Page 10-7
Pilot’s Operating Manual
Revision A2: Nov, 2004
Sub-section 10
ENVIRONMENTAL
Hawker 800XP Pro Line 21
Section III - SYSTEMS DESCRIPTION
OVERSPEED PROTECTION
A pressure switch downstream of the PRSOV, protects the ACM from overspeed should the PRSOV
fail. The pressure switch operates on a rising pressure of 40 psi to signal the No. 2 MAV to close.
The No. 1 MAV remains open and the maximum flow to the CAU is restricted by the venturi in the
No. 1 LP bleed duct.
Closure of the No. 2 MAV will be indicated by the MWS panel annunciator, MAIN AIR VLV 2 illumination.
NOTE:
The illuminated MAIN AIR VLV 2 annunciator indicates that the position of the main air valve
No. 2 does not agree with the selection made on the MAIN AIR VLV 2 switch.
The closure of No. 2 MAV results in a decrease in system pressure. To avoid cycling of the valve, a latch
circuit maintains the closed signal. The system can be reset by selecting the MAIN AIR VLV 2 switch
to CLOSE, this action causes the MAIN AIR VLV 2 annunciator to extinguish. If the system pressure
has fallen, No. 2 MAV will subsequently open when the switch is selected OPEN.
COLD AIR UNIT (CAU)
The CAU consists of a fan, a compressor and a turbine mounted on a common shaft. Bleed Air from
the venturi enters the ACM via the primary heat exchanger, which cools the air to an acceptable level
for the CAU.
The primary exchanger is cooled by ram air from an intake at the base of the fin. The cooled air from
the primary heat exchanger enters the CAU compressor, which raises its pressure and temperature
before it is cooled by the secondary heat exchanger. The secondary heat exchanger uses ram air as
the cooling medium.
From the secondary heat exchanger, the air is expanded and cooled through the turbine. The energy
extracted in this process is used to drive the compressor and the fan. The fan is used to draw cooling
air through the ram air system.
At the exit from the turbine, the air temperature is below dewpoint and water is condensed out in the
form of fog. To prevent the formation of ice, the temperature of the air before entry into the water
separator is controlled by the low limit temperature control system. The water separator extracts
approximately 2/3 of the water from the airflow.
The drained water is injected back into the heat exchanger cooling air to enhance the cooling capacity.
The temperature of the air down stream of the water separator is controlled by the cabin temperature
control valve.