GMC 1976 ZEO 6083 Maintenance Manual page 66

been properly purged or evacuated. Air is also intro-
duced into the system by unpurged gauge and charg-
ing 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-tight-
ening 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 suffi-
ciently exceeds the capacity of the desiccant, it can
break down the desiccant and cause it to powder .
The powder passes through the dehydrator screen
with the refrigerant liquid and is carried to the ex-
pansion 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 pre-
serving 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 abra-
sion or wedging, discharge and expansion valve fail-
ure, decomposition of refrigerant 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 important role servicemen play in
maintaining chemical stability.
The major cause of corrosion is moisture .
Moisture
AIR CONDITIONING SYSTEM 1- 49
Moisture is the greatest enemy of refrigerating
systems. Combined with metal, it produces oxide,
Iron Hydroxide, and Aluminum Hydroxide. Com
bined with R-12, it produces Carbonic acid, Hydro-
chloric acid, and Hydro-fluoric acid . Moisture can
also cause freeze-up of an expansion valve and pow-
dered desiccant.
Although high temperature and dirt are responsi-
ble for 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 da-
maging 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 re-
stricts 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 com-
pletely 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 important and that this sealed condi-
tion 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 re-
frigerant and oil.