Temperature Control
Temperature control of the engine bleed air
entering the cabin area is accomplished by
varying the position of the H-valve butterfly.
As the valve opens, less bleed air is directed
to the heat exchanger for cooling, while more
bleed air is bypassed and mixed with the cooled
air. Manual and automatic operation of the
H-valve is achieved by controls on the CABIN
C L I M AT E s w i t c h p a n e l , l o c a t e d o n t h e
copilot’s lower instrument panel (Figure 11-5).
Figure 11-5.
Cabin Climate Switch Panel
On SNs 24-350 and subsequent and 25-227 and
subsequent, the H-valve is positioned pneu-
matically by servo bleed air (see Chapter 9), and
n o e l e c t r i c a l c i r c u i t s a r e i nvo l v e d . T h e
AUTO–MAN knob is actually a servo bleed-air
selector valve. The COLD–HOT knob is a nee-
dle valve that controls the servo air pressure ap-
plied to the H-valve butterfly (spring-loaded to
the full cold position). Other system compo-
nents include a temperature sensor located in
the upper forward cabin, a duct temperature
sensor, and a duct temperature limiter located
in the air duct downstream of the H-valve (see
Figure 11-2). The control system consists of an
interconnected servo bleed-air network.
With the AUTO–MAN knob in the MAN
position, the selector valve isolates the con-
trol system from the influences of the cabin
temperature sensor and the duct temperature
sensor. Servo air pressure is routed directly
through the needle valve (controlled by the
COLD–HOT knob) to the H-valve butterfly.
Changing the COLD–HOT knob position
simply changes the servo air pressure on the
H-valve butterfly. The H VALVE indicator
(see Figure 11-4) displays the relative position
of the H-valve which is the only component
in the system that requires DC electrical power.
DC power is provided through the H VAL IND
circuit breaker on the left main bus.
With the AUTO–MAN knob (selector valve) in
the AUTO position, the servo pressure control
network samples the needle valve setting
(COLD–HOT knob position), the cabin temper-
ature sensor (existing cabin temperature), and
the duct temperature sensor (actual temperature
of the bleed air inside the duct). Servo air pres-
sures are modulated by the control system,
which causes the H-valve butterfly to modulate,
thereby keeping the cabin temperature constant.
Whether the system is being operated manu-
ally or automatically, the duct temperature
limiter signals the control unit if the duct
temperature increases to approximately 350°F.
The control unit’s response is to drive the
H-valve to the full cold position, and direct all
bleed air through the heat exchanger.
AIR-CONDITIONING SYSTEM
(SNs 24-230 THROUGH -349 AND
SNs 25-003 THROUGH -226
Figure 11-6 illustrates the air-conditioning
system.
Bleed-Air Check Valves
A bleed-air check valve is installed in the
pylon bleed-air duct for each engine. Each
check valve is a flapper check valve allowing
airflow in one direction and closes with air-
flow in the opposite direction. The purpose of
the check valves is to prevent loss of bleed air
in the event of single-engine operation.
Bleed-Air Manifold
The bleed-air manifold is a metal duct which
serves as a collection point for bleed air from
either or both engines. From the bleed-air
11-6
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
AUTO
MAN
COOL
COLD
HOT
FAN
O
F
F
C
A
B
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