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Cabin air.....Recirculated or Fresh?

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Cabin air.....Recirculated or Fresh?

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Old 28th Jan 2018, 22:13
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Cabin air.....Recirculated or Fresh?

I was discussing this with someone and it's way out of my pay scale but he did mention that some manufacturers are going ever higher with cabin air recirculation percentage as opposed to in the past.

He wasn't a big fan of that.

Any ideas?

Last edited by IcePaq; 28th Jan 2018 at 22:40.
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Old 28th Jan 2018, 22:37
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You should advise your friend to be a little more selective as to which orifice he uses for speech.

Since the general trend to non-smoking flights there has been a tendency to use more recirculated air to save fuel, and you'll find a number of threads about that (and concerns that the pressurised air tapped from the engines' compressors may be contaminated), but it's nothing to do with flying higher or lower. Cruise altitude will be selected to be as high as possible (for reduced drag and thus lower fuel consumption) commensurate with the performance characteristics of the aeroplane, ambient atmospheric conditions and available slots at the desired altitudes on the particular route. Sometimes aeroplanes will "cruise climb" gently through the flight as the reducing fuel weight makes its optimum cruise altitude higher.

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Old 29th Jan 2018, 03:48
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IcePaq - If I understand you correctly you are asking if it is possible for manufacturers to increase the percentage of re-circulated air to fresh air in the passenger cabin, therefore making that percentage higher than it is at the moment? I don't know but I suspect manufacturers cannot arbitrarily change the percentage of re-circulated air to fresh air and would have to have the numbers agreed by the regulating authority in the country the aircraft was being made, inevitably I believe, aero medicine and health and safety organisations will be involved and it is from these quarters that you may find the answers you seek. I think PDR1 may have been having a puff of medicinal weed and misunderstood your original question, especially that bit about 'higher'!
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Old 29th Jan 2018, 06:34
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PDR1:
You should advise your friend to be a little more selective as to which orifice he uses for speech.
By itself, a rather horrible way to reply to someone....

... the more so when you actually stop to read what was ACTUALLY written:
are going ever higher with cabin air recirculation percentage
Nevertheless, thanks for showing us which orifice you speak from.
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Old 29th Jan 2018, 07:44
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The bottom line is that the partial pressure of oxygen in the cabin, plus the percentage of CO2, will almost certainly be regulated somewhere, as they are both physiological limits, based on the worst case passenger (in health terms) and the maximum cabin altitude of the aircraft.

As oxygen partial pressure is determined (in this case) by the combination of cabin altitude plus percentage oxygen in the cabin air, then I would guess that the max allowable recirculation percentage would be type-dependent to some degree (more passengers per unit cabin volume = less oxygen + more CO2 in recirculated air).

There are other possible contaminants to be considered, as already mentioned, which again will be type-dependent (not all aircraft use the same systems for pressurisation air), but the main considerations are as above, to make sure that the CO2 level is low enough to not cause breathing difficulties (CO2 indirectly regulates breathing rate via blood pH, in effect) and a high enough partial pressure of oxygen to ensure that the risk of hypoxia is minimised for the most at-risk normal passenger (i.e. not one that needs supplemental oxygen normally).

There's no simple answer to your question, but you can be pretty sure that manufacturers will be on the ball when it comes to anything that can reduce operating cost whilst staying within the allowable limits. Those limits are bound to be defined somewhere, and are probably slightly different between different certification authorities.
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Old 29th Jan 2018, 08:15
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I was curious about whether ozone (O3) could be a problem at altitude, and, and am looking at this NIH study that mentions it. As an aside, they mention that the air change rate is much higher in aircraft than in (say) an office:
Outdoor-air exchange rates tend to be much greater in aircraft than in homes or commercial buildings. Specifically, exchange rates in aircraft range from 9.7 to 27.3 exchanges per hour (Hocking 1998), and in U.S. residences and office buildings from 0.2 to 2 exchanges per hour (Murray and Burmaster 1995; Persily 1989). The greater air-exchange rate in an aircraft limits the consequences of homogeneous O3 chemistry. However, the potentially high O3 concentrations in aircraft cabins compared with ordinary buildings somewhat offset the effect of faster air exchange.The compounds that are known to react with O3 at a rate competitive with aircraft air-exchange rates and that are most likely to be encountered in an aircraft environment include d-limonene, α-pinene, and isoprene. Sources of these compounds include solvents, cleaning fluids, and “synthetic” natural rubber materials (Budavari et al. 1989).
In other words, ozone could be a problem because it forms naturally at cruising altitudes, and the concern is about its reactions with other chemicals found in aircraft, such as solvents in cleaning agents. But even if the outdoor air exchange rate gets reduced for cost-cutting reasons or whatever, it's still much higher than found in buildings on the ground.
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Old 29th Jan 2018, 12:00
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FAR 25.831 Ventilation - For normal operating conditions, the ventilation system must be designed to provide each occupant with an airflow containing at least 0.55 pounds of fresh air per minute.

