Learjet 20 Series Pilot Training Manual page 215

manifold, bleed air is distributed to the flow
control valve for pressurization and cabin
heating, to the pressurization jet pump, to the
windshield heat (defog) valve, and to the wing
anti-ice valve.
Flow Control Valve
The flow control valve regulates the flow of
bleed air to the cabin for normal pressuriza-
tion and heating. It is controlled by a three-
position AIR BLEED switch labeled "OFF,"
"NORM," and "MAX" on the pressurization
module (Figure 11-7). The OFF position closes
the valve completely. MAX position opens
the valve to full flow. NORM position allows
sense line pressure from the venturi (located
downstream) to modulate the airflow and main-
tain a constant airflow into the cabin.
Figure 11-7. AIR BLEED Switch
Hot Air Bypass Valve (H-Valve)
The bypass valve is connected between the
bleed-air and precooled ducts adjacent to the
heat exchanger and is operated by 28 VDC.
The bypass valve either routes bleed air
through the heat exchanger for cooling or by-
passes the heat exchanger for cabin heating.
By varying the combination of heated and
cooled air, cabin temperature may be main-
tained at a comfortable level. This valve
requires 35 to 55 seconds to travel through its
11-8
LEARJET 20 SERIES PILOT TRAINING MANUAL
FOR TRAINING PURPOSES ONLY
entire range (full heat to full cold) and is con-
trolled by the cabin heat system in both the
automatic and manual modes. A potentiome-
ter and the temperature control circuit provide
a balancing signal. The cabin temperature
control indicator (H VALVE indicator) on the
copilot's subpanel indicates the position of he
H-valve and is operated by 28 VDC.
Ram-Air Heat Exchanger
The heat exchanger is located inside he tail-
cone. It consists of a bleed-air core surrounded
by a ram-air plenum. Cool air enters the room-
air inlet in the dorsal fin and flows through the
plenum, across the bleed-air core, thus cool-
ing the bleed air. The ram air then exhausts into
the tail compartment.
The cooled bleed air flowing out of the heat
exchanger core is ducted back to the bypass
side of the H-valve where it mixes with hot by-
passed bleed air. The resulting conditioned
air is then directed into the cabin and cockpit
distribution system.
CAUTION
When the airplane is on the ground,
do not perform extended engine
operation above idle with the switch
positioned to NORM. Since there is
no ram air for cooling of the bleed
a i r, p o s s i b l e d a m a g e t o t h e a i r-
c o n d i t i o n i n g c o m p o n e n t s c o u l d
result. Damage might also occur to
interior cabin furnishings, as well as
overheating the tailcone area.
Ram-Air Ventilation
In the event that the airplane is unpressurized
in flight, air for circulation and ventilation of
the cabin and cockpit areas is provided by ram
air, which is ducted into the conditioned bleed-
air distribution system.
During normal operation, a one-way check
valve in the connecting ram-air duct prevents
loss of conditioned pressurization bleed air
through the ram-air plenum exhaust port.
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