Keith Products TR-134 System Service Manual - page 9
Products, INC.
TR-134 System Service Manual
March 6/02
Page 4
2. ELECTRICAL DRIVEN COMPRESSOR SYSTEMS
Electrically operated systems use the aircraft electrical system and are operable in
most normal flight modes. Air conditioning may be operated with ground external
power or the aircraft electrical system providing voltage to the proper buss.
The compressor and condenser are manufactured on an assembly pallet. In most
cases, the compressor drive and condenser fan drive are both driven by a
common, double shafted motor. The compressor is belt driven from the shortest
shaft while the condenser fan is attached directly to the longer motor shaft.
Condenser cooling air (ambient air) is drawn over the compressor and drive motor
to provide cooling airflow for those components prior to passing through the
condenser coil to remove heat from the system. Some electrically driven systems
have separate compressor drive and condenser fan motors. After passing through
the condenser coil, the air is exhausted to the outside. The compressor takes low
pressure refrigerant gas and compresses it to a higher pressure and temperature.
On the ground, the electrical system allows operation of the air conditioning system
from either aircraft generator or from an active GPU prior to engine start. Some
system safety features include electrical interlocking and load shedding. In flight,
the air conditioning system can be operated from the aircraft electrical system only
with aircraft generators on line. Loss of aircraft generator power will automatically
shed the air conditioning system electrical loads except for the minimal loads of the
evaporator fans.
The entire air conditioning refrigerant loop is protected against over pressure and
over temperature conditions by two separate safety devices. The first device is a
binary high/low pressure switch that activates in the event of an overpressure and
is on the compressor discharge port. This switch will open at approximately 350
PSIG and will interrupt power to the compressor control circuit. This in turn will de-
energize the compressor motor relay and remove power to the compressor motor.
The refrigerant system pressures will then drop. The switch will also interrupt
power to the compressor control circuit under low pressure conditions. The
second safety overpressure device is a fuse plug which will vent the system
refrigerant safely overboard in the event of a system pressure in excess of 425
psig. It is located on the receiver/drier.