Why refrigeration cycle is required in aircrafts?

" ... the air is already at very high pressure at such height ... "

What air?

The atmospheric air which is introduced in the refrigeration cycle.

Does not air pressure decrease as the altitude increases?

Hey,

Atmospheric pressure increases as the altitude increases.

Thats the reason m asking the why thre is a need of refrigeration cycle in aircrafts.

Thanks......

I think i got my answer.

Can you show us?

And can you explain , how the pressure increase when altituded increase.?

What planet are you from where, "Atmospheric pressure increases as the altitude increases?"

Wow, He's in final year of college, amazing. I never went to college and knew pressure decreases the higher you go, hence complete vacuum in space.

Don't worry about it Kramarat but from next year on just ask for the manufacturer name of everything you ride on. LOL

Uh NO! air density (and therefore PRESSURE) decreases as yo move farther away from the center of the earth.

That is why mountain climbers need oxygen on very tall mountains and why even on lower mountains you can get altitude sickness. Anything above 10,000ft. you need oxygen because the air is too thin. Aircraft fly at 30,000-35,000. do the math young man.

Rorschach wrote: ". . . . you need oxygen because the air is too thin."

I agree with everything else you wrote. If, by "being thin", you mean low pressure than I agree with that too.

The fact of the matter is that for the blood system to extract the oxygen from the air through your lungs requires a minimal pressure differential. I used to remember this stuff but I believe the pressure differential needed is a minimum of 7 PSIG.

Yes, coming from a college engineering student, this display would be embarrassing.

I suspect what we really have is a youngster on our hands who wants to play with grownups. No college student, I don't care what school or what country, could make it past freshman finals much less to to senior year on this demonstrated "knowledge".

L.J.

Yes LJ that was exactly my intent.

Come on folks.

The OP is from Mumbai in India which at least is where he is studying.

But if you watch their English language (Mostly) TV channels, for instance during that recent attack, you discover that they speak at least two or more other languages and that India is not so well integrated that the English language imposed by British imperialism is a first spoken language especially for a young man who has come to Mumbai to study engineering.

j.

I think he means that the atmospheric pressure in the aircraft increases, is increased, as the plane ascends.

Then his question and confusion, which I am not qualified to clear up not being a refrigeration specialist, is understandable.

Why can't the process of introducing higher pressure air than is outside the aircraft be used to cool the air sort of as an air conditioning system does?

No insult intended but I sometimes think some here like to play around about meanings when they already know the meaning.

I can't imagine anybody but perhaps the flat earth nuts not knowing that air pressure lowers as we ascend.

j.

Actually Jack, the cabin pressure is slowly reduced as you ascend from ground level to the pressure you'd find at around 10K Ft. That is why your coffee or tea is never very warm or strong, it boils at too low a temp because of the reduced pressure. (well there are safety issues too, we wouldn't want a "mcdonalds" type incident now would we?).

It is increased again slowly as you descend. This is done to limit the hoop stresses on the airframe.

I realize the OP is in India and may not be well versed in English, but his statement was pretty clear cut and hard to misunderstand. He said that air pressure increased as you ascend, and there is no way that is true no matter if you mean cabin pressure, bleed air pressure or atmospheric pressure. All three fall as you ascend.

Another good indication of lower cabin pressure when at high altitude is the way sealed packets (biscuits, peanuts etc) swell.

I agree his English is fairly good but it is almost certain he is not, as first language, an English speaker. British imperialism imposed English on a region with several other, native, languages, and I for one recognize, since I only speak English, the great skill with which some peoples juggle multiple languages.

Nonetheless, he was being abused for an error, the truth of which it is almost certain he knows, without giving him the benefit of the doubt or querying to find out if in fact he does not know.

As to the details I am not expert in those matters, just the generalities although having on occasion flown I would have realized it is a multi-facet issue.

My main point here though was to stop those abusing him for misstating what it should be almost certain he knows, else he would not have been able to divine the probable process and state it.

j.

DEAD WRONG BUDDY. Atm. pressure is inversely proportional to altitude. Ex: air pressure on Mt. Everest's peak is so low,oxygen is frequently a requirement.

PLEASE PLEASE PLEASE !!!!!!!!!!!

Once you graduate, PLEASE, let us ALL know what you Engineered.

So as we can get out our video cameras.

Also print a copy of this to give to your instructors. I am very sure they would like to see this so as they can give you extra credit. Might even get you out of school sooner.

<....Atmospheric pressure increases as the altitude increases....>

This is incorrect. Atmospheric pressure decreases with increasing altitude.

Google the "international standard atmosphere" for a table of values. As an alternative, try Mayhew & Rogers "Thermodynamic properties of fluids", any edition, as there is a table towards the end of the book.

Dear Mr JohnDG,

You are Right. As the Altitude increases, the Pressure decreases. How others are saying that the Pressure will increase - is not known.

DHAYANANDHAN.S

Final year? Have you ever taken a flight and gotten the safety brief? Oxygen masks etc?

Air pressure is substantially lower at altitude - think mountain climbers and oxygen bottles.

