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 TM 55-1905-220-14-8
4-29. AIR CONDITIONING SYSTEM - MAINTENANCE INSTRUCTIONS (Continued).
(b) After pulldown, the capacity control system should be set at the control point. The control
point is the suction pressure at which the first step of unloading occurs. The control point will depend on the
lowest temperature load being handled by the compressor and, usually, it is 0 to 5 psi (O to 34.5 kPa) below the
normal operating suction pressure of a fully loaded system. The recommended control point is 36 psi (248.2
kPa).
NOTE
In practice, the capacity control should be set at a control point
which will maintain the refrigerated compartments or other
medium being cooled at the required temperatures while
making full use of the unloading feature. That is, the control
point should be high enough so that the compressor will be
fully unloaded before it is stopped on low pressure cut-out.
(c) The control point is set by operating the compressor at the suction pressure corresponding to
the control point and, then, slowly turning the adjusting stem clockwise until the first step of unloading takes
place. If the suction pressure is higher than the control point, reduce it to control point by slowly throttling the
compressor suction stop valve. If the suction pressure is below the control point, stop the compressor for
several minutes and allow suction pressure to rise above control point. Then, restart compressor and reduce
suction pressure to control point by throttling the suction stop.
(d) To check the control point adjustment, open the compressor suction stop valve and then
slowly close it. Observe suction pressure gauge reading and listen for the first cylinder to unload when the
suction pressure reaches the control point. Open the suction stop.
i. Thermal Expansion Valves.
(1) Operation of Thermal Expansion Valve.
(a) The thermal expansion valve meters the flow of liquid refrigerant to the evaporator in
response to changes in superheat of the refrigerant suction gas leaving the evaporator. A gas is said to be
superheated whenever its temperature is higher than the saturation temperature corresponding to its pressure.
The amount of superheat is the increase in temperature above the saturation temperature at the existing
pressure.
4-883
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