Editing Environmental control system (section)

== Air supply ==
{{main|Bleed air}}
[[File:ECSB737-300ENGINES ON.svg|thumb|Environmental control system (ECS) schematic of [[Boeing 737-300]]]]
On [[jetliners]], air is supplied to the ECS by being [[bleed air|bled]] from a [[Gas compressor|compressor]] stage of each [[gas turbine engine]], upstream of the [[combustor]]. The [[temperature]] and [[pressure]] of this [[bleed air]] varies according to which compressor stage is used, and the power setting of the engine. A manifold pressure regulating shut-off valve (MPRSOV) restricts the flow as necessary to maintain the desired pressure for downstream systems.

A certain minimum supply pressure is needed to drive the air through the system, but it is desired to use as low a supply pressure as possible, because the energy the engine uses to compress the bleed air is not available for propulsion, and fuel consumption suffers.  For this reason, air is commonly drawn from one of two (or in some cases such as the [[Boeing 777]], three) bleed ports at different compressor stage locations. When the engine is at low pressure (low thrust or high altitude), the air is drawn from the highest pressure bleed port.  As pressure is increased (more thrust or lower altitude) and reaches a predetermined crossover point, the high pressure shut-off valve (HPSOV) closes and air is selected from a lower pressure port to minimize the fuel performance loss. The reverse happens as engine pressure decreases.

To achieve the desired temperature, the bleed-air is passed through a [[heat exchanger]] called a '''pre-cooler'''.  Air bled from the engine fan is blown across the pre-cooler, located in the engine [[strut]], and absorbs excess heat from the service bleed air.  A fan air modulating valve (FAMV) varies the cooling airflow to control the final air temperature of the service bleed air.

Notably, the Boeing 787 does not use bleed air to pressurize the cabin. The aircraft instead draws air from dedicated inlets, located ahead of the wings. <ref>{{Cite web|title=AERO - 787 No-Bleed Systems|url=https://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/article_02_2.html|access-date=2021-02-20|website=www.boeing.com}}</ref><ref>{{Cite news|title=The Innovative 787 Carries Boeing, And Aviation, Ahead|language=en-us|magazine=Wired|url=https://www.wired.com/2009/12/boeing-787-dreamliner/|access-date=2021-02-20|issn=1059-1028}}</ref>