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 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.
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They have to pressurize their cabins because at the altitude they fly, the air pressure is too low for people to be able to breath. Basically, if the partial pressure of oxygen in the air is lower than the partial pressure of oxygen in your blood, you can take air into your lungs just fine, but oxygen will go out of your blood rather than into it.
As for the temperature issue, think about the equation PV=nRT. I assume you've seen that equation before? They can increase the pressure just by pushing more air into the same amount of space, but what happens to temperature when they do that? It goes up, right? Well, even though it's cold outside, they increase the pressure so much that it would actually be too hot if they didn't have some way to cool things down. You can use that equation to work out what the temperature would be if they didn't have any cooling.
Keep in mind that they can't pressurize the cabin just by cooling it because the outside temperature is already so low that their passengers would freeze to death. They have to do it by increasing the number of air molecules inside the plane. The cooling is just to deal with the side effects of that.
Cooling the cabin and pressurizing them is for different reasons. Pressurization is so the passengers can breath. At alt there is less O2 in the air and thus people go into hypoxia. The aircraft can fly (in tact) without being pressurized with no problems. We have military aircraft that fly unpressurized still today.
Cooling the air is going to be for passenger comfort, but I have been on planes when at altitude we are running the heat because the outside air temp is so low. The aircraft does not have a tremendous amount of insulation so that cold air cools the cabin quickly. Does this help any?
I'm not an expert on this, but planes flying at altitude need to pressurize the air in the cabin to about one atmosphere in order to increase the oxygen concentration...higher altitudes, less air pressure, therefore less oxygen concentration, thus <gasp> can't breathe!
Now, as you are aware, anytime you apply pressure to anything, heat is a byproduct. So in the process of pressurizing the cabin, heat is released from that pressurized air, and thus the need for a heat sink in order to make the cabin comfortable for the passengers.
That's all pretty much a guess. I'm a math person, and don't know the equations that you will need to back that statement up with.
At 40,000 there would not be enough oxygen to sustain life and even with oxygen the pressure would be so low that your blood would boil if the pressure were removed suddenly. The outside air temperature is about -55 degrees even during the summer time so some heat is required to make the cabin comfortable. The entropy & adaibatic-processes you refer to is the normal reduction of pressure and temperature as altitude increases. The temperature drops 2 degrees centigrade per 1000 feet and the pressure drops one inch of mercury. Normal pressure at sea level is 29.92" Hq, 1013.2 mb, and 14.7 psi.
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.
#1 Because nobody could breath because the air would be too thin
#2 If they were not pressurised people ears would bleed from the sudden changes in pressure
#3 They need to be air conditioned because it very cold at high altitudes
to equalize the pressure so your ears do not break
and so the body of the plane stays intact
Dont know...but it sure makes your ears pop..ouch...lol
so you can breathe
so they do not go dowe and crash