TM 55-203
CHAPTER 12
AIR CONDITIONING
compressor into the refrigerating condenser, or heat
12-1. General
exchanger, where heat is transferred from the refrigerant
Three types of air-conditioning systems have been
to the cooling medium passing over the condenser coils.
developed for use on railway passenger equipment: the
The cooling medium is either outside air or a combination
ice-activated, the steam-ejector, and the mechanical-
of air and water. Thus, the refrigerant is condensed to a
compression types. In the ice-activated system, cooling
liquid state. The liquid refrigerant at high pressure flows
is accomplished by bringing the air to be cooled in contact
from the refrigerating condenser into the storage
with coils through which cold water is pumped.
reservoir, after which the cycle is repeated. Diagram of
Mechanical refrigeration is not used in this system In the
one kind of mechanical-compression system is shown in
steam-ejector system, water, the refrigerating system, is
evaporated rapidly in a vacuum to produce the cooling
effect. Unevaporated water which is cooled as a result of
12-3. Control Equipment
evaporation is circulated through the cooling coil. These
systems are largely obsolete, and only the mechanical-
a.
General.
Pneumatic-electric
temperature
compression type is discussed in this manual (fig 12-1).
Any military passenger equipment, such as ambulance
control is based on the use of compressed air and electric
cars, required to be air conditioned would be equipped
motors
operated
through
accurately
calibrated
with this type of air-conditioning system. The Surgeon
thermostats. The latter selects the proper coolness, heat,
General in the continental United States (CONUS) or the
and humidity required by inside and outside conditions.
theater surgeon in oversea areas would probably
Whether air conditioning depends upon a single
prescribe conditions when excessive heat and humidity
thermostat operating a damper motor or upon a more
require air conditioning of rail equipment.
elaborate system with compensated temperature and
humidity controls, the pneumatic-electric system can be
used to control it.
12-2. Mechanical-Compression System
Practically all the mechanical-compression systems now
b. Components.
The system consists of the
in use employ Freon (dichlorodifluoro-methane) as a
following parts:
refrigerant. The operation of the mechanical-compression
system is as follows: the refrigerant in a liquid state and at
(1) A main thermostat, subject to changes in
high pressure in a receiver or storage reservoir is forced
temperature inside the car.
by its pressure through an expansion valve. This valve
(2) An inside or humidity compensator which
controls the flow of the liquid-refrigerant from the high-
is exposed to the same temperature as the main
pressure receiver to the air-cooling coil or refrigerating
thermostat and an outside compensator which is exposed
evaporator where a lower pressure exists. Because of the
to the outdoor temperature in the air intake.
reduction in pressure, a portion of the refrigerant
(3) A parking thermostat to control
evaporates, thereby lowering the temperature. The low
temperatures when the car is in the yard.
temperature refrigerant in flowing through the air-cooling
(4) Damper and valve motors, controlled by
coil absorbs heat from the air passing over the coil. This
the main thermostat.
absorption of heat vaporizes the refrigerant.
The
(5) A closed system of piping connected with
refrigerant vapor flows to the suction side of the
the compressed-air system of the train, supplemented by
refrigerating compressor where it is compressed and its
a storage cell of compressed air or by an air pump on the
temperature increased.
The high-pressure, high-
underframe.
temperature refrigerant vapor is discharged from the
12-1
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