Pressurized air for the cabin comes from the compressor stages in the aircraft's jet engines. Moving through the compressor, the outside air gets very hot as it becomes pressurized. The portion drawn off for the passenger cabin is first cooled by heat exchangers in the engine struts and then, after flowing through ducting in the wing, is further cooled by the main air conditioning units under the floor of the cabin.
The cooled air then flows to a chamber where it is mixed with an approximately equal amount of highly filtered air from the passenger cabin. The combined outside and filtered air is ducted to the cabin and distributed through overhead outlets.
Inside the cabin, the air flows in a circular pattern and exits through floor grilles on either side of the cabin or, on some airplanes, through overhead intakes. The exiting air goes below the cabin floor into the lower lobe of the fuselage. The airflow is continuous and quickly dilutes odors while also maintaining a comfortable cabin temperature.
About half of the air exiting the cabin is immediately exhausted from the airplane through an outflow valve in the lower lobe, which also controls the cabin pressure. The other half is drawn by fans through special filters under the cabin floor, and then is mixed with the outside air coming in from the engine compressors.
These high efficiency filters are similar to those used to keep the air clean in hospitals. Such filters are very effective at trapping microscopic particles as small as bacteria and viruses. It is estimated that between 94 and 99.9 percent of the airborne microbes reaching these filters are captured.
Key Characteristics and Overall Effectiveness
There are several characteristics of the cabin air system that deserve special emphasis:
* Air circulation is continuous. Air is always flowing into and out of the cabin.
* The cabin has a high air-change rate. All of the air in the cabin is replaced by the incoming mixture of outside air and filtered air during intervals of only two to three minutes, depending on airplane size. That's 20 to 30 air changes per hour.
* Outside-air mixing replenishes the cabin air constantly. The outside-air content keeps carbon dioxide and other contaminants well within standard limits and replaces oxygen far faster than the rate at which it is consumed. Replenishment also assures that the recirculated portion of the air does not endlessly recirculate but is rapidly diluted and replaced with outside air.
[Edited 2005-11-09 21:31:35]
African Civil Aviation Commission president "You don't want to fly out as a passenger and come back as cargo."