Pneumatic systems keep jet cabins comfortable
Pneumatic (compressed) air is used to operate a variety of important jet aircraft components. These include such things as air conditioning, pressurization, engine start, and wing and engine anti-ice systems. It can serve less obvious functions, too, like pressurizing water systems to operate the all-important coffee makers. For passenger comfort, however, temperature and pressurization control are what matter most.
Before engine start, pneumatically powered systems are supplied with compressed air from an auxiliary power unit or ground source. Once engines are up and running, so-called bleed air is extracted from the engines to operate these systems. Engine bleed air is taken first from a low-pressure stage of the engine compressor. However, this source may not be sufficient to meet system demands when the engine is at idle or low power settings. In that case, additional bleed air is taken from a high-pressure stage of the compressor.
Engine bleed air straight from the source is hot. Generally, the higher the compressor stage, the hotter the air. To be useful for pressurizing and air conditioning the aircraft, its temperature and pressure must be tamed. Although each aircraft system varies, this process typically begins when the bleed air is routed through a heat exchanger, where it is cooled by fan-stage engine air that bypasses the compressor. This precooling is only the first step in conditioning the air for use in aircraft environmental systems. After being sent through a valve that controls the air pressure to within a desired range, it eventually arrives at the aircraft's air conditioning assemblies, commonly called packs. A pack is a generic name for a collection of different components that work together to maintain cabin temperature within a comfortable range. Jets generally have two or three separate packs that are more or less identical in operation.
When the precooled bleed air arrives at the pack, it is still too warm for air conditioning purposes. A portion of this air is sent through a primary heat exchanger (an air-to-air radiator), where it is cooled further. In flight, cooling is provided by outside ram air sent across the heat exchanger. On the ground or at low airspeed with insufficient ram effect, cooling air is supplied by a fan. After exiting the primary heat exchanger, the bleed air is then sent through an air cycle machine. This is essentially an expansion turbine that cools the air further by turning heat energy into mechanical energy. Finally, the bleed air is sent through a secondary heat exchanger, where it is cooled even more.
Throughout this progressive cooling process, water vapor in the bleed air supply can condense and form droplets. This water is removed from the system by a water separator. The water separator is heated by warm bleed air to keep it from freezing and clogging.
Not all of the bleed air is cooled as described. Some is routed around the heat exchangers and air cycle machine. This much hotter air is remixed later with the conditioned air to adjust cabin temperature as required. Because jets operate in an extremely wide range of ambient temperatures, the amount of hot and cold air mixed together is continuously adjusted to maintain a constant cabin temperature. This mixing of hot and cold air usually occurs automatically in the packs, but the crew can manually control the process if so desired.
Compared with temperature control, cabin pressurization control is a simpler matter. The packs supply a relatively constant flow of air into the cabin. Desired cabin pressure is maintained by varying how much of this air is vented overboard. The pressurization controller accomplishes this by signaling the aircraft outflow valve to move in a range between full-closed and full-open positions, depending upon aircraft altitude.
Pneumatic air serves many purposes in a jet, not least of which is to make passengers and crew feel comfortable high above it all.
