thumb|200px|Control panel for a Boeing 737-800 ECS

In aeronautics, an environmental control system (ECS) of an aircraft is an essential component which provides air supply, thermal control and cabin pressurization for the crew and passengers. Additional functions include the cooling of avionics, smoke detection, and fire suppression.

Overview

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.

Air supply

thumb|Environmental control system (ECS) schematic of [[Boeing 737-300]]

On jetliners, air is supplied to the ECS by being bled from a compressor stage of each gas turbine engine, upstream of the combustor. The temperature and pressure of this bleed air varies according to which compressor stage is used, and the power setting of the engine. A manifold pressure regulating shut-off valve (MPRSOV) restricts the flow as necessary to maintain the desired pressure for downstream systems.

A certain minimum supply pressure is needed to drive the air through the system, but it is desired to use as low a supply pressure as possible, because the energy the engine uses to compress the bleed air is not available for propulsion, and fuel consumption suffers. For this reason, air is commonly drawn from one of two (or in some cases such as the Boeing 777, three) bleed ports at different compressor stage locations. When the engine is at low pressure (low thrust or high altitude), the air is drawn from the highest pressure bleed port. As pressure is increased (more thrust or lower altitude) and reaches a predetermined crossover point, the high pressure shut-off valve (HPSOV) closes and air is selected from a lower pressure port to minimize the fuel performance loss. The reverse happens as engine pressure decreases.

To achieve the desired temperature, the bleed-air is passed through a heat exchanger called a pre-cooler. Air bled from the engine fan is blown across the pre-cooler, located in the engine strut, and absorbs excess heat from the service bleed air. A fan air modulating valve (FAMV) varies the cooling airflow to control the final air temperature of the service bleed air.

Notably, the Boeing 787 does not use bleed air to pressurize the cabin. The aircraft instead draws air from dedicated inlets, located ahead of the wings.

Cold air unit

The primary component for the functioning of the cold air unit (CAU) is the Air Cycle Machine (ACM) cooling device. Some aircraft, including early Boeing 707 aircraft, used vapor-compression refrigeration like that used in home air conditioners.

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.

Most jetliners are equipped with PACKs Meaning of abbreviation see here. The location of the air conditioning (AC) PACK(s) depends on the design of the aircraft. In some designs, they are installed in the wing-to-body fairing between the two wings beneath the fuselage. On other aircraft (Douglas Aircraft DC-9 Series) the AC PACKs are located in the tail. The aircraft 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 AC 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 vice versa), although this valve may be opened in the event of loss of one bleed system.

Downstream of the FCV is the cold-air unit (CAU), also referred to as the refrigeration unit. There are many various types of CAUs; 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 repressurized, 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 further 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 flow then is directed to the reheater before it passes to the condenser to be ready for water extraction by water extractor.

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.

Ram air system

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.

Air distribution

The AC 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 more than 99% of all bacteria and clustered viruses.

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 AC PACK upstream of the TCV. Air is also supplied to individual gasper vents. A revolving control on the vent can be turned to adjust ventilation between no air output at all and a fairly substantial breeze.

thumb|Gasper vent over passenger seats of a [[Boeing 737-800]]

Gaspers In a statement to US 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 A350, by using more corrosion-resistant composites in their construction, can operate with a cabin relative humidity of 16% on long flights.

Health concerns

The bleed air comes from the engines but is bled from the engine upstream of the combustor. Air cannot flow backwards through 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 carbon seals can leak oil (containing potentially hazardous chemicals) into the bleed air, in what is known in the industry as a fume event. This is generally dealt with quickly since failed oil seals will reduce the engine life.

Oil contamination from this and other sources within the engine bay has led to health concerns from some advocacy groups and has triggered research by several academic institutions and regulatory agencies. However, no credible research has yielded evidence for the existence of a medical condition caused by fume events.

Footnotes

References

  • HVAC Applications volume of the ASHRAE Handbook, American Society of Heating, Ventilating and Air-Conditioning Engineers, Inc. (ASHRAE), Atlanta, GA, 1999.