normal operation, the evaporator tubes become so cold
that moisture in the air condenses on the tubes and
drains off as water. This accounts for the puddle we
often see under recently parked cars in the summer,
especially in humid weather. But if refrigerant pressure
inside the evaporator should fall too low, the evaporator
fin temperature can drop below 32 degrees F, and the
condensation on the external surface of the evaporator's
fins will actually freeze. This, in turn, reduces heat
eliminate this problem, the A/C system must be controlled
to keep evaporator temperature above a certain level.
In many systems, the control scheme takes advantage
of the fact that refrigerant temperature and pressure
are linked. As pressure rises, so does temperature.
operation in most mobile A/C systems is controlled by
cycling a clutch on the compressor drive pulley on and
off. When evaprator temerature falls too low, the compressor
is cycled on, raising the pressure (and thus, the temperature).
When temperature rises to a satisfactory level, the
compressor is cycled off again. This process can repeat
itself many times each minute, but it happens automatically,
so we're rarely aware of it.
systems, instead of using a fixed orifice and cycling
the compressor on and off, use an expansion valve that
modulates the pressure drop across the valve to regulate
evaporator pressure. The principle is the same, though
the components used in the system are different.
A/C system's operation is also affected by the operation
of the vehicle's cooling fan, which affects the volume
and rate of air flow over the condenser, and by the
blower fan, which controls the flow of air over the
evaporator and into the vehicle's interior.
the blower fan at too low a speed, especially on humid
days, can lead to evaporator icing, and a loss of cabin
basic functional requirements for an A/C system refrigerant
are relatively straight-forward. It must condense (become
liquid) at temperatures significantly higher than the
outside air's when reasonable pressure is applied (so
that heat can be transfered out of the system, to the
outside air). It must evaporate readily at 32 degrees
F to 40 degrees F when the pressure is reduced (so that
air destined for the cabin can transfer heat into the
system). It must not corrode or otherwise harm aluminum,
steel, plastic, rubber, or the other materials from
which system components are normally made.
Beyond these, there are other practical requirements,
including that it not cause ozone depletion, that it
not be toxic to humans or animals in case it should
leak into the air flowing into the passenger compartment,
and that it be available at an economically acceptable
these latter characteristics don't actually affect it's
ability to provide cooling, they are the factors that
have driven refrigerant selection in the last half-decade.
With the exception of depleting the ozone, Freon, or
R-12 offered high performance in all categories. Of
course, causing huge holes in the ozone is no small
problem, so we now face the transition to R-134a, and
perhaps to other alternative refrigerants.
we aren't going to discuss the pros and cons of the
various refrigerants here, but the choice of refrigerant
does have some practical impact on the A/C system hardware.
The most notable and obvious is that all fittings must
be exclusive to each refrigerant. This is an EPA requirement,
and if a system is retrofitted from R-12 to another
refrigerant, every fitting in the system must also by
problem of cross-contamination, that is, getting one
refrigerant into the recycling and/or reclamation equipment
that's supposed to be dedicated to another refrigerant,
can cost the service provider much money and aggravation.
The unique fittings provide a physical reminder that
each refrigerant must be kept and handled separately
from the others.