But the reason you have a refrigeration cycle is to cool passengers on the ground. Just like the injterior of a car, the interior of an aircraft gets very warm on ground.

mostly the air conditioning is done with ground units, those big yellow ducts..the packs do supply , but with the apu running were spending 500 lbs of fuel per hour.

so depending on the carrier, some prohibit / restrict apu ops till the last 30 minutes of ground time, some don't care at all. 20 ~ 25 degree difference between oat and cabin temp , when not conditioned, isn't rare. very nice in Jan, not so in Aug..

In Phoenix AZ a few years ago the temperature reached 122F. Imagine sitting on the ground for two hours in a plane with no air conditioning. You'd die.

Think!

BHR,

That is obviously the answer the op was looking for and he was right just as I suspected.

Refrigeration is achieved by using the high pressure bleed of air from the engines to achieve a refrigeration process without a closed cycle liquid-vapor- heat exchanger as on standard ground based air conditioning.

Thanks for providing the answer without all the language games.

I thought the system would be something like you have outlined but now I have the details of it, as does the op who did anticipate the system, which is nice.

j.

Refrigeration systems in aircraft are different than the conventional one which is normally used for domestic applications. Gas cycle is used. The main considerations involved are weight, space and operating power. Though power per ton of refrigeration is considerably more for air-cycle refrigeration than for a vapour –compression systems, the bulk and weight advantages of the air-cycle system, due to no heat exchanger at the cold end and a common turbo-compressor for both the engine and refrigeration unit, result in a greater overall power saving in the air craft.

I like the last few answers. It does not surprise me that the author is in his last year; I meet lots of these new "college" students. Makes me want to get back into teaching.

Anyway, the final thought I would add is the refrigeration cycle is required to maintain a CONSISTENCY in temperature in the aircraft cabin.

Mr swamy is absolutely right. In addition i would like to say that Air-conditioning system is used to Control temperature, Air circulation, Humidity control. These factors are useful for human comfort.

Regards..

Whatever system they use on commercial aircraft could stand substantial improvement. Whenever I fly it feels as if I'm breathing used air out of a paper bag, which is probably pretty accurate.

Actually, you are breathing recycled air. Long ago and far away, aircraft did not recirculate air much, but because compressing fresh air takes engine power, and therefore more fuel, all new aircraft now recirculate the air a number of times to minimize the amount of fresh air that must be compressed, thereby increasing fuel economy.

They do not recycle the air a number of times, but they do recycle some of it through HEPA filters with complete air exchange with outside air about every 5-10 mins.
They must keep a continuous in-flow and out-flow of air to prevent the build up of moisture, co², and to help with odor control from stinky passengers.
If they didn't, moisture in the air coming from the passengers exhaling, would condense and build up on the cold inner skin of the fuselage behind the trim panels. The insulation between the panels and skin would collect the water and retain it. This would cause corrosion in the aluminum, and could get into the wiring connectors and cause a fault.

The paper bag or stale air smell is more likely caused by a dirty "sock" in the ACM water separator. This is more likely to happen when the aircraft fly's short hops or in and out of pollution filled cities.
It can also occur when flying in damp weather when the water separator becomes saturated or the drain is plugged up. Much the same as when you sometimes get that dirty sock smell from the car A/C on humid or wet days.

Indeed avi-tech. A basis calculation is 1 ppm per person inside the aircraft, more or less. To allow the other ones to know, the outflow valves, that actually control the pressure inside the cabin, are always allowing some amount of air outflow to renew the inside air. I also have heard a lot of curiosity and surprise in people that hears about the cycle in aircraft. I did learn it myself only during my apprentice/trainee period. I also had some classes of students to introduce the system along with pressurization, water and waste, oxygen, etc, etc and etc. CUrrently involved with turboprop engines and APUs. Nice job. Its cool to make other people loose too ;-)

Mansi.

The refrigeration system or A/C system on the big planes, regulary using Turbine engine, take the adventage that they have great quantity of compresed air, it is taked from the LP compressor on the engine and then is crossed thru a expansion turbine , taking control of the quality (Filtered) and the temperature to be used inside the plane (crew and Pax.). The air to high levels is not usable for humans beens breathing. We are designed to no more than 14000 feets.

PIlar

mansilakhani

When I was young I thought no fridges were needed at the poles. It would be cold enough, I thought. I was wrong. This only scratches the problem you are having with AC units on planes but is part of it. We would be eating and drinking at -40deg C and kisses would last for ever if you get past the splattered parts of human tissue debris (maybe only blood) at that altitude.

Avi-tech has shown the graphs. I thought I'd give you some images.

Hope it helps, Ky.

GA my friend. GA.

One must also remember that our actions have consequences, this is true of everyone, but for engineers, those consequences can be especially dire. Human lives are often at risk when we make mistakes. One can only "fake it" so far, but eventually you will find yourself in a position of having to design something that, if it were to fail, could kill someone. Do you want to take that responsibility on when you do not even understand such basic physical concepts such as air pressure and altitude?

