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temperature control system. The system consists of a duct temperature
sensor, a low limit temperature controller, an electromagnetic control
valve and a low limit modulating valve. In response to temperature signals
from the duct sensor, the temperature controller operates the
electromagnetic control valve to vary the amount of servo pressure
delivered to the low limit modulating valve. The low limit modulating
valve allows air from the ACU precooler to enter the turbine section of the
cold air unit to maintain the cold air at the desired temperature.
The servo pressure required for the operation of the low limit modulating
valve is tapped from precooled air via a servo pressure regulating valve.
Servo pressure from this valve is also supplied to the temperature control
modulating valve of the temperature control system (refer to paragraph E).
SECTION 2
Page 7
Mar 01/85
canadair
chanenQer
OPERATING KANUAL
PSP 606
When the aircraft is on the ground, bleed air from the ACU precooler is
passed via a solenoid operated shutoff valve to an ejector in the ram air
duct to induce a flow of cooling air across the heat exchangers. The
shutoff valve opens whenever the ACU is selected on and the aircraft is on
the ground or when an overheat anticipator thermal switch, between the cold
air unit and the secondary heat exchanger senses that the air temperature
is near the overheat trip point (see below for details of the ACU
overtemperature protection system). In this case, the action of the
ejector prevents automatic shutdown of the ACU caused by high transient
temperatures usually associated with single ACU operation at high engine
power settings (high engine bleed temperatures).
The ACU is protected against overpressures by an overpressure sensor
located on the line between the primary heat exchanger and the cold air
unit. If the pressure in this line exceeds a preset limit, the switch
operates to close the associated pressure regulator and shutoff valve
thereby shutting off the supply of bleed air to the ACU. At the same time,
the FAIL light on the appropriate ACU switch light comes on and remains on
until the pressure drops to a preset level below the overpressure trip
point. After an overpressure, the system can be reset by pressing out the
affected ACU switch/light and opening the L or R AIR COND SHUT OFF circuit
breaker or pressing out the L or R ENG switch/light. If the FAIL light on
the ACU switch/light goes out, the system can return to operation by
closing the circuit breaker or pressing in the L or R ENG switch/light and
pressing in the ACU switch/light.
Protection against ACU overtemperature is provided by an overheat sensor
located on the line between the cold air unit compressor section and the
secondary heat exchanger. If the sensor detects an overheat condition, the
ACU pressure regulator and shutoff valve closes and the FAIL light on the
affected ACU switch/light comes on. The FAIL light remains on until the
temperature in the line drops to a preset level below the overtemperature
trip point. When this occurs, the ACU switch/light, which is normally
pressed out immediately after the overtemperature occurs, can be pressed in
to open the pressure regulator and shutoff valve and return the system to
operation.
On aircraft incorporating Canadair Service Bulletin 600-0476,
an additional thermal switch is installed in each of the cold air ducts
upstream from the point where the cold air is mixed with precooled air from
the temperature control modulating valve. The thermal switch causes the
appropriate (L or R) ACU FAIL light to come on if bleed air continues to
enter the ACU when the ACU switch light is pressed out. This condition can
only occur if the ACU pressure regulating and shutoff valve fails open when
the ACU is selected off.
SECTION 2
Page 8
Feb 12/88
cacnhaadiiaeinr qer
OPERATING MANUAL
PSP 606
Ventilation System (Figure 2)
The cold air from the ACU is split into two lines. One line supplies cold
air to personal, eyeball-type air vents in the cabin and in the flight
compartment. A tapping is taken off this line to circulate air behind the
instrument panels for instrument cooling. The cold air lines from each ACU
are connected by crossover ducts enabling asymmetric supply to balance the
system. The other line mixes with warm air from the ACU to supply
conditioned air through the cabin and flight compartment air distribution
ducting. Balancing valves are located in the conditioned air ducting to
ensure even distribution of conditioned air. The conditioned air ducts
from both ACUs are connected by crossover ducts to provide adequate
　
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