One area of the vehicle that has
undergone drastic changes in recent years is the air-conditioning (A/C)
system. Here is a look at A/C operation, as well as some diagnostic and
service guidelines.By Jeff Nelson
Contributing EditorTo diagnose and repair automotive
air-conditioning (A/C) systems, you must first understand the principles
of how air conditioning works and what provides that delightfully cool air
from the car's vents on a hot summer day. These principles are the basis
of all air-conditioning and refrigeration systems, from home refrigerators
and window A/C units to the largest industrial applications.With the rapidly changing technology of
today's automobiles, it's nice to know that these principles do not
change, no matter what refrigerant is used or how system controls may
change. This is because A/C systems work according to fundamental
principles of thermodynamics-laws that never change. Thermodynamics deals
with heat and its movement, and that's the principle behind air
conditioning.Mike Thames, chief operating officer (COO)
and part owner of Joe Hudson's Collision Centers, which operates 13 stores
in the Southeast, believes repairers need an aggressive approach to be
successful in the collision repair industry, and this includes the A/C
aspect of the business. "We have all the equipment necessary to
service R-12 and R-134a systems," he says. "For a body shop to
send out A/C work would be impractical and cause delays." Thames
would like to see more collision repair technicians trained in A/C
service, and such training begins with an understanding of A/C system
operation.Heat and Heat TransferAll matter contains heat, a form of
energy. Heat causes the molecules in matter to move: The more heat, the
greater the movement of the molecules. The form matter takes-solid, liquid
or vapor-is dependent on the amount of heat it contains. For example, when
enough heat is added to water, its state changes to vapor, or steam.Even cold matter contains some heat. This
is how a home heat pump is able to operate. Though the outside air feels
cold to us in winter, a heat pump can remove heat from this air and
release the heat into the home. In an A/C system, heat is removed from the
air entering the passenger compartment and released from the condenser in
front of the radiator, into the atmosphere. Heat always moves from a
warmer area to a cooler one.Evaporation and CondensationUnlike the coolant in a car's radiator,
boiling of the refrigerant in the A/C system is a good thing. In fact,
when the system is operating, refrigerant is constantly boiling in the
evaporator and condensing back to liquid in the condenser. This is
desirable because of an amazing thing that happens when liquid changes to
a vapor state: It absorbs a tremendous amount of heat. Conversely, when
vapor condenses into liquid, it gives off a great deal of heat.Pressure, Temperature and Changes of
StateIf any given liquid boils and condenses at
the same temperature, how can heat be removed from one location and then
disposed of in another? The answer is in the relationships between
pressure, temperature and changes of state.If the pressure acting on a liquid is
increased, the boiling point of the liquid is also increased, and if the
pressure acting on a liquid is lowered, the boiling point is likewise
lowered. For example, putting an automobile's engine cooling system under
pressure increases the coolant's boiling point substantially over its
boiling point at atmospheric pressure. Conversely, water boils at a lower
temperature on top of a mountain, where air pressure is lower.
Pressurizing a vapor also increases its temperature. This is an important
factor in the condensation stage at the condenser. The higher temperature
increases the differential between the vapor and the ambient air, aiding
condensation and heat transfer to the atmosphere.Two Sides to the StoryAn A/C system is divided into two sides:
the low pressure side and the high pressure side. The low pressure, cold
side works to remove unwanted heat from the passenger compartment, and the
high pressure, hot side releases this heat to the atmosphere.The high pressure side begins with the
compressor output, continues through the condenser and terminates at a
flow restriction. At the flow restriction, the low side begins. The low
side includes the evaporator and it continues up to the suction side of
the compressor.High side lines are generally smaller than
low side lines. The high pressure lines will be warm or hot to the touch,
while the low side lines will be cool and may collect frost or water
droplets on hot, humid days. Frost buildup not in proximity to the
evaporator may indicate a line restriction on the upstream side of the
frost.A/C System ComponentsBefore we trace the refrigerant flow
through the system, let's take a closer look at some typical components of
an A/C system. First on the list is refrigerant.There are currently two refrigerants used
in different A/C systems: R-12 and R-134a. R-12, or Freon, was used for
many years and was efficient and inexpensive, but due to environmental
concerns, it is being phased out. Vehicles manufactured before 1992 use
R-12. Vehicles made in 1995 and later use R-134a. During the transition
years, both of the systems were used.The two refrigerants are not
interchangeable. An identification label can be found on the compressor or
elsewhere under the hood. To prevent contamination of systems and service
equipment, the service valve design was changed for the R-134a systems.
R-134a system service valves are either quick-connect fittings or
metric-threaded. Most R-12 systems can be retrofitted for R-134a, so be on
the lookout for converted systems. If an R-12 system has sustained
significant collision damage or other component failure, it may be a good
candidate for retrofitting. R-12 is becoming prohibitively expensive as
available quantities dwindle.For a refrigerant to be efficient, it must
have a very low boiling point. The boiling point for R-12 at sea level is
-21.7
