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 TM 55-1905-219-14-8
4-28.
AIR CONDITIONING SYSTEM-MAINTENANCE INSTRUCTIONS (Continued).
b.
Principles of Operation.
(1)
Principles of Refrigeration.
(a) Refrigeration is the process of extracting heat at one temperature level and discharging it
at a higher temperature level for the purpose of reducing and maintaining the temperature of an enclosed
space below that of the surrounding atmosphere.
(b) The refrigerant is the fluid which carries heat through the refrigeration cycle from the
evaporator where heat enters the refrigerant, to the condenser where the heat is discharged to the coolant. It
should be understood that heat will always flow from a warm body to a cool body, and the rate of transfer will
depend on the existing difference in temperature.
(c) The evaporation temperature, or boiling point, is that at which the refrigerant turns into a
gaseous state, and no matter at what temperature this occurs, in order to bring about the change, heat must be
added to the liquid. Conversely, the condensation temperature is that at which the gas returns to its liquid state
and, similarly, heat must be removed from it to obtain the change.
(d) Pressure also affects the boiling point. Thus, if the pressure is increased, the boiling
point is raised, and if the pressure is reduced, the boiling point is lowered. For example, refrigerant-12 boils at-
21F (-29.4 ) under normal atmospheric pressure, but if the pressure is increased to 117 psig (75.8 kPa), the
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boiling point (or condensing point) is raised to 100F (37.8 ).
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(e) By allowing the refrigerant to boil in the evaporator at a temperature sufficiently lower
than that of the surrounding medium, heat transfer from the surrounding medium to the refrigerant is
established. The vapor formed in the evaporator is then raised in pressure (by the compressor) until its new
boiling temperature exceeds the temperature of the condenser coolant; under these conditions heat transfer is
established from the refrigerant vapor to the condenser coolant with resultant condensation of the refrigerant.
When condensed, the high pressure liquid refrigerant is reduced in pressure and again allowed to boil in the
evaporator.
(f) The first column of the table gives the range of refrigerant-12 saturation temperatures
likely to occur in practice. The third column gives the saturation pressure expressed in pounds per square inch
gauge corresponding to each given temperature. These relationships hold true only when both liquid and gas
are present together, as is the case in the evaporator, condenser, and receiver. The gas will be in the
superheated state in all other parts of the system.
4-825
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