WD Catalog 2018

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Chapter 5 – System Controls
5-10 - -
AMBIENT SWITCH
GROUND
COMPRESSOR
FUSE
HEATER
THERMAL LIMITER
CLUTCH COIL
SUPERHEAT SWITCH
POWER IN
There are a variety of other heat, air and pressure activated valves or
switches. These are shown in Figure 5-9 and described in the paragraphs
that follow.
LOW PRESSURE CUTOUT SWITCH–NORMALLY
OPEN, CLOSES AT 34 PSI, OPENS ON PRESSURE
DROP OF 10 PSI
HIGH PRESSURE CUTOUT SWITCH–NORMALLY CLOSED,
OPENS AT 350 PSI AND CLOSES WHEN PRESSURE
DROPS BELOW 250 PSI
HEAT SENSING SWITCH (CLAMPS TO TUBE) AS
HIGH TEMPERATURE SAFETY SWITCH, SET TO
OPEN AT 200° F
SOLENOID VAVLE–PROVIDES ELECTRICAL CONTROL
FOR OPERATING AIR CYLINDERS, AIR ACTUATED
SHUTTERS, AND AIR CONTROLLED CLUTCHING FANS.
THE VALVE CAN BE NORMALLY OPEN OR CLOSE
DEPENDING ON WHICH CONTROL PORT IS USED
A pressure relief valve may be installed on or near the compressor high
side or on the receiver-drier fitting. This valve “bleeds” excess refrigerant
pressure when malfunction or restrictions inside the system increase
pressure to dangerous levels.
A low pressure cutout switch may be installed in either the high or
low pressure side of the system. It is designed to sense low pressure due to
refrigerant loss or system restriction. It is wired into the clutch circuit,
and interrupts clutch operation to protect the compressor.
A low side low pressure switch is preferred, especially with axial
compressors having little or no oil storage capacity. A low side low pres-
sure switch is much more sensitive to problems such as a blocked expan-
sion valve or other situations involving a lack of refrigerant and oil
circulation.
High pressure cutout switches are frequently installed to protect
the compressor. They are positioned on the high side to sense pressure
inside the system, and are wired into the compressor clutch electrical
circuit. These switches complete the circuit to the clutch when “closed.”
They “open” when preset pressures are reached. Opening shuts the sys-
tem down by breaking the circuit to the clutch.
Figure 5-8
This drawing shows a
Frigidaire thermal limiter
and superheat switch sche-
matic. It works by inter-
rupting the circuit
(electrical power) to the
clutch and stops the com-
pressor.
Figure 5-9
Various types of heat, air
and pressure valves and
switches are illustrated.