Air conditioning is the treatment of air to ensure
control of temperature, humidity, and dust (or foreign
particles) at levels most suitable to personal comfort. A
good example is the air-conditioning system used by
astronauts; their air-conditioning units must supply all
life-sustaining conditions to support their existence. In
this chapter, we examine the basic principles of
refrigeration, system components, troubleshooting, and
the repair of these systems. Furthermore, in closing, the
changes to automotive air systems and how they may
affect you as a mechanic are also examined.
PRINCIPLES OF REFRIGERATION
Refrigeration is the process of producing low
temperatures. It is usually associated with refrigerators
or freezers rather than with vehicles. An understanding
of heat transfer, basic refrigeration, pressure-
temperature relationship, and the qualities of
refrigerants is essential for a working knowledge of the
It may seem a bit silly to cover heat transfer in
connection with air conditioning. Keep in mind,
however, that heat, like light, is a form of energy. As you
remove light, a room grows darker. Likewise, when you
remove heat, an area becomes colder. The process of
transferring heat is the basis for air conditioning.
Generally, when two objects of different temperatures
are close to each other, heat energy will leave the warmer
object and travel to the cooler. This is quite clearly
illustrated in North America each fall and winter. As the
rays of the sun become less direct and consequently give
off less heat, we experience a drop in temperature.
Cooler weather (refrigeration) results from this removal
Refrigeration applies a physical principle that is
known to most of us through our everyday experiences.
We have experienced the application of rubbing alcohol
and its cooling effect. This example illustrates that an
evaporating liquid absorbs heat. The evaporating
moisture in the air on a hot day soaks up heat like a
sponge. This removal of heat is exactly the same process
used in automotive air conditioning. Heat is removed
from the vehicle by an evaporating refrigerant and
transferred into the atmosphere.
Different liquids have different boiling evaporating
points; however, the boiling pint of any liquid increases
when pressure is increased. When pressure is decreased,
the boiling point is then decreased. This process of
removing the pressure and allowing the coolant to boil
is a vital part of any refrigeration system.
With the exception of changes in state, gases used
in refrigeration are recycled much like engine coolant.
Different pressures and temperatures cause the gas to
change state from liquid togas and back to a liquid again.
The boiling point of the refrigerant changes with system
pressure. High pressure raises the boiling point and low
pressure reduces it. These gases also provide good heat
transfer qualities and do not deteriorate system
components. Two gases commonly used in the
refrigeration process are Refrigerant-12 and
Refrigerant-22. Use extreme caution when handling
them. Refrigerant-12, otherwise known as R-12,
Freon-12, or F-12, boils at 21.7°F (29.8°C) when at
sea level. Because of this low boiling point and its ability
to pass through the system endlessly, R- 12 is almost the
ideal refrigerant. (R-12 is currently being replaced by a
refrigerant that is less harmful to our environment).
When you are working with R-22, keep in
mind that this refrigerant contains methyl
alcohol which can be a fire hazard. For this
reason, automotive air-conditioning systems
R-12 is classified as a safe refrigerant because it is
nonexplosive, nonflammable, and noncorrosive;
however, you must observe certain precautions when