The environmental control system (ECS) of an aircraft provides air supply, thermal control and cabin pressurization for the crew and passengers. Avionics cooling, smoke detection, and fire suppression are also commonly considered part of an aircraft's environmental control system.
An aircraft is a machine that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines. Common examples of aircraft include airplanes, helicopters, airships, gliders, and hot air balloons.
Cabin pressurization is a process in which conditioned air is pumped into the cabin of an aircraft or spacecraft, in order to create a safe and comfortable environment for passengers and crew flying at high altitudes. For aircraft, this air is usually bled off from the gas turbine engines at the compressor stage, and for spacecraft, it is carried in high-pressure, often cryogenic tanks. The air is cooled, humidified, and mixed with recirculated air if necessary, before it is distributed to the cabin by one or more environmental control systems. The cabin pressure is regulated by the outflow valve.
Avionics are the electronic systems used on aircraft, artificial satellites, and spacecraft. Avionic systems include communications, navigation, the display and management of multiple systems, and the hundreds of systems that are fitted to aircraft to perform individual functions. These can be as simple as a searchlight for a police helicopter or as complicated as the tactical system for an airborne early warning platform. The term avionics is a portmanteau of the words aviation and electronics.
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.
Boeing Commercial Airplanes (BCA) is a division of the Boeing Company. It designs, assembles, markets, and sells jet airliners and business jets ; it also provides product-related maintenance and training to customers worldwide. It operates from division headquarters in Renton, Washington, with more than a dozen engineering, manufacturing, and assembly facilities located throughout the U.S. and internationally. BCA includes the assets of the Douglas Aircraft division of the former McDonnell Douglas Corporation, which merged with Boeing in 1997. In late 2016, BCA was home to some 78,000 employees.
Airbus SE, from 2000 to 2014 known as the European Aeronautic Defence and Space Company (EADS), is a European aerospace corporation, registered in the Netherlands and trading shares in France, Germany and Spain. It designs, manufactures and sells civil and military aerospace products worldwide and manufactures in the European Union and various other countries. The company has three divisions: Commercial Aircraft, Defence and Space, and Helicopters, the third being the largest in its industry in terms of revenues and turbine helicopter deliveries.
The Aérospatiale/BAC Concorde is a British-French turbojet-powered supersonic passenger airliner that was operated from 1976 until 2003. It had a maximum speed over twice the speed of sound at Mach 2.04, with seating for 92 to 128 passengers. First flown in 1969, Concorde entered service in 1976 and continued flying for the next 27 years. It is one of only two supersonic transports to have been operated commercially; the other is the Soviet-built Tupolev Tu-144, which operated in service from 1977 to 1978.
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.
Bleed air is compressed air taken from the compressor stage of a gas turbine upstream of its fuel-burning sections. Automatic air supply and cabin pressure controller (ASCPCs) valves bleed air from high or low stage engine compressor sections. Low stage air is used during high power setting operation, and high during descent and other low power setting operations." Bleed air from that system can be utilized for internal cooling of the engine, cross-starting another engine, engine and airframe anti-icing, cabin pressurization, pneumatic actuators, air-driven motors, pressurizing the hydraulic reservoir, and waste and water storage tanks. Some engine maintenance manuals refer to such systems as "customer bleed air". Bleed air is valuable in an aircraft for two properties: high temperature and high pressure.
A combustor is a component or area of a gas turbine, ramjet, or scramjet engine where combustion takes place. It is also known as a burner, combustion chamber or flame holder. In a gas turbine engine, the combustor or combustion chamber is fed high pressure air by the compression system. The combustor then heats this air at constant pressure. After heating, air passes from the combustor through the nozzle guide vanes to the turbine. In the case of a ramjet or scramjet engines, the air is directly fed to the nozzle.
Temperature is a physical quantity expressing hot and cold. It is measured with a thermometer calibrated in one or more temperature scales. The most commonly used scales are the Celsius scale, Fahrenheit scale, and Kelvin scale. The kelvin is the unit of temperature in the International System of Units (SI), in which temperature is one of the seven fundamental base quantities. The Kelvin scale is widely used in science and technology.
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.
