch stops
the compressor motor. When the temperature in the
refrigerated spaces rises enough to operate one or more
of the solenoid valves, refrigerant is again admitted to
the cooling coils. This causes the compressor suction
pressure to buildup again. At the desired pressure, the
low-pressure cutout switch closes, starting the
compressor, and the cycle is repeated again.
High-Pressure Cutout Switch
A high-pressure cutout switch is connected to the
compressor discharge line to protect the high-pressure
side of the system against excessive pressures. The
design of this switch is essentially the same as that of
the low-pressure cutout switch. However, the
low-pressure cutout switch is made to CLOSE when the
suction pressure reaches its upper normal limit, while
the high-pressure cutout switch is made to OPEN when
the discharge pressure is too high. As you already have
learned, the low-pressure cutout switch is the
compressor control for the normal operation of the plant.
On the other hand, the high-pressure cutout switch is a
safety device only. It does not have control of the
compressor under normal conditions.
Water Failure Switch
A water failure switch stops the compressor if there
is a circulating water supply failure. The water failure
switch is a pressure-actuated switch. Its operation is
similar to the low- and high-pressure cutout switches
previously described. If the water failure cutout switch
fails to function, the refrigerant pressure in the
condenser quickly builds up to the point that the
high-pressure switch stops the compressor.
Strainer
Because of the solvent action of R-12, any particles
of grit, scale, dirt, or metal that the system may contain
are circulated through the refrigerant lines. To avoid
damaging the compressor from foreign matter, a strainer
is installed in the compressor suction connection.
Water Regulating Valve
A water regulating valve controls the quantity of
circulating water flowing through the refrigerant
condenser. The water regulating valve is actuated by the
refrigerant pressure in the compressor discharge line.
This pressure acts upon a diaphragm (or, in some valves,
a bellows arrangement) that transmits motion to the
valve stem.
The primary function of the water regulating valve
is to maintain a constant refrigerant condensing
pressure. Basically, the following two variable
conditions exist:
1. The amount of refrigerant to be condensed
2. Changing water temperatures
The valve maintains a constant refrigerant
condensing pressure by controlling the water flow
through the condenser. By sensing the refrigerant
pressure, the valve permits only enough water through
10-11
