1. What are the other aircraft systems that the pressurization system relies on to do its job? In a nutshell, the engine for 1, precooler/heat exchanger, Flow control units, bleed air valves, and outflow valves to name a few.
Great answer and more detailed than I was thinking. I was thinking about the larger systems that are indirectly associated with the pressurization system that most people don't realize:
(1) Engines. The air comes into the cabin from the engines as others have already pointed out. Once sucked into the engine, part of the air is taken away from the engine compressors via it's bleed air system; bleed air that is needed by other systems on the aircraft as I previously mentioned in answering question 2 the other day (recall we're talking about the engine and wing anti-icing systems, etc. in addition to the air conditioning systems)
(2) Air Conditioning systems. The following a bit long regarding the air conditioning system but it does relate to the pressurization system, so please bear with me. For those pilots who know about the air conditioning sytem and it's related ACM, you may want to skip this part. After the technical explanition I'll tie it all together for you.
The air coming into the cabin is directed through the air conditioning system; sometimes called the A/C Packs. There are usually 2 or 3 packs. Without going into a lot of detail, the core of an a/c pack is a device called an Air Cycle Machine or ACM. An ACM is pretty much like the turbo charger on your car; that is, it has a turbine wheel and a compressor wheel. Even on a large jumbo jet, this is a fairly small device. Hot, relatively high pressure bleed air enters the pack through an On/Off valve called a Pack Valve. This valve does not modulate; it's either open or closed. It is operated by the pack switch in the cockpit. Switch ON, the pack valve opens and the pack is working. Switch OFF and the pack valve closes and the pack stops working. It's very binary for you computer geeks.
Once the pack valve is open, the air first passes through a primary heat exchanger. The purpose of this primary heat exchanger is to remove some of the heat from the air but not reduce the pressure too much. The heat exchanger is like an air-to-air radiator vs an air-to-water radiator like we're used to seeing on our cars. Anyway, after passing through the primary heat exchanger the air finally enters the pack itself and specifically, it enters the compressor section. There the air gets compressed to a higher pressure and along with this change also an increase in temperature. There is a key temperature senor down stream of the compressor section called the Pack Temperature Senor, which feeds the Pack Temperature Gage in the cockpit. More on this important gage in a minute.
There are some additional temperature sensors in this duct (down stream of the compressor), including one that will shut down the pack (close the pack valve) if the temperature in this section overheats but we will not go into that much detail. The air, leaving the compressor, now passes through a secondary heat exchanger where much of the heat is removed but most of the pressure is still retained. The air then flows into the turbine section (of the ACM) where the air is now *expanded* across the turbine wheel and this is where the magic happens: As the air flow *expands* across the turbine, there is a very large pressure drop and along with that comes a very large temperature drop! The air temperature is now down to about 32 to 34 degrees F (approximately).
ACM's are wonderful little machines that are great for one thing and one thing only: making very cold air. I'll ignore the other parts of the air conditioning system that ensure the air stays above freezing as it is not uncommon for the air to come out of the turbine section at or slightly below freezing. Maintaining the air temperature above freezing is desirable as mositure (always present) in the air would freeze and the air flow making it's way into the cabin and cockpit, would turn into snow flakes or hail. If you have ever been in a jetliner and had snow flakes or hail come out (I have) of the over head vents, that indicates some malfunction of the sub-system which ensures enough hot air is bypassed around the compressor inlet of the ACM and added to the outlet of the turbine section to maintain the target 35 F temperature of the air flow.
Now, once this air leaves the ACM it enters into the air conditioning distribution ducts and that in turn finally delivers the air to the cockpit and cabin.
Putting It All Together (the A/C System)
Now, I know this is a long explanation about the air conditioning system but laying it's ground work is important in regards to the pressurization system. Once the air is now inside the cabin, pressurization is controlled by controlling how much of it we allow to escape through the ourflow valve(s). Remember, the engines are pumping a lot of air into the cabin so a computer will control how much the outflow valves move and modulate to release enough air from the cabin to maintain the target cabin altitude/pressure differential. The cabin altitude is usually set by the pilots or done automatically by the onboard computers based on the cruising altitude.
So, now, where does all of this air conditioning info come into play? Well, if you turn off a pack, the pack valve closes and no air can move through the ACM/air conditioning system and no air flows through the distribution ducting and into the cabin. Turn off all the packs at cruise and guess what? You'll loose your pressurization. Oh, not by alot but the older the plane the more leaks it will have. When I flew FE on the TriStar for Lockheed one of the production flying checks we had to do at FL350 was to turn off all 3 packs and make sure the cabin didn't climb more than 1,000 feet per minute. If it did then we had too many air leaks and they had to be found and sealed. And no, you don't do this in the airline flying world but I wanted to make the point that the air conditioning system is an important part of the pressurization system in that unless you have at least one pack on, you won't be able to pressurize the cabin.
Now, lets go back to the ACM's for a moment. Knowing that the packs only put out 35 F air, how does the air conditioning system provide for warm air when the plane is cruising at FL350 where it's -50F outside? On the ACM there is a bypass duct that takes the hot air coming from the engines via the bleed air system and through a computer controlled by the pilots in the cockpit as they set the cockpit and cabin desired temperatures, modulates this bypass valve so that when hot air is needed in the distribution ducts, it is added directly and mixed with the cold air for the desired temperature.
Now, picture the plane sitting on the ramp at the gate loading passengers in Las Vegas in July when it's 110F outside!! As the pilot, you've got one or probably two packs on and the temperature set to 65F or so. You're calling for cold air into the cockpit and cabin. The air coming into the packs, almost all of it is going through the compressor section because we want most of it to expand across that turbine section which produces the most (in volume) cold air. Very little air is being bypassed around the ACM in this condition. Now, remember our little friend the Pack Temperature Gage in the cockpit? What does that little jewel tell us? Since most of the air is being directed through the ACM machine in this situation, the pack temperature gage will read high; will read on the high end of the gage's temperature range. In other words, the Packs (and hence ACM's) are working their asses off and you *will* see high pack temperatures under this condition. (But not, normally, high enough to trip the over heat sensor and shut down the pack). High pack temperatures mean your ACM is working hard and therefore your asking for a lot of cold air!
Now you take off and start to climb out into cooler and cooler air. As this happens, the demand for cold air is reduced more and more and more hot air ends up being bypassed *around* the ACM now. Pack temperatures will also come down to verify this. By the time you reach cruise, the ACM's are receiving the minimum amount of air through them to keep them turnng. They are really just 'idling' in cruise as most of the bleed air is bypassing the ACM. Look up while in cruise and observe the pack temp gages: it will indicate a low temp on its range.
So, that's it for the air conditioning system, the Air Cycle Machines and how they relate to the pressurization system. Now if you find yourself in a simulator for an airline or other operator getting initial training or recurrent training and the instructor gives you a Pack over heat problem, as you work through the checklist, hopefully you'll understand what is happening to the pack as you try to bring it back online and why you want to bring it back online!
(3) Outflow Valves. Controlling the outflow valves means controlling how much air we allow to 'leak' out. There is usually multiple outflow valves and one pressure relief valve. If the modulating outflow valves were to ever slam close and stay closed (malfunction), pressure would build up inside the cabin so to prevent structural damage, a maximum pressure relief valve would begin to open to relieve excess pressures.
