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timun · 05-16-2022, 01:21 PM

I don't work on aircraft but I do work in HVAC design, so upfront I admit I'm not a subject matter expert on aircraft HVAC, but ventilation in aircraft has come up in a lot of industry literature. The manufacturers made some dubious claims in the last couple years about the efficacy of their ventilation systems to assuage worries about Covid and virus dispersion. Airbus and Boeing made a lot of rah-rah ad copy about how their planes have HEPA filters and high ventilation rates and that obviated the need for social distancing and they could still fill planes to the brim without issue, but the actual efficiency of the ventilation air distribution in passenger aircraft is questionable.

Ideally you'd ventilate the cabin with air from outside, exchanging "fresh" air in and relieving "spent" air out. However, the aircraft HVAC system doesn't distribute pure outdoor air but instead recirculates air inside the cabin and mixes it with outdoor air bled off the engine compressors. This means 1) not all of the supply air to the cabin consists of "fresh" air from outside, and 2) the "fresh" air coming from the engines may be of dubious quality to begin with. It's kind of analogous to using air from the intake side of a turbocharger to ventilate a car's passenger cabin: it could be contaminated with oil...

(Building ventilation works broadly the same way, but careful consideration is usually made when figuring out where the outdoor air intakes will be located so that the outdoor source is as "clean" as possible. E.g. we typically don't put outdoor air intakes at ground level, and certainly won't put them anywhere near an alley or street. And while the air handling units will often be of a recirculating design, they will be sized with heating/cooling to allow for a minimum amount of outdoor air intake at peak weather conditions.)

Air is supplied overhead at the aisle seats and by design must pass across the adjacent seating to the return side of the system along the outer edge of the floor. There are a lot of obstructions to efficient air distribution with all the seats and people and carry-on bags under seats. CO2 measurements could vary wildly depending on where you sat and where exactly you took the CO2 measurements from. At the window seat = probably higher CO2, and closer to the floor = probably higher CO2.

05-16-2022, 01:21 PM	#974
timun First Line Centre Join Date: May 2012 Exp:	I don't work on aircraft but I do work in HVAC design, so upfront I admit I'm not a subject matter expert on aircraft HVAC, but ventilation in aircraft has come up in a lot of industry literature. The manufacturers made some dubious claims in the last couple years about the efficacy of their ventilation systems to assuage worries about Covid and virus dispersion. Airbus and Boeing made a lot of rah-rah ad copy about how their planes have HEPA filters and high ventilation rates and that obviated the need for social distancing and they could still fill planes to the brim without issue, but the actual efficiency of the ventilation air distribution in passenger aircraft is questionable. Ideally you'd ventilate the cabin with air from outside, exchanging "fresh" air in and relieving "spent" air out. However, the aircraft HVAC system doesn't distribute pure outdoor air but instead recirculates air inside the cabin and mixes it with outdoor air bled off the engine compressors. This means 1) not all of the supply air to the cabin consists of "fresh" air from outside, and 2) the "fresh" air coming from the engines may be of dubious quality to begin with. It's kind of analogous to using air from the intake side of a turbocharger to ventilate a car's passenger cabin: it could be contaminated with oil... (Building ventilation works broadly the same way, but careful consideration is usually made when figuring out where the outdoor air intakes will be located so that the outdoor source is as "clean" as possible. E.g. we typically don't put outdoor air intakes at ground level, and certainly won't put them anywhere near an alley or street. And while the air handling units will often be of a recirculating design, they will be sized with heating/cooling to allow for a minimum amount of outdoor air intake at peak weather conditions.) Air is supplied overhead at the aisle seats and by design must pass across the adjacent seating to the return side of the system along the outer edge of the floor. There are a lot of obstructions to efficient air distribution with all the seats and people and carry-on bags under seats. CO2 measurements could vary wildly depending on where you sat and where exactly you took the CO2 measurements from. At the window seat = probably higher CO2, and closer to the floor = probably higher CO2.