Aircraft Cabin Air Systems

In order to pressurize the cabin and meet the regulatory requirement of 0.55 pound of air pressure per person, approximately half of the air within an aircraft cabin is recirculated and mixed with fresh air supplied from the engines. In a typical commercial cabin air recirculation system, the air in the cabin consists of approximately half outside air from the engine compressor or auxiliary power unit and half filtered, recirculated air.

The key to cabin air systems is the filtration systems that allow air to be recycled and funneled back into an aircraft. The two main types of filters are HEPA (High Efficiency Particulate Air) and the Pall A-CAF (Advanced Cabin Air Filter). The HEPA filter was first introduced to protect from dust, fibers, allergens, and microbes such as viruses and bacteria. The A-CAF is a combination HEPA/carbon filter that removes odors and volatile organic compounds (VOCs) that may result from engine oil degradation, ingestion of de-icing or hydraulic fluids, etc. These filters only clean the recirculated air, so some particulate contamination, odor causing components, and VOCs can still enter through the outside air system.

The location of the cabin air filters varies from aircraft to aircraft, but they are generally located just in front of the location where the recirculated and outside air are mixed. This is typically underneath or above the main passenger cabin. The materials used to make filters vary, but generally include glass fiber filter medium, activated carbon, epoxy resin sealant, aluminum hardware, and plastic hardware. The production of a filter is like a more sophisticated method of paper making, where fibers are suspended in a liquid which is then poured over a screen, allowing the liquid to drain away. The efficiency of the filtration medium, particularly its ability to filter smaller particles, is related to the thickness of the fibrous medium, the density of fibers, and the thickness of the individual fibers.

There are three different methods filters use to remove particles. These are direct interception, inertial impaction, and diffusional interception. The concept of direct interception is simple. Filters have defined pores. If the particles are larger than the pores, they will be unable to penetrate and are removed from the air stream by direct interception. Inertial impaction is used to remove particles of smaller size than the pores. Because particles have a higher density than the air, they will deviate from the air stream and come into contact with the surfaces of the pores formed by the fibers of the filter medium, where they adhere to the filter and are trapped.

Finally, diffusional interception is a process that catches very small particles. Brownian motion, the concept that physical quantities are constantly undergoing random, minute fluctuations, causes the particles to rapidly move around a nominal trajectory and come into contact with the fibers and pore walls of the filter. This method is effective for small particles below 0.2 microns. In fact, the smaller the particle, the greater the Brownian motion, and therefore the higher the capture efficiency.

Due to the large number of passengers in the average aircraft cabin, there may be significant amounts of dust, fibers, bacteria, and other particulates. As such, filters have an important role in protecting the health and comfort of all those onboard. For all types of aircraft filtration equipment and much more, look no further than Sourcing Streamlined, a trusted supplier of all kinds of aircraft parts. Owned and operated by ASAP Semiconductor, we are an online distributor of aircraft parts as well as parts pertaining to the aerospace, civil aviation, defense, industrial, and electronics markets. We’re always available and ready to help you find all the parts and equipment you need, 24/7-365. For a quick and competitive quote, call us at 1-763-401-8616 or email us at sales@sourcingstreamlined.com.


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April 22, 2021

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