The Environmental Control System of an airliner provides air supply, thermal control and cabin pressurization for the passengers and crew. Avionics cooling, smoke detection, and fire suppression are also commonly considered part of the Environmental Control System.
The systems described below are specific to current production Boeing airliners, although the details are essentially identical for passenger jets from Airbus and other companies. An exception was Concorde which had a supplementary air supply system fitted due to the higher altitudes at which it flew, and also the slightly higher cabin pressure it employed.
On most jetliners, air is supplied to the ECS by being "bled" from a compressor stage of each turbine engine, upstream of the combustor. The temperature and pressure of this "bleed air" varies widely depending upon which compressor stage and the RPM 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 achieve the desired temperature, the bleed-air is passed through a heat exchanger called a "pre-cooler." Air from the jet engine fan is blown across the pre-cooler, 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.
The Cold Air Unit, or "Airconditioning pack" is usually an air cycle machine (ACM) cooling device. Some aircraft, including early 707 jetliners, used vapour-compression refrigeration like that used in home air conditioners.
On most jetliners, the A/C packs are located in the "Wing to Body Fairing" between the two wings beneath the fuselage. On some jetliners (Douglas Aircraft DC-9 Series) the A/C Packs are located in the tail. The A/C Packs on the McDonnell Douglas DC-10/MD-11 and Lockheed L-1011 are located in the front of the aircraft beneath the flight deck. Nearly all jetliners have two packs, although larger aircraft such as the Boeing 747, Lockheed L-1011, and McDonnell-Douglas DC-10/MD-11 have 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 (and v.v.), although this valve may be opened in the event of loss of one bleed system.
Downstream of the FCV is the CAU (Cold Air Unit), also referred to as the refrigeration unit. There are many various types of CAu however they all use typical fundamentals. The bleed air enters the primary "Ram Air Heat Exchanger", where it is cooled by either ram air, expansion or a combination of both. The cold air then enters the 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. The air then passes through a turbine which expands the air to fuyrther reduce heat. Similar in operation to a turbo-charger unit, the compressor and turbine are on a single shaft. The energy extracted from the air passing through the turbine is used to power the compressor.
The air is then sent through a Water Separator, where the air is forced to spiral along its length and centrifugal forces cause the moisture to be flung through a sieve and toward the outer walls where it is channeled toward a drain and sent overboard. Then, the air usually will pass through a Water Separator Coalescer or, The Sock. The Sock retains the dirt and oil from the engine bleed air to keep the cabin air cleaner. This water removal process prevents ice from forming and clogging the system, and keeps the cockpit and cabin from fogging on ground operation and low altitudes.
For a Sub-zero Bootstrap CAU, the moisture is extracted before it reaches the turbine so that sub-zero temperatures may be reached.
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.
The "Ram Air Inlet" is a small scoop, generally located on the "Wing to Body Fairing." Nearly all jetliners use a modulating door on the ram air inlet to control the amount of cooling airflow through the primary and secondary ram air heat exchangers.
To increase ram air recovery, nearly all jetliners use modulating vanes on the ram air exhaust. A "Ram Air Fan" within the ram system provides ram air flow across the heat exchangers when the aircraft is on the ground. Nearly all modern fixed-wing aircraft use a fan on a common shaft with the ACM, powered by the ACM turbine.
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."
Air from the "mix manifold" is directed to overhead distribution nozzles in the various "zones" of the aircraft. 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.
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 aircraft 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.
Aircraft cabin pressure is commonly pressurized to a "cabin altitude" of 8000 feet or less. That means that the pressure is 10.9 psi (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 aircraft 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 airliners such as the Airbus A380 and Boeing 787 will have lower maximum cabin altitudes which help in fatigue reduction during flights.
The atmosphere at typical jetliner cruising altitudes is generally very dry and cold, and the outside air pumped into the cabin on a long flight typically has a relative humidity around 10%. The fact that cabin pressure is generally lower than the pressure at ground level does not of itself contribute to the dryness.
The low cabin humidity has advantages for the structure and avionics of the aircraft: condensation which might cause corrosion or electrical faults is eliminated. Consequently when humid air at lower altitudes is encountered and drawn in, the ECS dries it through the warming and cooling cycle and the water separator mentioned above, so that even with high external relative humidity, inside the cabin it will usually be not much higher than 10% relative humidity.
Although low cabin humidity has health benefits of preventing the growth of fungus and bacteria, the low humidity causes drying of the skin, eyes and mucosal membranes and contributes to dehydration, leading to fatigue, discomfort and health issues. In one study the majority of flight attendants reported discomfort and health issues from low humidity. In a statement to Congress in 2003 a member of the Committee on Air Quality in Passenger Cabins of Commercial Aircraft said "low relative humidity might cause some temporary discomfort (e.g., drying eyes, nasal passages, and skin), but other possible short- or long-term effects have not been established".
A cabin humidity control system may be added to the ECS of some aircraft to keep relative humidity from extremely low levels, consistent with the need to prevent condensation. Furthermore the Boeing 787 and Airbus 350, by using more corrosion-resistant composites in their construction, can operate with a cabin relative humidity of 16% on long flights.
The bleed air comes from the engines but 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 combustion contaminants from the normal running of the aircraft's own engines.
However, on occasions components can leak oil (containing highly toxic additives) into the bleed air. This is generally dealt with quickly since failed oil seals will reduce the engine life.