Cabin airflow

Fresh air, so-called bleed air, is taken directly from the compressor stages of the engines. The highly compressed, approx. 250° C hot air then enters the air-conditioning packs of the aircraft. There it is processed and fed into the cabin from above via distribution systems. The "old" air is sucked out downwards, so that an even circulation is created. The extracted air is either released into the external environment or returned through filters to the central air conditioning systems and fed back into the air cycle as recirculated air. All air in the cabin is renewed every 2-3 minutes, half of it by fresh and half by recirculated air.

Functionality of the filters

The filtering process is determined by the following three mechanisms:

• Direct interception: Filters consist of matrices with a precisely defined pore size. Like a sieve, particles larger than these pores are trapped directly on the surface of the filter system.
• Diffusional interception: Very small particles like viruses can pass through the empty spaces of filters. However, they are influenced by the "Brownian motion". The zigzag motion of the microscopic particles increases the probability of them colliding with a fiber of a filter element. The "Brownian motion" thus causes even the smallest particles to adhere to the fibres or pore walls. Particles with a size of about 0.1 μm (=1/10,000 millimetres) are detected with this principle.
• Inertial effect: Particles that have a higher density than air deviate from the actual air flow and settle on the surfaces and walls of the filters. This principle works best for particles ranging in size from 0.3 to 10 μm.