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Air in the cabin of airliners is a mix between:

  • new air (engine bleed air or ram air in some aircraft) and
  • recycled air pumped out of the cabin, filtered and re-injected.

How much air is recirculated? What are the reasons for recycling air instead of using bleed air only?

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In modern aircraft cabin, close to half of the cabin air is recircualted. As this IATA briefing paper on Cabin air quality - risk of contagious viruses notes:

The cabin air system is designed to operate most efficiently by delivering approximately 50 percent outside air and 50 percent filtered, recirculated air.

The AIVC also notes in Air quality in passenger aircraft:

In early commercial jet aircraft, passenger cabins were ventilated with 100% outside air. In more recent jet aircraft, approximately 50% of the ventilation air is outside air and the remaining 50% is filtered recirculated cabinair.

The reason for using recirculated air is quite simple- fuel savings. From the document Commercial Airliner Environmental Control System- Engineering Aspects of Cabin Air Quality:

At the beginning of the commercial jet airliner age, jet airplanes did not have cabin air recirculation systems,... The primary reason was that early jets were powered by highly inefficient turbojet engines.

As engine technology progressed, turbofans were developed with a core bypass ratio of approximately 2 to 1. Fuel economy improved and the cost of engine bleed air relative to overall fuel consumption was still sufficiently small to make 100% bleed air to the passenger cabin cost effective.

As modern turbofan engines with high 5 to 1 bypass ratios were developed, fuel consumption to provide engine thrust decreased. However, the fuel consumption relative to extracting bleed air dramatically increased, almost in direct proportion to the higher bypass ratio. For a 767 with P&W 4000 engines, the percent increase in fuel consumption due to bleed air only would be almost four times higher than an equivalently sized turbojet for the same amount of bleed air.

Fuel consumption

Increase in bleed air fuel consumption with modern jet engines; image from Commercial Airliner Environmental Control System- Engineering Aspects of Cabin Air Quality by Elwood H. Hun et. al.

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From an own work report, based on public web presentations for type rating and on The Boeing 737 Technical Guide by Chris Brady:

A recirculation system reduces bleed air requirements and pack loads, by filtering cabin air and re-introducing it to the mix manifold. Approximately 25% of the cabin air is recirculated. The recirculation fan will switch off if either pack is in HIGH flow, causing a net reduction in ventilation rate of about 15%. Total ventilation rate on a 737-800 is approx 1 m3/sec

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A portion of the conditioned air from the left pack flows directly into the flight deck. The remainder of the left pack air, all of the right pack air, and air from the recirculation system is combined into the mix manifold. Mix manifold air is distributed to the passenger cabin through different zones, via the side-wall risers on the left and right sides of the aircraft. On ground, pre-conditioned air can be pumped directly into the mix manifold

The B737 Technical Guide further mentions that on the B757 and B767, up to 50% of cabin air is recirculated, and on the MD80 none.

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