Why is it that airlines no longer take in fresh air, pressurize, heat and humidify it, instead we get the same, stale air for the entire flight? (yeah, I know, it is so they can make larger profits… the usual explanation of why any company does anything) Any connection to lack of fresh air and air rage?
Where do you get your facts, please?
The warm air in the cabin is bled off the compresser stages of the jet engines that power the plane, so more warm air in the cabin means less hot air out the tail pipe of the engines and more fuel burned per mile.
Peace
LIONsob
As you said, because it’s cheaper. It’s not completely recycled air, some fresh air is mixed in- that’s why it’s so dry (typically < 10% humidity-- desert levels).
Here’s one link about it:
http://www.flyana.com/air.html
Consumer Reports did a big article about airline air a few years ago- I’ll dig up the issue …
Arjuna34
I’ve always had a problem with this story. Usually you get a scandalised report about how the pilots and first class passengers are getting lots of wonderful mountain-fresh air whereas those in economy are breathing a barely life-sustaining mixture of farts and burnt rubber.
Trouble is, it doesn’t add up. No airliner I’ve ever been on has hermetically sealed doors between the cockpit and passenger compartments, or between economy and first class. As far as I can see, we’re all breathing the same air. Nor do I see the cabin crew stumbling half dead out of economy back to the hallowed high-oxygen air of first class, and let’s face it, they are the only ones doing anything remotely like exertion on an airliner.
The reports themselves don’t inspire me with confidence. Take a look at the link provided above by Arjuna34. Right at the start, it says:
**"Airline pilots get ten times more oxygen than economy passengers get, says a report mandated by the US Congress from the National Academy of Sciences.
Scientists have also proven that poor air quality results in lack of oxygen to the brain and impaired visual acuity."**
The first statement is bullshit, pure and simple. Even assuming that the pilots are getting the 0.2 bar partial pressure of oxygen which we all know and love (and they won’t be, because the plane isn’t pressurised to sea level) this implies that in economy we are getting only 0.02 bar, which would kill us all in a few minutes. Not physically possible. If you dig deep enough into the site you find they are talking about air FLOW, which has nothing to do with its oxygen content or quality. Use one of those little battery-powered fans in economy and you too can get “more air” than those in first class.
The second statement doesn’t relate to anything. The site doesn’t define “poor air quality” anywhere or give any figures about measured quality of air on airliners. It doesn’t even say that the air on an airliner IS of “poor quality.” It just complains that the airliners have extra air supply capacity which is often not switched on.
The site has exchanges of letters, reports and quotes from anonymous sources but again, no hard data. Some of the language is very emotive, referring to recirculated air as “recycled swill.” Many of the correspondents seem to imagine that the air they breathe in shouldn’t contain any components breathed out by the guy in the next seat - it should be whisked away by magic and ejected from the aircraft. Newsflash, boys and girls - in any space occupied by other people, in your house, in your car, in the cinema - you are breathing “recirculated air.” it doesn’t have to be pumped through a pipe to be breathed again.
There’s a fallacy in the idea that anything done by a company “to save money” is to the detriment of the consumer. You don’t run the demister in your car all the time because it isn’t necessary. In the same way, no airline wants to make its passengers ill, but they aren’t obliged to replace the cabin air several times faster than is neccessary for comfort and health either. A proper study with measured levels of oxygen and carbon dioxide, pollutants and airborne bacteria and comparisons with air from other environments might convince me otherwise, but not a bunch of misleading statements designed to sell someone’s book.
It does not cost anything to warm the outside air (as in your car). (Outside air is taken, the “bled air off compressor stages” in not taken). Pure oxygen costs more, but there is no need to pump it in the cockpit or cabin, it does not do anything to normal people or is it needed. The pressure inside the plane equals that at 3,000m altitude.
The answer to the OP. It’s not done because the complete air exchange ain’t necessary. You do not have in your house, or workplace, or movie theater. It would be much easier to do in a hermetized (sealed) airplane, but there is no need. Partial exchange is adequate.
Peace
On just about all turbine and jet engine powered aircraft cabin pressurization air comes off the cold side of the engine.
