WD Catalog 2018

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Chapter 5 – System Controls
5-4 - -
Pressure Switch Control
Most of the AC systems that have an expansion tube and accumula-
tor (instead of an expansion valve and receiver-drier) use a thermo-
stat to control clutch operation. However, there are some accumula-
tor systems with a pressure sensing switch instead of a thermostat.
The switch opens and closes an electrical circuit to the clutch. Figure
5-3 shows this type of automatic compressor clutch control.
HIGH PRESSURE
VAPOR
CONDENSER
HIGH PRESSURE LIQUID
ORIFICE
(EXPANSION)
TUBE
LOW PRESSURE
LIQUID
ACCUMULATOR
WITH PRESSURE SWITCH
LOW PRESSURE
VAPOR
CIRCUIT TO CLUTCH
COMPRESSOR CLUTCH
EVAPORATOR
When pressure (PSI) in the accumulator is high, the pressure sens-
ing switch closes an electrical circuit and the clutch is engaged. If the
pressure is low, the switch opens the circuit to disengage the clutch.
4. Refrigerant Flow Control
In Chapter 4 we described devices frequently used to control refrigerant
flow into and out of the evaporator. Most are controlled by pressure,
temperature or a combination of both inside the system. SomeAC systems
have two evaporators, one for the cab and the other for the sleeper
compartment. A freon solenoid valve may be used to split and direct part
of the refrigerant flow to a second evaporator. The solenoid valve is
electrically controlled and opens when the sleeper cab AC control is
operated. Figure 5-4 illustrates various refrigerant flow control devices
and explains what causes them to function.
Figure 5-3
The accumulator system
schematic uses a pressure
sensitive electrical switch to
control the compressor
clutch. The switch senses
refrigerant pressure (PSI)
inside the accumulator.