FAR 25.832 Cabin ozone concentration.

(a) The airplane cabin ozone concentration during flight must be shown not to exceed—

(1) 0.25 parts per million by volume, sea level equivalent, at any time above flight level 320; and

(2) 0.1 parts per million by volume, sea level equivalent, time-weighted average during any 3-hour interval above flight level 270.
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Old 12th Feb 2018, 04:16
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Originally Posted by PDR1 View Post
You should advise your friend to be a little more selective as to which orifice he uses for speech.

Since the general trend to non-smoking flights there has been a tendency to use more recirculated air to save fuel, and you'll find a number of threads about that (and concerns that the pressurised air tapped from the engines' compressors may be contaminated), but it's nothing to do with flying higher or lower. Cruise altitude will be selected to be as high as possible (for reduced drag and thus lower fuel consumption) commensurate with the performance characteristics of the aeroplane, ambient atmospheric conditions and available slots at the desired altitudes on the particular route. Sometimes aeroplanes will "cruise climb" gently through the flight as the reducing fuel weight makes its optimum cruise altitude higher.

PDR

He's got a mig kill and over 30,000 hours before retiring as head line check pilot for a major carrier.

How about you?


I believe he was stating that recirculation percentage is higher than it used to be.
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Old 12th Feb 2018, 06:11
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I believe he was stating that recirculation percentage is higher than it used to be.
Your friend (with a bit of flight time under his belt) would be correct, but the changeover to fuel-saving recirculating systems happened so long ago, most aircraft built with 100% bleed feeds have long since been retired, and the remaining fleet is likely hauling cargo with a few still delivering passengers in places like Africa. What types? Think anything that was built with a low-bypass fan or pure turbo jet during the 1960s through the 70s along with some early wide bodies.

Fuel savings come about through the reduction in bleed load requirements for recirculating systems. Packs on LOW can bump up the recirc percentage, but this is still dramatically better ventilated than most office spaces as Bnt and Megan's data show. With pack flow on NORMAL, today's transport aircraft provide a complete change of air within a mean range of once every 2-3 minutes.

Last edited by vapilot2004; 12th Feb 2018 at 08:35. Reason: added attribution
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Old 12th Feb 2018, 07:51
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Megan, make you wonder what we breathed in the 60s. High 40s, 8k cabin altitude and no thought to ozone or atomic particles. The later, in particular, was always a concern to ground crew washing the aircraft. An adjacent thought, how were linneys' overalls laundered. At home, special treatment, or bulk wash at a commercial laundry?

I know our flying suits were laundered in flying clothing section.
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Old 12th Feb 2018, 12:36
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An adjacent thought, how were linneys' overalls laundered.
Our overalls were changed weekly and washed at the local laundry. The uncontrolled bit was our cold weather gear and blue uniforms. Most of us worked out on the line in our hairy blue trousers with a pre-loved anorak as a top coat. All our cold weather kit was ancient hand-me-down: you were issued from a pile of discarded kit on joining the Line Maintenance Squadron and handed it back for re-issue when you left. That even included the sea-boot socks! In warmer weather we just worked in No.3 rig - shirt-sleeve order. In early 1968 we had a visit from AWRE boffins who took radiation measurements of our kit. On my shift, all anoraks were subsequently taken away in sealed drums, for disposal. We had to buy our own after that as there were no RAF stocks available for issue. I believe Fighter Command ground crew had new cold weather kit but in No.1 Group the budget was prioritised on maintaining the flying programme.
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Old 12th Feb 2018, 12:59
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Thanks Vapilot. Your answer makes sense since he started at delta in 1968.
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Old 12th Feb 2018, 16:23
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There is another reason to maximize recirculation of air: humidity. 787 does NOT use aircraft bleed air for pressurization, has high recirculation per centage, and (compared to other aircraft) has high cabin humidity.