Its cheaper and more reliable than having a seperate piston or turbine motor driving a compressor and only causes a small decrease in main powerplant efficiency
Merry Christmas
Peace
LIONsob
Yeah, but your house, workplace, and movie theater aren’t up at high altitude and completely sealed against the environment.
Your comments are incorrect. Airlines DO need to exchange the air more frequently. There was a famous incident recently where someone with tuberculosis flew on a plane, and the air was not replenished enough (as per usual). The airline tracked down almost all the 100+ passengers on the plane, and 7 of them tested positive for TB.
If you can catch TB from the air in a room, that room DEFINITELY needs more ventilation.
Getting proper unbiased information on this subject seems to be difficult.
In the older designs of airliner, outside air was bled in from the compressor stage, cooled in the air packs (since the compression actually made it quite hot) and run into the air circulation system. It would mix with the cabin air in the cabin. At the same time, air was valved out of the plane to the outside. What you were breathing was a mixture of “used” and “fresh” air. Always.
Contrary to the beliefs of various campaigners, there is no way to prevent “used” air from mixing with “fresh” air, or to only vent “used” air overboard. Their objections are based on the fact that the air circulation systems on newer planes mix “used” air with fresh air, whereas on the older planes such mixing only takes place within the cabin itself. But I repeat - there is no way to avoid breathing “used” air, in a plane, in an office, in your home. It isn’t a problem as long as the rate of exchange is sufficient.
As far as health issues go - a plane is probably not the healthiest environment in the world. You’re in close confinement with several hundred other people for hours at a time, the air pressure is below what you’re used to, the air is extremely dry which I’ve heard weakens the ability of your mucus membranes to fight some infections (no cite. Maybe someone could confirm or refute?), you’re cramped and you have a good chance of becoming dehydrated. In these conditions it’s not surprising that someone with a notoriously infectious respiritory disease was able to infect a few others. Whether refreshing the air faster would have prevented some or all of these infections is not a question I’m qualified to answer.
For a fairly balanced review of some studies, claims and counter-claims, try:
http://www.prioritiesforhealth.com/1102/air.html
An extract re tuberculosis:
"In studies conducted between 1992 and 1996 whose subjects were seven persons with active (infectious) TB and 2,600 passengers and crew members, the CDC found a few cases of TB transmission on planes. The CDC also found, however, that these cases were not linked to cabin-air quality or to the type of ventilation system the aircraft had, but rather to proximity to someone with active TB and to the duration of exposure to the pathogen."
The CDC is the Centre for Disease Control and prevention.
Thanks for the info, matt. The link I posted earlier seems to be somewhat biased towards selling a book.
Arjuna34
Bagkitty loosing her interest nonwithstanding, in my defence:
Lion, I misspoke. What I meant to say was that the cold outside air is warmed (heat exchanged) and pumped inside. The turbin air compressor is used for that (as you implied). Warm air (as you said) from any turbine stage is never pumped in, as any “warm” air does not exist: if it is “warm” (heated), it is already modified by noxious combustion products.
A few hours exposure to a TB patient at a rarified atmosrhere of a plane is very unlikely to cause infection. The CDC data seems to confirm it. I would tend to believe the sentence “… seven of them turned positive for TB.”, not as it was given. Besides, it did not say that the infection, if it occured, was due to poor air quality.
The whole question seems to be of the kind: "Why did they stop to put oil in car engines?’
Not so (and I design this stuff for a living, OK?). Bleed air is taken from the engines’ compressor, where it is already heated just from being compressed, NOT from downstream of the combustor. The heat of compression alone is enough to get the air up to the 800 deg. F. range or higher in a typical engine. I think you’re confusing the turbines with the compressor just because they’re on the same shaft. That approach is simpler and lighter than using a separate compressor just for cabin air, and the effect on engine performance is pretty slight, as has been noted. It is at least possible to save a little fuel by reducing air flow, but not without health consequences (the crew has to breathe what’s in the cabin too).
The compressed bleed air is cooled in air-conditioning packs before being metered for flow into the cabin. It’s pretty dry because there’s so little humidity at 32,000 feet.