Environmental Control

The Environmental Control System of an airliner provides air supply, thermal control and cabin pressurization for the passengers and crew. Avionics cooling, smoke detection, and fire suppression are also commonly considered part of the Environmental Control System.
The systems described below are specific to current production Boeing airliners, although the details are essentially identical for passenger jets from Airbus and other companies. An exception was Concorde which had a supplementary air supply system fitted due to the higher altitudes at which it flew, and also the slightly higher cabin pressure it employed.
On most jetliners, air is supplied to the ECS by being "bled" from a compressor stage of each turbine engine, upstream of the combustor. The temperature and pressure of this "bleed air" varies widely depending upon which compressor stage and the RPM of the engine.
A "Pressure Regulating Shutoff Valve" (PRSOV) restricts the flow as necessary to maintain the desired pressure for downstream systems. This flow restriction results in efficiency losses. To reduce the amount of restriction required, and thereby increase efficiency, air is commonly drawn from two bleed ports.
When the engine is at low thrust, the air is drawn from the "High Pressure Bleed Port." As thrust is increased, the pressure from this port rises until "crossover," where the "High Pressure Shutoff Valve" (HPSOV) closes and air is thereafter drawn from the "Low Pressure Bleed Port."