Chevrolet Light Duty Truck 1973 Service Manual page 62

1A-36 HEATER A N D AIR C O N D IT IO N IN G
uncapped even for a few minutes allows the oil to absorb
moisture from the air.
System failures can result if contaminated oil is added to
the system.
Desiccant (Dehydrating Agent)
An ideal desiccant must have the following
characteristics:
1. High capacity.
2. High efficiency.
3. Low tendency to powder.
4. Absorb moisture without reacting chemically with
it.
5. Allow refrigerant to flow through it with minimum
restriction.
6. Retain moisture at high temperature.
While some desiccants excel in several of the desirable
characteristics, they are unsatisfactory in others.
Activated Silica Alumina is a most satisfactory desiccant;
however, its ability to retain moisture is affected by its
temperature. As the temperature increases, its ability
decreases. This means that moisture which is retained at
a lower temperature may be put back into the system at
a higher temperature.
PRIMARY CAUSES OF SYSTEM FAILURES
Leaks
A shortage of refrigerant causes oil to be trapped in the
evaporator. Oil may be lost with the refrigerant at point
of leakage. Both of these can cause compressor seizure.
Oil circulates in the globules with the vapor. It leaves the
compressor by the action of the pistons and mixes with
the refrigerant liquid in the condenser. The oil then
enters the evaporator with the liquid and, with the
evaporator properly flooded, is returned to the
compressor through the low pressure line. Some of the
oil returns as globules in the vapor but more
importantly, it is swept as a liquid along the walls of the
tubing by the velocity of the vapor. If the evaporator is
starved, the oil cannot return in sufficient quantities to
keep the compressor properly lubricated.
High Temperature and Pressure
An increase in temperature causes an increase in
pressure. This accelerates chemical instability in clean
systems. Other results are brittle hoses, "O " ring gaskets,
and by-pass valve diaphragms with possible decomposi­
tion, broken compressor discharge reeds, and seized
compressor bearings.
A fundamental law of nature accounts for the fact that
when a substance, such as a refrigerant, is increased in
temperature, its pressure is also increased.
Any chemical reactions caused by contaminants already
in the system are greatly accelerated as the temperature
increases. A 15° rise in temperature doubles the chemical
action.
While temperature alone can cause the synthetic rubber
parts to become brittle and possibly to decompose, the
increased pressure can cause them to rupture or blow.
As the temperature and pressure increases, the stress and
strain on the discharge reeds also increases. This can
result in broken reeds. Due to the effect of the
contaminants caused by high temperature and pressure,
compressor bearings can be caused to seize.
High temperature and pressure is also caused by air in
the system.
Air in the System
Air results from a discharged system or careless servicing
procedures. This reduces system capacity and efficiency
and causes oxidation of oil into gum and varnish.
When a leak causes the system to become discharged, the
resulting vacuum within the system will cause air to be
drawn in. Air in a system is a non-condensable gas and
will build up in the condenser as it would in an air
compressor tank. The resultant heat produced will
contribute to the conditions discussed previously.
Many systems are contaminated and also reduced in
capacity and efficiency by careless servicing procedures.
Too frequently, systems which have been open to the
atmosphere during service operations have not been
properly purged or evacuated. Air is also introduced into
the system by unpurged gauge and charging lines.
Remember that any air in the system is too much air.
Poor Connections
Hose clamp type fittings must be properly made. Hose
should be installed over the sealing flanges and with the
end of the hose at the stop flange. The hose should never
extend beyond the stop flange. Locate the clamp
properly and torque as recommended. Be especially
careful that the sealing flanges are not nicked or scored
or a future leak will result.
When compression fittings are used, over-tightening can
cause physical damage to the "O " ring gasket and will
result in leaks. The use of torque and backing wrenches
is highly recommended. When making a connection with
compression fittings, the gaskets should always be first
placed over the tube before inserting it in the
connection.
Another precaution - inspect the fitting for burrs which
can cut the "O " ring.
Restrictions
Restrictions may be due to powdered desiccant or dirt
and foreign matter. This may result in starved
evaporator and loss of cooling, high temperature at the
bypass hose, or a seized compressor.
When the amount of moisture in a system sufficiently
exceeds the capacity of the desiccant, it can break down
LIGHT DUTY TRUCK SERVICE MANUAL