refrigerator and a vehicle air conditioning system have
a lot in common. In fact, the automotive air conditioning
system is basically a refrigerator. We don't call it
that, because most folks don't like the idea of riding
around in a mobile refrigerator, but technically, that's
what a car with the A/C running amounts to.
cooling cycle (or refrigeration cycle) takes advantage
of some inherent properties of matter. The first is
a basic heat transfer principle; Heat always flows from
areas of higher temperature to areas of lower temperature.
When you put your hand around a glass of ice, it feels
like the cold is flowing into your skin, but really,
heat is flowing from your skin to the glass faster than
your body can replace it.
systems also exploit changes of state. In order for
a liquid to change into a gas (boil), it must absorb
heat. Think for a minute about boiling a pot of water.
As you turn the flame up under the pot, a droplet of
water at the bottom of the pot absorbs enough heat to
change to steam (a gas). It flows up through the liquid
to escape into the air above.
normal atmospheric pressure boiling occurs at 212 degrees
F. As long as there is water in the pot, we can turn
the flame as high as we like, and the water (and the
pot's inside surface) will never rise above 212 degrees
F. Only when the water is gone does the pot burn, because
we've lost the cooling effect of the evaporating water.
an air conditioning system, we take advantage of the
same phenomenon by blowing air across a heat exchanger
(the evaporator) that has a pressurized liquid in it.
As the air passing over the evaporator coils gives up
heat to the cooler liquid inside, the liquid evaporates
(boils). Each drop of liquid that converts to a gas
absorbs a large amount of heat from the air flowing
across the outside of the heat exchanger. This cooled
air is conveyed into the passenger compartment of the
evaporated refrigerant, now a gas, flows into an accumulator,
which acts as a storage tank. The accumulator also separates
from the gaseous refrigerant any liquid fraction that
may still remain, and allows only the gas to go on to
the compressor inlet. The gas is drawn into the compressor,
which raises the pressure (and thus the temperature)
of the gas and pumps it through the system.
the compressor, the next stop for the hot, gaseous refrigerant
is the condenser, which is simply another heat exchanger.
In the condenser, the hot gas gives up its heat to the
cooler outside air flowing across the condenser tubes.
As the refrigerant cools at the high pressure, it condenses
again into a liquid.
it flows through a restriction of some kind (usually
an orifice tube), which lowers the liquid's pressure
before it returns to the evaporator to provide more
passenger compartment cooling.
that's the cycle. In the evaporator, refrigerant absorbs
heat from the air as the refrigerant changes state from
a liquid to a gas; the cooled air flows into the passenger
compartment. At the other end of the cycle, the gaseous
refrigerant gives up its latent heat to the outside
air as it changes state back into a liquid. The work
necessary to make this happen is provided by the vehicle's
engine, which drives the compressor (nothing is free)
by way of a drive belt and pulley assembly.
internal moving parts of the compressor are lubricated
by a special oil that dissolves into the refrigerant
and travels through the system with it. Different refrigerants
require different oils. Some newer system components
can withstand exposure to most of these oils, but many
use materials only compatible with one type or another.