Pneumatic, Air-Conditioning and Pressurisation Systems
Air sources
Transport aircraft bleed air from the engine compressor (HP and LP stages, with a pressure-regulating and shut-off valve), from the APU, or from a ground cart. Bleed air is hot and at high pressure, so it passes through a pre-cooler (fan air) and then the air-conditioning pack. The pack is an air cycle machine: bleed air is compressed, cooled in a primary and secondary heat exchanger, expanded through a turbine that drives the compressor and fan, and delivered cold. Water separation removes moisture; temperature is controlled by mixing hot trim air with cold pack output.
Cabin pressure
Cabin pressure is set by the outflow valve, which modulates how much air leaves the cabin. The differential pressure (cabin minus ambient) is limited by pressure relief valves and the negative pressure relief valve prevents outside pressure exceeding cabin pressure. Cabin altitude is normally held near 8000 ft or below; the maximum differential pressure, not the absolute cabin pressure, is the structural limit. Rate of change of cabin altitude is limited (typically around 500 ft/min climb and less on descent) for comfort.
Traps
Questions often confuse the roles: the outflow valve controls cabin pressure, not the inflow. Another trap is the 'dump' function, which opens the outflow valve to reduce differential pressure rapidly on the ground or in smoke removal. Also, the pressurisation controller uses ambient pressure and selected landing field elevation; selecting a high field elevation before descent prevents an uncomfortable pressure bump.
Worked example
If maximum differential pressure is 8 psi and ambient at cruise is 3 psi, maximum cabin pressure is 11 psi absolute, which corresponds to a cabin altitude well below sea level; the controller limits cabin altitude so the differential limit is not reached. Rule of thumb: differential pressure, not cabin altitude, breaks the fuselage.
Smoke and ventilation
Smoke removal uses the outflow valve and dedicated fans; the packs may be shut off and ram air used.