The high humidity made "rain in the plane" an issue, but the composite fuselage helped mitigate many of the corrosion/maintenance related issues associated with humidity inside the aircraft.

Last edited by KenV; 12th Feb 2018 at 17:23.
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Old 12th Feb 2018, 17:07
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Black sheep, ty.

Better not talk about having radioactive kit washed in a public laundry in case some Journo finds out.
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Old 12th Feb 2018, 18:39
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Megan and KenV have the important bits but during the 787 design period Boeing did formally attempt to request recirc component increased to acknowledge the engine bleed air no longer being in the loop and the need for greater humidity.

They were knocked back and the percentages remain as Megan wrote.

Rob
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Old 12th Feb 2018, 21:16
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Flying for Delta in the 1960s - I bet this man has some tales to tell. Cheers IcePaq.

787 - The primary reason this aircraft has higher cabin humidity is due to the replacement of hot bleed air from the engines with fresh outside air as the pressurization source. This air is still compressed, but not to the degree that the engine bleeds are. Further down the air path, the air cycle machines remove far less moisture than other aircraft types during their operation. As KenV states, the composite fuselage construction eliminates the need for low moisture.

I've been told the other plastic aeroplane also touts slightly higher cabin air humidity than its aluminum brethren due mostly to changes in the air cycle machine's water separators.

Last edited by vapilot2004; 12th Feb 2018 at 21:28.
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Old 13th Feb 2018, 03:48
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I thought part of the reason for recirculating the air was to keep the moisture level up, given that what's coming in from outside has almost none.
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Old 13th Feb 2018, 15:29
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Originally Posted by vapilot2004 View Post
787 - The primary reason this aircraft has higher cabin humidity is due to the replacement of hot bleed air from the engines with fresh outside air as the pressurization source.
The "fresh outside air" is the same, whether it gets compressed by an engine compressor section or a separate air compressor. And at the typical cruise altitudes of modern airliners, the "fresh outside air" is very very dry. Further, the only reason the engine bleed is hot is because it's been compressed, it essentially doesn't pick up heat from the engine's combustion section. The separate air compressor used on the 787 for cabin air heats the air just as much as the engine - its just plain physics. If the designers wanted cooler (i.e. less compressed) air from the engines, the designers could bleed air from an earlier compressor stage in the engine.

So what's the source of humidity on these airplanes? Are there humidifiers on board? Yes, and they're called passengers! The environmental control system separates the water from the air before it is expelled overboard and reintroduces that water into incoming air. The exact process is a little more complex that just described, but that's the gist of it. And is why the A350 has higher humidity even though it uses engine bleed air for cabin pressurization.
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Old 13th Feb 2018, 18:56
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The environmental control system separates the water from the air before it is expelled overboard and reintroduces that water into incoming air.
Air cycle machine doing its job, but I understand Boeing has (and likely Airbus followed) changed the air separator operation to remove less moisture to begin with.

Yes, and they're called passengers!
Indeed!

It should also be noted that because of the higher moisture content of the air coming from the packs, the 787 includes two dehumidifiers located above the fascia in the cabin.

The separate air compressor used on the 787 for cabin air heats the air just as much as the engine - its just plain physics
I'm not so sure, KenV. Considering legacy bleed systems provide concurrent volume and pressure for not just the packs, but also anti-ice, whereas the CAC fans are dedicated to just the packs. Also, as 9th stage air is not present, and that is some fairly hot stuff, leaving the bleed ducts at temps that can exceed 240C, I feel fairly sure the CACs on the 87 do not compress the air to that degree (pun intended).

While we are revisiting, it should also be noted that no commercial transport aircraft I know of pipes recirculated air up front. The cockpit always receives fresh outside air due to pressure differences (by design) in the distribution plenum.
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