decrease in temperature. When the temperature reaches
35°F (0.2 in.Hg, absolute), dry air should be admitted
through a chemical dehydrator into the system at a point
farthest from the pump. Continue operating the pump so
the dry air will mix with and dilute any remaining
moisture. Secure the opening that feeds the dry air into
the system. Continue evacuating the system until the
indicator again shows a temperature of 35°F. The
dehydration process is complete. Close the valves and
disconnect the vacuum pump.
Sometimes obtaining a temperature as low as 35°F
in the vacuum indicator will be impossible. The
probable reasons for such a failure and the corrective
procedures to take are as follows:
l Excess moisture in the system. The dehydration
procedure should be conducted for longer
periods.
l Absorbed refrigerant in the lubricating oil
contained in the compressor crankcase. Remove
the lubricating oil from the crankcase before
proceeding with the dehydration process.
l Leakage of air into the system. The leak must be
found and stopped. You must then repeat the
procedure required for detecting leaks in the
system.
l Inefficient vacuum or defective vacuum
indicator. The defective unit(s) should be
repaired or replaced.
Immediately after each period of use or after the
system has been opened for repairs, replace the drying
agent in the dehydrator. If a replacement cartridge is not
available, reactivate the drying agent and use it until a
replacement is available.
You can reactivate the drying agent by removing
and heating it for 12 hours at a temperature of 300°F to
bake out the moisture. Place the drying agent in an oven
or circulate a stream of hot air through the cartridge.
Both methods are satisfactory for reactivating
commonly used dehydrating agents such as activated
alumina or silica gel. The specific instructions furnished
by the manufacturer should be followed to reactivate
special drying agents.
After reactivation, replace the drying agent in the
dehydrator shell and seal it as quickly as possible. This
prevents absorption of atmospheric moisture. When the
drying agent becomes fouled or saturated with
lubricating oil, replace it with a fresh charge, or
dehydrator cartridge, taken from a sealed container.
Remember that the dehydrators permanently
installed in refrigeration systems of naval ships are
designed to remove only the minute quantities of
moisture unavoidably introduced into the system. You
must be careful to prevent moisture or moisture-laden
air from entering the system.
CLEANING THE SYSTEM
Systems may accumulate dirt and scale as a result
of improper techniques used during repair or installation
of the system. If such dirt is excessive and a tank-type
cleaner is available, connect the cleaner to the
compessor suction strainer. When such a cleaner is not
available, a hard, wool felt filter about five-sixteenths
inch thick should be inserted into the suction strainer
screen. Run the plant with an operator in attendance for
at least 36 hours or until the system is clean. The length
of time required for a clean system depends upon the
size and condition of the plant.
AIR-CONDITIONING SYSTEM
Most of the information presented so far applies to
the refrigeration side of a system, whether it is used for
a refrigeration plant or for air conditioning. The
compressor controls for both types of systems are nearly
identical; however, the devices used to control space
temperatures differ, The two-position dual control,
called 2PD, is used for the automatic control of most
shipboard air-conditioning systems.
TWO-POSITION DUAL CONTROL (2PD)
This control is used on three types of systems:
Type 1.
Systems employing a simple thermo-
statically controlled single-pole switch
to control flow of refrigerant to the
cooling coil
Type 2.
Systems using reheaters, employing a
ther- mostatic element actuating two
interlocked switches
Type 3.
Systems using reheaters in the same
manner as those in type 2, with control
of humidity added where specified
The type 1 system, because of its simplicity, requires
little explanation. The thermostat consists of a
temperature-sensing element actuating a single-pole,
single-throw switch. It opens and closes a magnetic
valve to start and stop the flow of refrigerant-chilled
water or commercial refrigerant. This type of control is
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