The present invention relates to aircraft environmental control systems. In particular, the invention relates to bleed air systems for supplying compressed air to an aircraft environmental control system.
Aircraft environmental control systems maintain aircraft cabin air pressures and temperatures within a target range for the safety and comfort of aircraft passengers. This is done through the use of compressed air taken from two compressor stages (bleed air) of the turbine engines propelling the aircraft. Each of the two air pressures available from the compressor, low pressure (LP) and high pressure (HP), are directed to the environmental control system (ECS) through pressure lines or plenums. A pneumatic valve controller operates a series of pneumatically operated bleed valves in response to electronic control signals from the ECS (or from a dedicated ECS bleed valve controller) to control the relative flows of LP and HP compressed air flowing to the ECS. The LP and HP bleed air as taken from the compressor is at a very elevated temperature due to the natural increase in the temperature of a gas as it is compressed. Thus, before the bleed air flows to the ECS, where it is cooled to a desired cabin temperature, it is “pre-cooled” by flowing through an air-to-air heat exchanger known as a “pre-cooler.” Cool fan air from the bypass region of the engine also flows through the pre-cooler to cool the bleed air. Air pressure in the bleed air lines is measured by at least one pressure sensor which provides this information to the ECS. Similarly, air temperature in the bleed air lines is measured by at least one temperature sensor which provides this information to ECS. The ECS uses the air pressure and temperature information along with other information from around the aircraft to direct the pneumatic valve controller to provide bleed air at a desired pressure to the environmental control system. The ECS also directs a fan air valve to adjust the flow of fan air to the pre-cooler to provide bleed air to the ECS at a desired temperature.