Because of continual changes in the
environmental thermal loads as well as individual human comfort
preferences, air conditioners are fitted with means to control their
output. As noted above, the Thermal Expansion Valve already is part of
the control system in that it provides the opportunity for the
Evaporator to deliver its best performance. However, after the
temperature within the air conditioned space has reached a desired
level, the output of the air conditioner must be controlled to match the
"steady" thermal load. This is often achieved both by lowering the air
flow rate over the Evaporator and reducing the rate of refrigerant flow
within the a/c loop.
It is quite easy to reduce the flow of air across the Evaporator
simply by, for example, lowering the voltage to the electric fan motor
and, therefore, lowering the speed of the fan. On the other hand, it is
much more difficult to reduce the refrigerant flow rate to exactly match
a reduction in Evaporator air fan flow. Instead, in the case of a mobile
air conditioning system, the compressor speed (and, therefore, the
refrigerant flow rate) is controlled by the vehicle engine speed.
However, engine speed rarely bears any relationship whatever with the
cooling capacity requirement. For this reason, electromagnetic clutches
have been developed that independently cycle the compressor on and off
in response to the cooling demand.
For example, when the Evaporator fan speed is reduced, the immediate
effect is a reduction in the refrigerant pressure in the Evaporator
(because its boiling point is reduced). When this pressure drops below a
pre-determined (low) value due to reduced air flow, an electrical relay,
wired in series with the electromagnet of the clutch, is opened through
the action of the refrigerant-charged sensing bulb. This disengages the
Compressor and, therefore, it ceases rotation and stops the circulation
of refrigerant.
Because the Evaporator fans are still operating, the pressure in the
Evaporator (and sensing bulb) will begin to rise due to the fact that no
refrigerant is flowing but heat is still being added. When this pressure
exceeds a pre-set (high) value, the elevated sensing bulb pressure
causes the electrical relay to close, thus re-engaging the
electromagnetic clutch and restarting the Compressor. Due to the
"thermal inertia" of the evaporator, this on/off cycling process
delivers a relatively constant Evaporator air temperature and humidity.
