Learjet 60 - Page 256

Learjet 60

Pilot's Manual
28 VDC through the 15-amp CREW FAN circuit breaker on the copilot's
circuit
breaker
panel .
R-134A COOUNG SYSTEM (Aircraft 60-174
and Subsequent)
The R-134A vapor cycle cooling system is installed for cockpit and
cab-
in cooling during ground operations, inflight cooling, and cabin dehu-
midification . On the ground, power must be supplied by an engine
generator, APU or ground power
unit . In flight, the air conditioning
system must be powered by both engine generators
.
When
the COOL-
OFF switch is set to COOL, power is supplied to the compressor motor
and the system refrigerant is compressed and circulated under high
pressure through a receiver/dehydrator
(dryer) to the cockpit and cab-
in evaporators . A cockpit blower, located below
the cockpit floor, and a
cabin blower, located in the aft cabin overhead, circulate air through
the
system evaporators to provide cooling . Also, pressurization bleed air is
used to provide airflow through
the cabin evaporator .
The system is protected against overpressure conditions
by
two
sepa-
rate safety devices . The first is a binary high /low pressure switch locat-
ed on the
compressor
discharge port . This switch will open at
approximately 350 prig and will
interrupt power to the compressor
control circuit . This in turn will de-energize
the compressor motor relay
andremove power to the compressor motor . The
system pressure
will
then drop . The switch will also interrupt power to the compressor con-
trol circuit under
low
pressure conditions . This low pressure switch
may shut down the compressor if the
average refrigerant temperature
between the cabin and tailcone is 35°F (1 .7°C)
or less . The second over-
pressure safety device is a fuse plug located on the receiver /dehydra-
tor bottle . This plug will vent the system refrigerant safely overboard
in the event of a system pressure in excess of 425 psig . The compressor
motor
is
automatically
cut
out during
engine start, STAB WING HEAT
operation, and inflight when only one
generator is operating . When the
aircraft is on external power, the compressor motor is powered by
28
VDC
suppliedthrough a 175-amp current limiter connected to the bat-
tery charging bus and a power contactor .
When the generators are op-
erating, the compressor motor is powered by
28
VDC
supplied through
two power contactors and two 175-amp current limiters connected
to
the generator buses .
A
fault isolator will remove power from the com-
pressor motor should a fault occur which
causes the compressor load
to become unequally shared between the
generators (except during
single generator operation on the ground) .
System control circuits, including the cabin blowers, are powered by
28
VDC supplied through
the 5-amp COOLCONTROL circuit breaker on
the pilot's circuit breaker panel . The
cabin blowers are powered by 28
VDC through a 50-amp current limiter .
Speed control circuits for the
PM-M
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