TM 9-4120-360-14
2 Bypass Cycle. The compressor (1) operates continuously when the selector switch is set at
COOL. When actual cooling is not required the system goes into bypass operation to prevent build-up of excessive pressur-
es. Bypass operation is initiated when the temperature control thermostat causes the liquid line solenoid valve (8) to shut off
refrigerant flow to the evaporator coil (12), and as the suction pressure drops, hot gas bypasses thru the pressure regulating
valve (15). In the bypass configuration. vapor is piped from the discharge side of the compressor to the suction side. To
prevent the development of excessive heat by constant recompression, a second expansion valve is used in the system. This
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expansion valve (13) injects liquid refrigerant into the suction side of the bypass circuit to reduce, or quench the heat.
.(c) Heating Mode. When the selector switch is set at HI HEAT, six heating elements, located behind he
evaporator coil, are energized. These elements are protected from overheating by a thermal cutout witch, Three of the ele-
ments are controlled by the temperature control thermostat, while the remaining three are on at all times. The ventilating fan
also operates at all times. When the selector switch is set at LO HEAT, only the three thermostatically controlled elements
are energized.
(2) Temperature Control Thermostat. The temperature control thermostat is a rotary, continuously variable
control located in the middle of the control panel. It is marked DECREASE TEMP INCREASE above a semi-circular
double-ended pointer. The thermostat controls the degree of heating and cooling and should be set at a point at which alI
inhabitants of the area agree, or the ranking person decides, is comfortable.
(3) Two-speed Fan Switch. The two-speed fan switch is located at the left-hand end of the contro lpanel. It is a
two-position toggle switch, marked HI SPEED and LO SPEED. When set at LO SPEED the evaporator/condenser fan mo-
tor operates at 1725 rpm. At HI SPEED, the fan speed is increased to 3450 rpm. The switch may be set at either position, as
desired, for any mode of operation; however, if it is set at LO SPEED when the air conditioner is operating in the COOL
mode, an automatic pressure switch may over-ride the manual switch to operate the fans at the higher speed.
b, Airflow Controls. Airflow is controlled by the proper adjustment of louver blades in the evaporator intake and
discharge grilles, the fresh air damper, and when installed, the chemical-biological-radiological (CBR) filtering system.
(1) Evaporator Intake Grille. When the air conditioner is operating in any mode, and is recirculating room air
exclusively, the evaporator intake louvers should be in their fully open position. When either the fresh air damper is open or
the CBR filter is attached, the evaporator intake grille should be partially closed to compensate for the outside air being
introduced. (see figure 2-l).
(2) Evaporator Discharge Grille. The evaporator discharge grille, located at the top part of the air conditioner,
should always be open. The louvers are provided to control the direction of airflow, and should never be closed to the extent
that they would obstruct free passage of air. Two sets of louvers are incorporated in the grille: vertical and horizontal. The
vertical louvers should be individually adjusted to direct the conditioned air to one or both sides, as desired. The inner hori-
zontal louvers should be adjusted to direct the conditioned air upward or downward. (See figure 2-l).
NOTE
Cool air is denser than warm air, so it tends to sink downward; therefore, it is usually desirable to direct
cool air slightly upward and warm air slightly downward for maximum comfort and coverage.
(3) Fresh Air Damper. Fresh air is introduced through the rectangular screened oprning in the upper right-hand
corner of the rear surface of the air conditioner. A damper inside the screened opening controls the volume of air admitted.
The variable opening of the damper located to the left of the evaporator intake grille is controlled by means of a ball chain
and detent slot to retain its position. The evaporator intake grille should be closed about halfway to promote the introduction
of fresh air. (See figure 2-l).
NOTE
Under all but extreme weather conditions, it is desirable to introduce about 10 percent of fresh air into the
system. This will create a slight positive pressure, and will help to eliminate the musty odors associated
with stale air.
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c. Resetting Automatic Controls. Three automatic controls can stop the air conditioner, and can be manually reset.
They are the following:
2-4
