Chevrolet Light Duty Truck 1973 Service Manual page 54

1A-28 HEATER A N D AIR C O N D IT IO N IN G
though they do contain quantities of heat. Consequently,
we can't expect to remove heat from sub-freezing vapors
by "cooling" them in air temperatures that usually range
between 60° and 100°...heat refuses to flow from a cold
object toward a warmer object.
But with a pump, we can squeeze the heat-laden vapor
into a smaller space. And, when we compress the vapor,
we also concentrate the heat it contains. In this way, we
can make the vapor hotter without adding any heat.
Then we can cool it in comparatively warm air.
That is the only responsibility of a compressor in an air
conditioning system. It is not intended to be a pump just
for circulating the refrigerant. Rather, its job is to exert
pressure for two reasons. Pressure makes the vapor hot
enough to cool off in warm air. At the same time, the
compressor raises the refrigerant's pressure above the
condensing point at the temperature of the surrounding
air so it will condense.
As the refrigerant leaves the compressor, it is still a
vapor although it is now quite hot and ready to give up
the heat that it absorbed in the evaporator. One of the
easiest ways to help refrigerant vapor discharge its heat
is to send it through a radiator-like component known as
a condenser.
The condenser really is a very simple device having no
moving parts. It does exactly the same job as the familiar
radiator in a typical home steam-heating system. There,
the steam is nothing more than water vapor. In passing
through the radiator, the steam gives up its heat and
condenses back into water.
The purpose of the condenser, as the name implies, is to
condense the high pressure, high temperature refrigerant
vapor discharged by the compressor into a high pressure
liquid refrigerant. This occurs when the high pressure,
high tem perature refrigerant is subjected to the
considerably cooler metal surfaces of the condenser. This
is due to the fundamental laws, covered earlier, which
state that "heat travels from the warmer to the cooler
surface," and that "when heat is removed from vapor,
liquid is produced."
When the refrigerant condenses into a liquid, it again is
ready for boiling in the evaporator. So, we run a pipe
from the condenser back to the evaporator.
MAIN UNITS OF THE SYSTEM
These three units then; the evaporator, the compressor,
and the condenser...are the main working parts in any
typical air conditioning system. We have the evaporator
where the refrigerant boils and changes into a vapor,
absorbing heat as it does so. We have the pump or
compressor to put pressure on the refrigerant so it can
get rid of its heat. And we have a condenser outside the
car body to help discharge the heat into the surrounding
air.
Now let's look at the compressor in detail, and some of
the components that work with these main units to
complete the air conditioning system.
Compressor
The prime purpose of the compressor (fig. 34) is to take
the low pressure refrigerant vapor produced by the
evaporator and compress it into a high pressure, high
temperature vapor which will be sent on to the
condenser.
It utilizes the principle that "when a vapor is
compressed, both its pressure and temperature are
raised" which we have already discussed. The compres­
sor is mounted above the engine in a special rubber
mounted bracket and is belt driven from the engine
through an electromagnetic clutch pulley on the
compressor.
The compressor has three double-acting pistons, making
it a six cylinder compressor. The compressor has a 1.5
inch bore and 1.1875 inch stroke, giving it a total
displacement of 12.6 cu. in. Identification of the
compressor is by model and serial num ber stamped on a
plate on top of the compressor.
Clutch-Pulley
The movable part of the clutch drive plate is in front of
the pulley and bearing assembly. The armature plate, the
movable member, is attached to the drive hub through
driver springs and is riveted to both members. The hub
of the drive plate is pressed over a square drive key
located in the compressor shaft. A spacer and retainer
ring are assembled to the shaft and the assembly is held
in place with a self-locking nut. The pulley rim, power
element ring and pulley hub are formed into a final
assembly by molding a frictional material between the
rim and the hub with the power element ring imbedded
in the forward face of the assembly.
A two-row ball bearing is pressed into the pulley hub
and held in place by a retainer ring. The entire pulley
and bearing assembly is then pressed over the front head
of the compressor and secured by a retainer ring.
Clutch Coil
The coil is molded into the coil housing with a filled
epoxy resin and must be replaced as a complete
assembly. Three protrusions on the rear of the housing
fit into alignment holes in the compressor front head. A
retainer ring secures the coil and housing in place. The
coil has 3.85 ohms resistance at 80°F. ambient
temperature and will require no more than 3.2 amperes
at 12 volts D.C. Since the clutch coil is not grounded
internally, a ground lead is required as well as a "hot"
lead.
Shaft Seal
The main shaft seal, located in the neck of the
compressor front head, consists of the seal assembly with
its ceramic seal face in a spring loaded cage. An "O "
ring seal, located within the ceramic seal, provides a seal
to the shaft surface. The contact surface of the shaft seal
seat is finished to a high polish and must be protected
against nicks, scratches and even fingerprints. Any
surface damage will cause a poor seal. An "O " ring,
LIGHT DUTY TRUCK SERVICE MANUAL