WD Catalog 2018

865 / 1003

5-3 - -
2. Passive Air Conditioner Controls
Most automatic controls function in response to AC system temperature
(degrees Fahrenheit) or pressure (PSI) conditions. The thermostat is a
good example. It uses temperature to automatically cycle the compressor
clutch.
3. Compressor Clutch Control
The different types of compressor clutch control are described below.
Thermostatic Control
A thermostat in a basic air conditioning system controls the electri-
cal circuit to the clutch. Most thermostats have a capillary tube that
is inserted between the fins of the evaporator coil. The depth of tube
insertion and location in the coil are critical to system performance.
Thermostats are either fixed set or adjustable. A fixed set control
will operate in the 32 degrees to 38 degrees Fahrenheit range. An
adjustable thermostat functions between 32 degrees and 50 degrees
Fahrenheit and may be controlled with a knob or push-pull cable.
Figure 5-2 shows these types of thermostats.
The thermostats shown have sealed capillary tubes that sense tem-
perature. The capillary tubes contain a small amount of heat sensi-
tive gas or liquid like R-12. System temperature variations raise and
lower pressure in the sealed capillary tube. This pressure activates a
switch inside the thermostat to open and close the electrical circuit
to the clutch. These thermostats may sense temperature in the
evaporator coil or at the evaporator outlet.
Another type of user adjustable rotary thermostat has a bimetal
heat sensing strip and has been used to control the temperature in
sleeper cab AC or HVAC systems. The bimetal strip senses air
temperature. When the sleeper box air is too warm (in AC mode), the
bimetal thermostat electrical circuit activates a blower motor and
opens a solenoid valve. The valve allows refrigerant to flow to a
separate evaporator used to cool air circulating in the sleeper com-
partment. This type of system design is illustrated in Chapter 11
(Figure 11-7).
Air Conditioner Controls
CABLE CONTROLLED THERMO-
STAT VARIABLE TEMPERATURE
Figure 5-2
Two thermostats are shown.
They are used to cycle the
compressor clutch on and off
at selected high and low
temperatures.
FIXED SETTING THERMOSTAT,
PRESET TEMPERATURE