If you are going to be an engineer, you better learn to recognize what is the right answer when you are told the right answer. As of now you have impressed no one with your "final year of diploma" knowledge of mechanical engineering

Let's give Mansi the benefit of the doubt. He may be having trouble translating...

No mam, it was no minor mistake, and the sooner you grasp that the better. As to whether anyone should doubt your knowledge, why shouldn't we? We have been given no evidence to the contrary.

Judging by the number of engineering failures that have taken place in just the last month in India, the number of really stupid engineering questions I have seen, not just on this forum but other forums as well, from OP's from the sub-continent, I seriously question the quality of your educational system. I question not only your knowledge, but the knowledge of your professors as well. I get the distinct impression it more closely resembles the for profit degree mills here in the states than the public or private university system.

I am saddened that you are unable to argue your points based upon the points of the OP's post, instead resorting to rhetoric unsupported by fact, and using biases and concidence to attempt to make a case.

I would suggest a review of the Canons and Rules of Practice for Engineers which address these types of situations, especially debate and argument.

Well, what I read from ed didn't have any attack on this student's race or nationality.

I think he was being really fair to this student.

You need to grow up, go back and really read what was posted.

One more thing, your buddy gushas calls people "niggers" in another post; well, as a mixed race American, including black, you don't need to start throwing mud at people unless you can take it. It's guilt by association. Makes you and your buddy just as guilty as being a bigot as someone can be.

Grow up and become an adult. Just because you're a guru doesn't excuse you calling folks insults when you can't take it as well.

39

One more thing, your buddy gushas calls people "niggers" in another post; well,

Please let me know where I used such words.

39, you are keeping mum.

standarded

Some of us have given in, although the points Jack is making are relevant, they have the signature of a bean counter and are very disruptive, abrasive and some times just plain rude. Go to any of the discussions jj is involved in and you will always have a pedantic over tone in his replies. It is not against what you say but against you or some one not in line with his standards. While coming from a predictable source I would not worry. Just agree and let him count the beans, he seems good at it and all is in place, so we can turn the page and be helpful with out dictating terms.

As I type this I can see him blasting away with out thought and making his point with out really considering what emancipation means. Free of dominance by one party be it male or female, left or right.

Like your spirit Mate, Ky.

Rorschach,

Perhaps your name is reminiscent of mental problems of your own.

Any fool should see that the question posed could only have been so put except by somebody that understood, at base, these issues of pressure.

I am not usually so directly insulting except you persist when the op has indeed told you it was a simple error in statement.

Of course if you yourself are a mental case then you would not be able to empathize, or be disposed, to understand how such an error of mind, not knowledge of fact, gets expressed.

j.

Great comment. I've often been put down re: criticizing spelling and language mistakes on this site. I agree with you strongly! Mistakes and lack of attention to detail, not to mention failure to grasp the BASIC engineering concepts basically leads to injury/ deaths and costs increases.

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2151817

well, lets start with the ambient air temp at 35 k feet is about minus 40 degrees.

want that on your tootsies?...

engine bleed from either the low psi or the hi psi ( some use 8th stage and 13 stage air ) needs to be " conditioned ".. `cause you don't want 470 degree air on those tootsies , either. and we reduce the psi to around 20 psi..so we don't overpressure the cabin.

and another reason : human beings don't so well above 10 k feet w/o assistance.

so the cabin altitude (psi) is very sensitive. like when your ears won't "pop".

so using ram air intakes to feed directly into the cabin...at altitudes above 10 k feet..

ok ..see??

Hello Mansi

If you want to maintain your aircraft Cooling continously you must install a different unit for your air\carft even when air conditioning is required at bottom level they are also required seperate unit.

After certain high level the air is thick and creat a vaccum so this vaccum air we can not use directly in our refregeration unit.... ok

I hope i understand waht i mean to say ........ toy you..

Bhargav Thakkar

hello, i am sridhar, student of engineering at university in united states. i read sad words from mr. jack towards mr. ed on this blog. i as student engineer work with mr. ed in engineering factory and find him to be most kind man towards all people.

last year after bombing in mumbai, i am frightened for family at home in india and want to see my family. being student, i have small amount of money, not enough to buy ticket for airplane flight to home. mr.ed find me sad and ask as to my problem. when he find out i have family who may be hurt in bombing, he buy me and two of my friends airplane tickets so we may be with family after bombing.

mr. ed is man who mother is what was called in america american indian, now to call native american. mr. ed is man who cares for people of all color. his kindness to me and others says to me that his is good man, not person who do not like indians as you are saying mr. jack. if mr. ed says things he is trying just to help make mansi better engineer.

Who sent you to USA Shridhar? Your english is too horrible!

First let you know that high pressure air is not available at such height, there is a low pressure air. In that case, breathing of person travel in plane was no more alive. To overcome this problem, we must develop a high pressure air by compressing the natural air. And after compressing the natural air, it becomes hot and you must aware that hot air will not make you comfortable to work. So to you must have to cool the air. For that reason a whole refrigeration unit was installed in every aircraft plane