Chevrolet Light Duty Truck 1973 Service Manual page 63

the desiccant and cause it to powder. The powder passes
through the dehydrator screen with the refrigerant liquid
and is carried to the expansion valve screen. While some
of it may pass through the valve screen into the
evaporator, it may quickly build up to cause a restriction.
Due to the fact that sufficient oil then cannot be returned
to the compressor, it may seize.
Dirt
Dirt, which is any foreign material, may come from
cleaner residues, cutting, machining, or preserving oils,
metal dust or chips, lint or dust, loose rust, soldering or
brazing fluxes, paint or loose oxide scale. These can also
cause seized bearings by abrasion or wedging, discharge
and expansion valve failure, decomposition of refriger­
ant and oil, or corrosion of metal parts.
Corrosion
Corrosion and its by-products can restrict valve and drier
screens, roughen bearing surfaces or hasten fatiguing of
discharge reeds. This can result in high temperature and
pressure, decomposition or leaks. In any event, this
means a damaged compressor.
From this, we can see the vicious circle that can be
produced in a refrigerating system to cause its failure.
Corrosion can be the indirect cause of leaks and leaks
can be the direct cause of corrosion. We can also see the
im portant role servicemen play in m aintaining chemical
stability.
The major cause of corrosion is moisture.
Moisture
Moisture is the greatest enemy of refrigerating systems.
Combined with metal, it produces oxide, Iron Hydroxide,
and Aluminum Hydroxide. Combined with R-12, it
produces Carbonic acid, Hydrochloric acid, and Hydro­
fluoric acid. Moisture can also cause freeze-up of an
expansion valve and powdered desiccant.
Although high temperature and dirt are responsible for
Fig. 40-System Contaminants
HEATER A ND AIR C O N D IT IO N IN G
many difficulties in refrigerating systems, in most
instances it is the presence of moisture in the system that
accelerates these conditions. It can be said, therefore,
that moisture is the greatest problem of all. The acids
that it produces, in combination with both the metals
and the refrigerant, causes dam aging corrosion. While
the corrosion may not form as rapidly with R-12 as with
some other refrigerants, the eventual formation is as
damaging.
If the operating pressure and temperature in the
evaporator is reduced to the freezing point, moisture in
the refrigerant can collect at the orifice of the expansion
valve and freeze. This temporarily restricts the flow of
liquid causing erratic cooling.
As previously mentioned, moisture in excess of the
desiccant's capacity can cause it to powder.
Points to Remember
That the inside of the refrigerant system is completely
sealed from the outside world. If that seal remains
broken at any point—the system will soon be damaged.
That complete and positive sealing of the entire system
is vitally im portant and that this sealed condition is
absolutely necessary to retain the chemicals and keep
them in a pure and proper condition.
That all parts of the refrigerant system are under
pressure at all times, whether operating or idle, and that
any leakage points are continuously losing refrigerant
and oil.
That the leakage of refrigerant can be so silent that the
complete charge may be lost without warning.
That refrigerant gas is heavier than air and will rapidly
drop to the floor as it flows from a point of leakage.
That the pressure in the system may momentarily
become as high as 480 lbs. per square inch.
That the total refrigerant charge circulates through the
entire system at least once each minute.
That the compressor is continually giving up some
lubricating oil to the circulating refrigerant and depends
upon oil in the returning refrigerant for continuous
replenishment. Any stoppage or major loss of refrigerant
will therefore damage the compressor.
That the extreme internal dryness of a properly
processed system is a truly desert condition, with the
drying material in the receiver or accumulator holding
tightly onto the tiny droplets of residual moisture.
That the attraction of the drying m aterial for moisture is
so powerful that if the receiver or accumulator is left
open, moisture will be drawn in from the outside air.
That water added to the refrigerant will start chemical
changes that can result in corrosion and eventual
breakdown of the chemicals in the system. Hydrochloric
acid is one result of an R-12 mixture with water.
That air in the refrigerant system may start reactions
that can cause malfunctions.
1A-37
LIGHT DUTY TRUCK SERVICE MANUAL