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 TM 55-1905-219-14-8
4-28. AIR CONDITIONING SYSTEM - MAINTENANCE INSTRUCTIONS (Continued).
(c) Use only distilled water in the indicator. Be sure the wick is clean. Oil or dirt on the wick causes
erroneous readings. To prevent loss of oil from the vacuum pump and contamination of the indicator:
1 Install a shut-off valve in the suction line at the vacuum pump.
2 Install a shut-off valve in the suction line at the vacuum indicator.
3 When shutting off the pump, close the indicator valve, the pump valve and turn off the pump in
that sequence.
(3) Dehydration Procedure.
(a) Connect a tee to refrigerant liquid line charging valve. Install copper tubing from one branch of tee
to vacuum pump suction and from other branch to vacuum indicator. Provide shut-off valves in suction lines at
vacuum pump and at indicator.
(b) Open compressor stop valves and all line stop valves in system. Close all valves and connections
to atmosphere. Be sure to open hand expansion valves, coil return valves and any other line valves which will
connect the high and low sides of system and allow the pump to draw a vacuum on entire system. If
necessary, install a "jumper" line between the high and low sides of system.
(c) Open shut-off valve in pump suction line, start vacuum pump, and open charging valve.
(d) Open vacuum indicator shut-off valve occasionally and take a reading. Keep valve open at least
three minutes for each reading. (Keep valve closed at all other times to decrease amount of water pump must
handle and to hasten dehydration.) When the pressure in the system drops to a value corresponding to the
vapor pressure of the water in the indicator, the temperature will start to drop. In the example shown the
ambient temperature and the temperature of the water in the indicator is 60F (15.6 ). Starting at 60F
C
(15.6C) and zero time the temperature of the water in the indicator remains at 60 F (15.6C) until the
pressure in the system is pulled down to the pressure corresponding to the saturation temperature of the water
(60F (15.6C)). At this point the moisture in the system will start to boil. The temperature drops slowly until
the free moisture is removed (35F (1.7C)). Dehydration is nearly completed at this point, provided the
ambient temperature remains at 60F (15.6C) or higher. If the ambient temperature were lower than 60F
(15.6C), ice might form before moisture removal is complete.
4-868
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