TEMPERATURE AND PRESSURE REGULATION

The bleed air temperature is controlled by a pre-cooler that uses fan air. The bleed air pressure is regulated by the bleed valves, which are down-stream of the IP and HP valves. The bleed valves are electrically controlled and pneumatically operated valves act as pressure regulators as well as shutoff valves. 

There is an overpressure valve downstream of the bleed valves which will close to protect the system if the bleed valve fails to regulate high pressure.

A bleed valve will close pneumatically if: 

• The upstream pressure falls below 8 psi 

• There is reverse flow

A bleed valves will close electrically if: 

• The bleed pb is switched off 

• An engine fire pb is pressed

The BMC will close the engine bleed valves for the following: 

• APU bleed switch is ON 

• Engine start valve not closed 

• Over-temperature 

• System leakage 

• Over-pressure

APU 

When the APU bleed valve is opened, the BMC sends a signal to the cross-bleed valve to open and the engine bleed valves to close. The cross-bleed valve has two electric motors; one is controlled automatically by the BMC and one manually. During normal operations with the APU bleed valve closed, the cross-bleed will also close to isolate the two systems. The cross-bleed is opened manually during a cross-bleed engine start.

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