Cheyenne PA-31T CHEYENNE, Service Manual

Page: 946 / 1437

13-91. AIR CONDITIONING SYSTEM.
13-92. CABIN AIR CONDITIONING SYSTEM DESCRIPTION AND OPERATION. The air conditioner can
be operated independently or in conjunction with the heater through the use of an electronic controller
mounted in the lower right section of the instrument panel. The controller receives its signal from a
controllable mercury bulb thermostat. When the thermostat calls for cooling, the controller turns on the air
conditioner. Under all normal operations, the cabin thermostat will control the operation of the air conditioner
or heater.
The air conditioning system uses Freon 12 refrigerant which is drawn into an engine mounted, belt driven,
recirculating, axial type, six cylinder compressor. It is driven from a power takeoff on the rear of the right
engine at 3968 RPM compressor speed. The compression is set into operation through a magnetic clutch
mounted on the compressor. With the compressor in operation, Freon 12 is pumped under high pressure. The
Freon vapor is heated as a result of the compressor process. It then flows to the condenser which is a fin and
tubing heat exchanger mounted behind the right engine fire wall. The Freon vapor is cooled and condensed to a
liquid state by air entering through the lower right side of the nacelle. The air is drawn in and circulated by an
impeller fan. The liquid then flows through tubing to the receiver-dehydrator assembly which acts as a
reservoir and also functions as a filter to remove any trapped air or moisture that was in the system during the
initial charging. High pressure liquid Freon is supplied from the receiver to an expansion valve. This valve
meters the refrigerant into the evaporator core at a rate which allows the liquid refrigerant to evaporate. Heat
from the evaporator core surface is lost to the boiling and vaporizing Freon which is cooler than the core,
thereby cooling the core and the air passing through it. As this process is taking place, moisture (humidity) in
the air condenses on the outside surface of the evaporator core and is drained off as water. By the time the
refrigerant leaves the evaporator, it has completely vaporized. It then passes through the evaporator control
valve which is used to control the evaporator pressure. This is accomplished by restricting the outlet from the
evaporator so that the pressure within the evaporator is maintained at a predetermined valve to prevent the
freezing of atmospheric moisture on the evaporator. The refrigerant vapor then returns to the compressor where
the cycle is repeated. There is a overpressure switch located in the high pressure port of the freon compressor
and a thermal switch mounted on the condenser inlet line on various models. (Refer to Parts Catalog for serial
numbers.) The overpressure switch will engage the clutch at 375±10PSl and disengage the clutch at 200±10
PSl. The thermal switch will engage the clutch at 170°F and disengage the clutch at 150°F.
13-93. TROUBLESHOOTING. A table at the end of these instructions will assist in locating and correcting
malfunctions which may arise in this system.
13-94. MALFUNCTION DETECTION. The detection of system malfunction largely depends on the
mechanic’s ability to interpret the gauge pressure readings into system problems. A system operating normally
will have a low side gauge pressure reading that will correspond with the temperature of the refrigerant
evaporating in the evaporator, allowing for a few degrees temperature rise due to loss in the tube walls and fins.
The high side will have a gauge pressure that will correspond with the temperature of the refrigerant
condensing in the condenser, allowing for a few degrees temperature drop due to loss in the tube walls and fins.
Revised: 4/25/83
5D21
PIPER CHEYENNE SERVICE MANUAL
ENVIRONMENTAL CONTROL SYSTEM
PA-31T AND PA-31T1