Winnebago 1987-1989 LeSharo Service Manual page 604

EVACUATING THE SYSTEM
A system that has had the refrigerant removed to
perform repairs, or is excessively low on
refrigerant, must be evacuated with a vacuum
pump before new refrigerant is installed.
System must be evacuated to remove air and
moisture.
MOISTURE in any quantity (even a few drops of
water) is extremely harmful to the air
conditioning system and if not retained by the
desiccant in the receiver-drier will circulate with
the refrigerant. Moisture may collect and freeze in
the Thermostatic Expansion Valve orifice which
would block refrigerant flow and stop the cooling
action. Moisture will also react with
Refrigerant-12 to form hydrochloric acid which
will cause corrosion of the metal parts of the
system. Corrosion particles may become detached
and block the small passages and orifices in the
system. The desiccant in the receiver-drier can
absorb only a limited amount of moisture before it
becomes saturated and it is extremely important
to prevent moisture entering the system, and to
remove moisture which may have entered the
system through a leak or open connection.
Vacuum pump must be used for evacuation.
Unwanted air and moisture are removed from the
system by controlling the pressure; however,
instead of adding pressure, we remove pressure or
evacuate the system. A vacuum pump is the only
piece of equipment designed to lower the
pressure sufficiently so that the moisture boiling
temperature is reduced to a point where the
water will vaporize and can then be evacuated
from the system.
N3-5
Water boils at 212°F, at 14.7 psi (sea level). As the
vacuum pump lowers the pressure of the closed air
conditioning system, the boiling point of the
moisture in the system will also be lowered. In
evacuating the system, it is necessary to lower the
boiling point of any moisture in the system to a
point lower than the ambient temperature to
ensure that all moisture is boiled off. At an
ambient temperature of 75°F., when the desired
vacuum of 29.5 inches of Hg. is reached, water will
boil at approximately nOF and a complete boiling
off of all moisture in the system is assured when
this vacuum reading has been reached.
Altitude affects vacuum gauge readings.
At altitudes higher than sea level, it will not be
possible to obtain a vacuum reading of 29.5 inches
of Hg. on the low side compound gauge. For each
1000 feet of altitude, the vacuum gauge must be
corrected by 1 inch of Hg. to compensate for a
change in atmospheric pressure. For example, at
altitudes between 950 and 1,050 feet, a gauge
reading of 28.5 inches of Hg. will be the same as a
gauge reading of 29.5" of Hg. at sea level. When
this vacuum is reached, approximately 30 minutes
should be allowed in evacuating the system to
ensure complete moisture removal.
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