During flight, the aircraft cabin is a ventilated, enclosed environment whose occupants are totally dependent on the air provided by the ECS. The ECS is designed to provide a healthy and comfortable environment for the aircraft occupants from the time crew members and passengers first board for a flight until all passengers and crew members deplane after a flight. The ECS must pressurize the aircraft cabin and maintain its temperature within tolerable limits. Most other functions are subordinate to those requirements at cruise altitudes.
The aircraft ECS is different from ECSs used in most other applications, such as buildings and surface vehicles, in that it must be able to operate in extremes of temperature, ambient air quality, and air pressure. The primary role of an aircraft ECS is to protect the occupants of the aircraft from those extreme conditions. Commercial aircraft operate over a broad range of temperatures from −55°C to 50°C (−65°F to 122°F) at ground level and as low as −80°C (−112°F) at an altitude of 12,000 m (39,400 ft). As shown in Figure 2–1, at a typical cruise altitude of 11,000 m (36,000 ft), the air temperature is usually about −55°C (−65°F) but can range from about −70°C to −30°C (−92°F to −20°F) (ASHRAE 1999a).
More critically, at a typical cruise altitude of 11,000 m (36,000 ft), the atmospheric pressure is only about one-fifth that at sea level (Figure 2–2). Although the relative concentration of oxygen at that altitude is nearly the same as at sea level, the partial pressure of the oxygen (PO2) is only about 4.7 kPa(0.69 psi) compared with 21 kPa (3.1 psi) at sea level and is far below what is necessary to sustain human life. Furthermore, the ambient air quality on the ground and at low altitudes can range from pristine to extremely polluted in urban environments. The ECS meets those needs through integrated subsystems that pressurize the cabin when in flight, control thermal conditions in the cabin, and ventilate the cabin with outside air to prevent a buildup of contaminants that might cause discomfort or present a health hazard.
In flight, the ECS maintains the cabin pressure and therefore the oxygen partial pressure at acceptable levels by compressing the low-pressure outside air and supplying it to the cabin. The air pressure in aircraft cabins is com-