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Edited on Mon Jun-05-06 06:54 PM by Cannikin
Air supply
On most jetliners, air is supplied to the ECS by being "bled" off each the compressor of each jet engine upstream of the combustor. The temperature and pressure of this "bleed air" varies widely depending upon the thrust setting of the engine.
A "Pressure Regulating Shutoff Valve" (PRSOV) restricts the flow as necessary to maintain the desired pressure for downstream systems. This flow restriction results in efficiency losses. To reduce the amount of restriction required, and thereby increase efficiency, air is commonly drawn from two bleed ports (3 on the Boeing 777).
When the engine is at low thrust, the air is drawn from the "High Pressure Bleed Port." As thrust is increased, the pressure from this port rises until "crossover," where the "High Pressure Shutoff Valve" (HPSOV) closes and air is thereafter drawn from the "Low Pressure Bleed Port."
To maintain the desired bleed air temperature, the air is passed through a heat exchanger called a "precooler." Air from the jet engine fan is blown across the precooler, which is located in the engine strut. A "Fan Air Modulating Valve" (FAMV) varies the cooling airflow, and thereby controls the final air temperature of the bleed air.
On the new Boeing 787, the bleed air will instead be provided by electrically-driven compressors, thereby eliminating the inefficiencies caused by bleed port system.
Air conditioning pack
The air conditioning package, or "A/C pack" is usually an air cycle machine (ACM) cooling device. Some aircraft, including early 707 jetliners, used Vapor Cycle cooling devices like those used in your home air conditioner.
An ACM uses no Freon: the air itself is the refrigerant. The ACM is preferred over vapor cycle devices because of reduced weight and maintenance requirements.
On most jetliners, the A/C packs are located in the "Wing to Body Fairing" between the two wings beneath the fuselage. On most jetliners with tail-mounted engines (Boeing 727, Douglas Aircraft DC-9, etc) the A/C Packs are located in the tail, although the A/C Packs on the McDonnell Douglas DC-10 are awkwardly located in the front of the airplane beneath the flight deck. Nearly all jetliners have two packs, except for the Boeing 747, which has three.
The quantity of bleed air flowing to the A/C Pack is regulated by the "Flow Control Valve" (FCV). One FCV is installed for each pack. A normally closed "isolation valve" prevents air from the left bleed system from reaching the right pack, although this valve may be opened in the event of loss of one bleed system.
Downstream of the FCV, the bleed air enters the primary "Ram Air Heat Exchanger", where it is cooled by ambient air. The cold air then enters the ACM compressor, where it is re-pressurized, which reheats the air. A pass through the secondary "Ram Air Heat Exchanger" cools the air while maintaining the high pressure. When this cool, high-pressure air is expanded through the ACM turbine, the expanding air can be chilled to sub-zero temperatures.
The temperature of the Pack Outlet Air is controlled by the adjusting flow through the "Ram Air System" (below), and modulating a "Temperature Control Valve" (TCV) which bypasses a portion of the hot bleed air around the ACM and mixes it with the cold air downstream of the ACM turbine.
Just like a jet engine, the compressor and turbine are on a single shaft. The energy extracted from the air in the turbine is used to power the compressor - the system has no motor.
Air Distribution
The A/C Pack exhaust air is ducted into the pressurized fuselage, where it is mixed with filtered air from the recirculation fans, and fed into the "mix manifold". On nearly all modern jetliners, the airflow is approximately 50% "outside air" and 50% "filtered air."
Modern jetliners use "High Efficiency Particulate Arresting" HEPA filters, which trap >99% of all bacteria and clustered viruses.
Air from the "mix manifold" is directed to overhead distribution nozzles in the various "zones" of the airplane. Temperature in each zone may be adjusted by adding small amounts of "Trim Air", which is low-pressure, high temperature air tapped off the A/C Pack upstream of the TCV.
Pressurization
Airflow into the fuselage is approximately constant, and pressure is maintained by varying the opening of the "Out Flow Valve" (OFV). Most modern jetliners have a single OFV located near the bottom aft end of the fuselage, although some larger airplanes like the 747 and 777 have two.
In the event the OFV should fail closed, at least two Positive Pressure Relief Valves (PPRV) and at least one Negative Pressure Relief Valve (NPRV) are provided to protect the fuselage from over- and under- pressurization.
Airplane cabin pressure is commonly pressurized to a "cabin altitude" of 8000 feet or less. That means that the pressure is 10.9 psia (75 kPa), which is the ambient pressure at 8000 feet (2,400 m). Note that a lower cabin altitude is a higher pressure. The cabin pressure is controlled by a "Cabin Pressure Schedule," which associates each airplane altitude with a cabin altitude. Since jetliners do not always fly at their maximum rated altitude, the cabin altitude is also generally lower than the maximum permitted. For example, domestic flights rarely exceed a 5500 ft cabin altitude. The new Airbus A380 and 787 jetliners will have lower maximum cabin altitudes.
Myths
Do crews turn off one A/C Pack during flight to save fuel?
When one A/C Pack fails or is turned off, the other pack increases flow to ~185% of normal. This is required for safety reasons to maintain cabin pressurization. This may actually increase fuel consumption because the bleed flow is taken asymmetically from the engines.
Is there a switch for the crew to provide less air to the cabin unless the passengers complain?
One of the oldest 747s has a feature to turn off one of the three packs. No recently produced jetliner has this feature. Jetliners are designed to operate with all packs operating at all times.
The bleed air comes from the engines - is there fuel vapor or jet exhaust in the cabin air?
The air is "bled" from the engine upstream of the combustor. Air cannot flow backwards though the engine except during a compressor stall (essentially a jet engine backfire), thus the bleed air should be free of these contaminants from the airplane's own engines.
The bleed air has the same composition as the outside air, thus, when the airplane is on the ground, you are breathing the same air, and the same outdoor contaminants, as the ground crew on the runway.
On rare occasions, jet engine bearing seals can leak oil into the bleed air, but this is generally dealt with quickly since failed seals will reduce the engine life.
Is the air in first class better?
Boeing and Airbus jetliners supply constant flow per unit length of the cabin. The seats in first class are spaced farther apart, resulting in more air per seat, but the nozzles provide the same amount of air at all locations.
Since all the air in the main cabin comes from the same manifold, first class receives 50% outside air and 50% filtered recirculated air just like the rest of the cabin.
Is the air in the flight deck better?
Most jetliners supply 100% outside air to the flight deck. This is because the flight deck has the highest concentration of avionics and the most glass per unit volume, making the flight deck very hard to keep cool on hot days. By providing 100% outside air to the flight deck, the air supply temperature can be near freezing if required, much cooler than if the air was mixed with recirculated air. A drawback is that the air in the flight deck is much drier on these airplanes.
Some jetliners provide 50% recirculated air to the flight deck, to increase pilot comfort by raising the humidity.
Are the cargo compartments pressurized?
The cargo compartment is pressurized to the same level as the cabin, although the temperature is generally colder.
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