Why do airplanes pressurize their cabins?

Question

This a thermodynamic question from a chapter on specific heats, entropy & adaibatic-processes: "Commerical jet aircraft fly at cruising altitudes between 30,000 & 40,000 ft. Yet such aircraft carry heat exchangers to cool cabin air while flying at these altitudes. (a) Estimate the air temperature at which jet aircraft fly. (b) Explain why it is necessary to cool cabin air at cruising altitudes. Hint: The ratio of the heat capacities of air at constant pressure and volume is about 1.4." I alrealy have the answer to (a). It's 181 Kelvins. I need help with (b). Even if you could just give me a verbal answer for why airplanes have to cool their cabins and hence pressurize them would be helpful. Thanks.

Answer 1

Pressurization is required for physiological reasons. Some of the answers above say that the plane's fuselage will crush. This isn't true because the same pressures would exist on both the inside and outside of the airplane just like it is on the ground with the aircraft doors open. Furthermore, following the same logic, if this were true then a rapid decompression would mean the aircraft fuselage would collapse and that also isn't true. Anyway, the atmosphere has constant percentages of nitrogen, oxygen, and other trace gases, regardless of altitude. However, the distance between molecules increases the higher up you go in altitude. This is what causes atmospheric pressure to decrease with an increase in altitude. As such, with each constant volumetric breath, you would have fewer oxygen molecules in your lungs. You reach a point where you aren't getting enough oxygen to survive. Interestingly, at altitudes above 25,000 you can be oxygen deprived even if you are breathing 100% oxygen. People that climb Mount Everist are frequently hypoxic even if they are breathing 100% oxyen.

As for your other question. air for pressurization is taken from the compressor. The air is very hot so it has to be cooled. That's the down and dirty answer!

As for further information, as far as outside temperature is concerned, a standard atmospheric day starts off at 15 degrees celcius at 0 feet MSL. The temperature cools at the rate of 1.5 degrees celcius with each one thousand feet of altitude all the way up until reaching the tropopause.

The efficiency of an internal combustion engine is directly proportional to the temperature difference of the "hot sink" and "cold sink". In this case, the hot sink is the combustion chamber and the cold sink is the outside air. This is one of the reasons why it is more efficient to fly higher. Furthermore, at any constant fan or compressor RPM, a specific ratio of fuel to air must be introduced into the combustion chamber. The higher the airplane is, the less air there is and as such, the less the amount of fuel that must be sequenced into the combustion chamber. This is the second reason why it is more efficient to fly higher.

Answer 2

The reason why airplanes pressurize cabins is to allow the oxygen in the cabin to be pushed from your lungs into your blood. Without sufficient pressure, your blood could not get the oxygen even while breathing 100% oxygen. At 35,000 ft, the outside air pressure measures about 7 inches of mercury or only about 23% of sea level pressure. In the Citation X, the automatic cabin pressure system sets the cabin altitude at 4,000 feet at 35,000 feet. A typical cruise altitude on a trip of more than 1 hour would be 40,000 to 45,000 feet, and the resulting cabin pressure would be 5,500 to 6,800 feet respectively.

Hot air tapped from the engines, called bleed air, is sent to air cycle machines to be cooled and trapped or released by outflow valves. Normally, in the Citation X, the 8th stage, engine low-pressure air is sufficient for pressurization and air conditioning operation (PAC). High pressure air from the 14th stage can also be supplied to the PACs when low pressure is not sufficient, such as reduced power for descent at high altitudes.

The hot air in the engine stages can get up over 700 degrees celsius and higher and so has to be cooled with outside air to be used in the cabin. the ouside air at 35,000 feet and higher is typically -54 degrees celsius.

Answer 3

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RE:
Why do airplanes pressurize their cabins?
This a thermodynamic question from a chapter on specific heats, entropy & adaibatic-processes: "Commerical jet aircraft fly at cruising altitudes between 30,000 & 40,000 ft. Yet such aircraft carry heat exchangers to cool cabin air while flying at these altitudes. (a) Estimate the air...

Answer 4

You wanna die?

Its to feel like that you have almost normal pressure from below 12,000 ft. Above that you'd be lucky to live past 1/2 an hour or so. Remember that golfer that died from his plane(jet) losing cabin pressure? All the passenger and pilots died.

Answer 5

The short answer to your long question is ..
The airplane is pressurized so that your brains wont run out of your ears.and by the way the heat exchangers heat the cabin not cool it as its about -40 deg outside at 40,000 ft. whatever the season.

Answer 6

Most of the people above only read the first part of your question.

a) It's about -30 degrees Farenheit up there.

b) The air to pressurize the cabin comes from the engine bypass air that has been compressed, and therefore heated, by the engine. The air goes thru the heat exchanger to cool down the air so the passengers won't be roasted.

Answer 7

Air is thinner at high altitudes, people would pass out. The air must be cooled because compressed air heats up. I believe air is considered a liquid when considering it's properties. A diesel engine compresses fuel at high ratios to ignite solely from heat generated by the pressure. I'm not very scientific though sorry.

Answer 8

airplanes pressurise their cabins at high altitudes because the higher up you go, the less oxygen you have, the oxygen level i believe is maintained until around 12,000 feet and then starts to decrease from there, airplanes therefore pressurise their cabins at that level so the passengers do not suffocate from the lack of oxygen at higher levels

Answer 9

Cabin altitude is maintained at 8000 feet, this is so the passengers can breath without supplemental oxygen (masks).

Answer 10

Jerry L is right, the engine air is up to several hundred degrees F*. They cool it, and make it do work in an Air Cycle machine to provide climate control.