The cabin air system in today's jetliners is designed to provide a safe, comfortable cabin environment at cruising altitudes that can reach upwards of 40,000 feet.
At those altitudes, the cabin must be pressurized to enable passengers and crew to breathe normally. By government regulation, the cabin pressure cannot be less than the equivalent of outside air pressure at 8,000 feet.
Here's briefly how the system works:
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.
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 is used for maintaining a comfortable cabin temperature. About half of the air exiting the cabin is 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 have similar performance to those filters used to keep the air clean in hospitals. Such filters are very effective at trapping microscopic particles such as bacteria and viruses.
There are several characteristics of the cabin air system that deserve special emphasis:
There are multiple factors associated with the aircraft cabin environment that can influence comfort. Symptoms occasionally reported by passengers and crew, including headache and fatigue, can be caused by complex interactions of factors including the individual's health, jet lag, medications, alcohol consumption and motion sickness in combination with factors such as cabin altitude effects and low humidity. Boeing supports industry efforts to develop a better understanding of how these factors interact
Engines that produced all or most of their thrust directly from the engine core powered early-generation jetliners. Air extracted from the compressor in these older aircraft provided the cabin with 100 percent outside air with only a modest impact on fuel economy. But by today's standards, the engines themselves were very noisy, emitted much higher levels of pollutants into the atmosphere and were much less fuel-efficient.
By contrast, most newer jetliners are powered by high-bypass-ratio fan engines which are much quieter, much cleaner burning, more powerful and much more efficient. At the front end of this engine type is a large-diameter fan, which is powered by the core. The fan moves a large volume of air past the core rather than through it, and actually generates most of the thrust. Every unit of pressurized air extracted from the engine core has the effect of reducing fan thrust by an even greater amount, and that degrades fuel efficiency more severely on this type of engine than on the older type. By providing the cabin with a mixture of about 50 percent outside air taken from the compressor and 50 percent recirculated air, a balance has been achieved that maintains a high level of cabin air quality, good fuel efficiency and less impact to our environment.
However, that's only part of the rationale for the current design of cabin air systems. Cabin air is typically quite dry at cruise altitudes. With 50 percent recirculation, the cabin is provided with at least a modest level of humidity in newer jetliners compared to the very low levels in earlier models.