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 TM 55-1905-220-14-8
4-29. AIR CONDITIONING SYSTEM - MAINTENANCE INSTRUCTIONS (Continued).
g. Compressor Lubrication.
(1) A typical compressor lubrication system is shown below.
(2) Oil Circulation in System.
(a) A certain amount of compressor oil will always circulate through the refrigerant system
because oil is miscible in refrigerant-12. To allow for oil circulation, systems requiring a large refrigerant
charge will require the addition of oil in excess of the normal compressor crankcase oil charge.
(b) When the system is first placed in operation, closely observe the oil level in the compressor
crankcase. Add oil whenever the oil level drops below normal (half-way up on bull's-eye sight glass). Allow
sufficient time for the system to balance after adding oil, since some of the circulating oil may return to the
compressor. Then, again check oil level. Generally, the addition of one quart of oil for each fifty pounds (22.7
kg) of refrigerant charge will be an adequate allowance for oil circulation.
(c) After adding oil, if the oil level in the crankcase still falls below normal, oil is not returning to
the compressor. The oil is probably being trapped in the cooling coils by an improperly adjusted thermal
expansion valve.
4-875
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