bleed-air is cooled in an air-to-air heat ex-
changer and cooling turbine within the ECU.
The refrigerated bleed-air is then mixed with
hot bleed air, that bypasses the heat exchanger,
to obtain the desired temperature for distri-
bution. The mixing of conditioned air is reg-
ulated by the dual-zone (cockpit and cabin)
temperature control system, a function of the
integrated ECS temperature controller.
AIR CONDITIONING
SYSTEM (TAILCONE
COMPONENTS)
GENERAL
The major components of the air conditioning
system that are located in the tailcone are the
Pack Bi-level PRSOV, a pre-cooler, the ECU,
and a water separator.
PACK BI-LEVEL PRESSURE
REGULATING SHUTOFF VALVE
(PRSOV)
Engine bleed-air enters the airplane venti-
lating system through the pack bi-level
PRSOV (Figure 11-4). The pressure regulat-
ing valve controls ECU inlet air to either 18
or 26 psig depending upon the position of the
HI FLOW solenoid. The low setting insures
bleed-air economy, while the high setting en-
sures sufficient pack inlet pressure for cool-
ing and heating. The valve is controlled by the
PACK and the HI-FLOW switches located on
the copilot’s pressurization and oxygen con-
trol panel (Figure 11-1).
For the airplane air-conditioning system to
operate, the PACK switch must be on. When
selected to the on position, the switch/indicator
(S/I) will be blank, as this is the normal posi-
tion. When selected off, an OFF caption is il-
luminated within the S/I.
The HI-FLOW switch signals the bi-level
PRSOV to the 26 psig opening position. This
allows bleed airflow to the ECU and air dis-
tribution systems to increase. This feature, as
per AFM, shall be OFF for takeoff and land-
ing, when using anti-ice, and above 30,000'.
The pack bi-level valve is powered by the left
essential bus and is protected by the PACK cir-
cuit breaker located on the pilot circuit breaker
panel. The valve is depowered open and re-
quires DC electrical power to close. During an
electrical system malfunction, the valve re-
mains open with the loss of electrical power. The
HI FLOW selection requires 28 VDC to acti-
vate and loss of electrical power will cause it
to deactivate.
ENVIRONMENTAL CONTROL
UNIT (ECU)
The environmental control unit (ECU) takes
hot high pressure bleed-air and provides de-
humidified, cold air to pressurize, cool and pro-
vide ventilation throughout the cabin and
cockpit.
The ECU consists of a pre-cooler, primary
and secondary heat exchangers, a three-wheel
air cycle machine (ACM), ducting, a low limit
temperature control valve and a bypass shut-
off valve. The water separator, though not an
integral component of the ECU is included in
this discussion.
PRE-COOLER AND
PRIMARY/SECONDARY HEAT
EXCHANGERS
When the airplane is in flight, ram air enters
the ACM airscoop (Figure 11-2) and flows
through the heat exchangers and pre-cooler
where the ram air cools the hot engine bleed
air. The ram air flows overboard through a
port located above the right engine pylon. On
the ground the impeller fan draws air into the
ACM through the airscoop and then through
the ECU for cooling (Figure 11-5).
When the PACK bi-level PRSOV is open, en-
gine/APU bleed-air passes through it and en-
11-2
FOR TRAINING PURPOSES ONLY
LEARJET 45 PILOT TRAINING MANUAL
FlightSafety
international
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