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 TM 55-1905-220-14-8
4-29. 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 60 (15.6 ). Starting at 60
F
C
F
(15.6 ) and zero time the temperature of the water in the indicator remains at 60 (15.6 ) until the pressure
C
F
C
in the system is pulled down to the pressure corresponding to the saturation temperature of the water (60
F
(15,6 )). At this point the moisture in the system will start to boil. The temperature drops slowly until the free
C
moisture is removed (35 (1.7 )). Dehydration is nearly completed at this point, provided the ambient
F
C
temperature remains at 60 (15.6 ) or higher. If the ambient temperature were lower than 60 (15.6 ), ice
F
C
F
C
might form before moisture removal is complete.
4-898
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