External air tappings:
Engines vary as to the number of external air tappings and their usage. The following notes are taken from the Pratt and Whitney JT9D but have been simplified to provide a more generic coverage.
Utilised for the pre-cooling of air conditioning air, cooling the ignition system and on some engines, the Passive and Active tip clearance control.
HP Compressor - IP Air (8th and 9th Stage)
Utilised for pneumatic cabin bleeds at concise RPM's on the JT9D, this can also supply air for nose cowl anti-icing on other engines. The nose cowl anti-icing may have a separate manifold from another compressor stage.
Should the high pressure stage bleed valve fail in the open position, a pressure relief valve is provided to protect the pre-cooler from over-pressure damage. The valve normally would include a pressure switch connected to a PRESS RELIEF warning on the pneumatics display on the flight deck. The operating pressure would be in the region of 100 psi. If the valve opens the vented air escapes through a spring-loaded door on the cowl (blow out panel).
The system normally consists of a pre-cooler temperature sensor and controller, pre-cooler and control valves. This system stabilises the air going to the airframe system, by keeping it constant at a value that the engine can achieve at all power settings. The valves are normally part of the pre-cooler and flow of the fan air is regulated by the opening or closing of the valves.
When temperature at the bleed air outlet of the pre-cooler exceeds its limit (1600-1800C) the pneumatic pressure is vented from the actuators to move the cooling air valves toward the open position.