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:
Cabin Air System Operation
Pressurized air for the cabin in today's jetliners 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 generally circular patterns and exits through floor grilles on either side of the cabin and on airplanes with overhead recirculation, the air may exit through the overhead. 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, pressurization and overall air quality. About half of the air exiting the cabin is exhausted from the airplane through one or more outflow valves in the lower lobe, which also controls the cabin pressure. The other half is drawn by fans through High Efficiency Particulate Air (HEPA) filters under the cabin floor, and then is mixed with the outside air coming in from the engine compressors. (Some airplanes are equipped with an overhead recirculation system in combination with the under floor system).
The HEPA filters are very effective at trapping microscopic particles such as bacteria and viruses and can provide essentially particle free air in the recirculation system.
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.
- Outside-air mixing replenishes the cabin air constantly. The outside-air content keeps carbon dioxide and other contaminants below standard limits and supplies oxygen far faster than the rate at which it is consumed.
There are multiple factors associated with the aircraft cabin environment that can influence comfort. Symptoms occasionally reported by passengers and crew, including respiratory irritation and fatigue, can be caused by complex interactions of cabin environmental factors (cabin altitude, low humidity, noise, motion and temperature, odors and contaminants (personal hygiene, perfumes, etc), among other factors), individual health factors and work-related factors for cabin crew (duty-schedule impacts, jet lag, etc.). Boeing is supporting industry efforts to develop a better understanding of how these factors interact
Differences Between Older and Newer Cabin Air Systems
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 early-generation 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. 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 with respect to particulates, good fuel efficiency and less impact to our environment.