safe level, the relief valve reseals and stops venting
MAGNETIC
BREAKAWAY
refrigerant.
COIL
PIN
HIGH- AND LOW-PRESSURE CUTOUTS.--
High- and low-pressure cutout switches are
incorporated in refrigeration units to control the
operation of the compressor. The controls prevent the
PLUNGER
discharge pressure of the compressor from building up
VALVE
beyond a reasonable limit and the suction pressure of
STEM
the compressor from falling below the safe limit.
The control unit, illustrated in figure 11-17,
contains controls for both the high- and low-pressure
VALVE
PISTON
gases. In some installations the high- and low-pressure
SEAT
controls are mounted in separate units; however, their
operation is essentially the same.
INLET
OUTLET
CURRENT LIMITERS.--Current limiters are
ASf11016
usually fuses or circuit breakers. They are used in
electrical circuits to prevent the current from going
Figure 11-16.--Solenoid valve.
beyond reasonable limits in the event of a malfunction,
such as shorted or grounded circuits. The current
FUSIBLE PLUG.--The fusible plug is provided
limiters open and interrupt the circuit current if their
to prevent excessive buildup of refrigerant pressure
rated current is exceeded for a short period of time. The
within the confined area of the equipment. The
exact amount of time the current limiter will carry the
refrigerants commonly used today act in accordance
excessive current depends upon the type and rating of
with normal liquid/gas laws; that is, increased
the current limiter and the amount of overload.
temperature causes increased pressure. Should the
DUCT AIR PRESSURE SWITCH.--The duct
air-conditioning equipment be in close proximity to a
air pressure switch is designed to shut down the en-
fire, a positive method of releasing vapor and liquid
tire unit if an overpressure condition occurs in the
must be provided to prevent the buildup of excessive
conditioned air discharge duct. The pickup tube for the
pressure within the system. The fusible plug is
commonly used for this purpose. The fusible plug is
usually threaded into the receiver tank. (See figure
11-8.) When excessive external heat is present, due to
fire or other similar causes, the plug melts, allowing the
refrigerant gas to escape from the system. The
reduction of gas pressure prevents possible rupture or
explosion of the equipment. Extreme caution must be
exercised if this situation occurs.
WARNING
When refrigerant R-22 comes in contact with a
flame, phosgene gas forms. Phosgene gas is a deadly
nerve gas. Whenever you work with this refrigerant,
the working space must be thoroughly ventilated.
HIGH-PRESSURE R E L I E F VA LV E . -- A
high-pressure relief valve that functions similar to a
fusible plug may be installed in the receiver. When the
pressure in the receiver increases above a preset value,
ASf11017
the relief valve opens, venting excess pressure. Unlike
Figure 11-17.--High- and low-pressure safety control cutouts.
the fusible plug, however, after the pressure reaches a
11-17
