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About Airplanes
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.
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.
Secret Seats on Airplanes
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