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temperature sensors enable and disable the blanket heaters. The heaters
will not activate unless the temperature falls below the established
low value and will deactivate when the temperature reaches the established
high value. The heaters are self-regulating whereby power consumption
is reduced as temperature increases.
The baggage compartment heaters are powered by 28-vdc through the
R NON-ESS bus. A baggage heat circuit breaker is located on the aft left
power distribution panel within the tailcone.
Pilot’s Manual
PM-126A 6-13
ENVIRONMENTAL CONTROL SYSTEM (ECS)
The environmental control systems provide:
- conditioned air to the passenger and crew compartments for
normal ventilation and pressurization,
- unconditioned air for emergency pressurization,
and
- temperature control of passenger and crew compartments.
ECS CONTROLLER
A dual channel ECS controller provides the circuitry required to operate
the ECS valves, HP shutoff valves and the emergency pressurization
valves. The left and right channels independently control their
respective shutoff valves. The controller receives and monitors signals
from pressure sensors, the cabin pressurization controller, thrust lever
angle, L and R BLEED switch positions and EMER PRESS switch position.
The ECS controller channels are powered by 28-vdc from the respective
L and R essential buses through the L and R BLEED circuit breakers located
in their respective pilot and copilot circuit breaker panels (ENVIRONMENTAL/
ENVIR group).
ECS SHUTOFF VALVE
Normally during flight the ECS shutoff valves will be de-energized
open. When a L or R BLEED switch is set to OFF, the respective ECS
shutoff valve will be energized to the closed position. When the L and
R BLEED switches are On and the EMER PRESS switch is activated, the
bleed air shutoff valves will close and the emergency pressurization
valves will be energized open and the HP bleed air will be shut off. The
ECS shutoff valves will close automatically whenever emergency pressurization
or a fire switch (respective ENGINE panel) is activated.
ENVIRONMENTAL CONTROL UNIT (ECU) OR PACK
The ECU or PACK consists of a precooler, primary and secondary heat
exchangers, an Air Cycle Machine (ACM), and various pneumatic control
valves and sensors. System control is accomplished by the Pressure
Regulating Shutoff Valve (PRSOV). A water separator is installed to reduce
humidity of the discharged air.
PRECOOLER AND HEAT EXCHANGERS
Ram air from a PACK airscoop is used to provide the cooling media for
the precooler, primary and secondary heat exchangers. In the absence
of ram air (ground operations), a fan wheel within the air cycle machine
is used to create airflow across the precooler and heat exchangers.
Pilot’s Manual
6-14 PM-126A
AIR CYCLE MACHINE (ACM)
An Air Cycle Machine (ACM) is used to refrigerate bleed air, providing
a source of cool air. The ACM is a three-wheel (fan, compressor, and
cooling turbine) unit installed in the tailcone. Bleed air from the aircraft
engines is first cooled by a precooler, then the amount of air is regulated
by the PRSOV before being admitted into the ACM. The ACM then
cools the bleed air further with the primary heat exchanger before compressing
it in its compressor section. After the bleed air is compressed,
heat is again extracted with a secondary heat exchanger before entering
the turbine section of the machine. Additional heat is extracted from
the compressed air by converting some of its heat energy to work (driving
the cooling turbine) and rapid expansion of the air as it exits the turbine.
This refrigerated air is passed through a water separator to
remove the water before being supplied to the cabin and crew air distribution
systems.
ACM BYPASS VALVE
When pressure at the compressor inlet exceeds the pressure at the compressor
outlet, airflow is permitted around the compressor section
through the ACM bypass check valve. This prevents the compressor
from restricting airflow during operation of the ACM at low speeds.
LOW LIMIT TEMPERATURE SENSOR
A temperature sensor, at the water separator outlet, is used to modulate
an anti-ice modulating valve. The anti-ice modulating valve is incorporated
into the ACM to maintain a constant outlet temperature and prevent
icing at the turbine outlet. To accomplish this, the anti-ice
modulating valve mixes warm bleed air with the cooling turbine discharge
air to produce the desired temperature.
PACK OVERTEMP SENSOR
Overheat conditions of the ACM are monitored by the pack overtemp
sensor. The pack overtemp sensor, mounted downstream of the ACM
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