The Boeing 777 is a long-range wide-body twin-engine jet airliner developed and manufactured by Boeing Commercial Airplanes. It is the world's largest twinjet and has a typical seating capacity of 314 to 396 passengers, with a range of 5,240 to 8,555 nautical miles. Commonly referred to as the "Triple Seven", its distinguishing features include the large–diameter turbofan engines, long raked wings, six wheels on each main landing gear, fully circular fuselage cross-section, and a blade-shaped tail cone. Developed in consultation with eight major airlines, the 777 was designed to replace older wide-body airliners and bridge the capacity difference between Boeing's 767 and 747. As Boeing's first fly-by-wire airliner, it has computer-mediated controls. It was also the first commercial aircraft to be designed entirely with computer-aided design.
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 absorbes 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.
A heat exchanger is a device used to transfer heat between two or more fluids. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural-gas processing, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air. Another example is the heat sink, which is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant.
A strut is a structural component commonly found in engineering, aeronautics, architecture and anatomy. Struts generally work by resisting longitudinal compression, but they may also serve in tension.
An air cycle machine (ACM) is the refrigeration unit of the environmental control system (ECS) used in pressurized gas turbine-powered aircraft. Normally an aircraft has two or three of these ACM. Each ACM and its components are often referred as an air conditioning pack. The air cycle cooling process uses air instead of a phase changing material such as Freon in the gas cycle. No condensation or evaporation of a refrigerant is involved, and the cooled air output from the process is used directly for cabin ventilation or for cooling electronic equipment.
The Boeing 707 is a mid-sized, long-range, narrow-body, four-engine jet airliner built by Boeing Commercial Airplanes from 1958 to 1979. Versions of the aircraft have a capacity from 140 to 219 passengers and a range of 2,500 to 5,750 nautical miles.
Vapor-compression refrigeration or vapor-compression refrigeration system (VCRS), in which the refrigerant undergoes phase changes, is one of the many refrigeration cycles and is the most widely used method for air-conditioning of buildings and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for chilled or frozen storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. Oil refineries, petrochemical and chemical processing plants, and natural gas processing plants are among the many types of industrial plants that often utilize large vapor-compression refrigeration systems.
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" which stands for Pressurization Air Conditioning Kits.[citation needed] The air conditioning (A/C) packs are located in the "wing to body fairing" between the two wings beneath the fuselage. On some jetliners (Douglas AircraftDC-9 Series) the A/C packs are located in the tail. The aircraft packs on the McDonnell DouglasDC-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 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 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 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 Re-heater 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 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 more than 99% of all bacteria and clustered viruses.
Air from the "mix manifold" is directed to overhead distribution nozzles [1] 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. Air is also supplied to individual gaspers: small, circular vents above each passenger seat that can be adjusted by passengers for their personal comfort. A revolving control on the vent can be turned to adjust ventilation between no air output at all and a fairly substantial breeze.
Gaspers usually receive their air from the A/C packs aboard the aircraft, which in turn receive compressed, clean air from the compressor stages of the aircraft's jet engines or when on the ground from the auxiliary power unit (APU) or a ground source. A master control for gaspers is located in the cockpit, and gaspers may be temporarily turned off during certain phases of flight, when the load on the engines from bleed-air demands must be minimized (e.g. take-off and climb).
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 pounds per square inch (75kPa), which is the ambient pressure at 8,000 feet (2,400m). 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. The new airliners such as the Airbus A350 and Boeing 787 will have lower maximum cabin altitudes which help in passenger fatigue reduction during flights.
The atmosphere at typical jetliner cruising altitudes is generally very dry and cold; the outside air pumped into the cabin on a long flight has the potential to cause condensation which might in turn cause corrosion or electrical faults, and is thus 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.[2] 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".[3]
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.[4] 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 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".[5] 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.[6][7][8]
Related Research Articles
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↑ "Cabin Air Quality." Statement of William W. Nazaroff, Ph.D. Professor of Environmental Engineering, University of California, Berkeley and Member, Committee on Air Quality in Passenger Cabins of Commercial Aircraft. (June 5, 2003)
↑ Bagshaw, Michael (September 2008). "The Aerotoxic Syndrome"(PDF). European Society of Aerospace Medicine. Archived from the original(PDF) on February 27, 2012. Retrieved December 31, 2012.
↑ Select Committee on Science and Technology (2000). "Chapter 4: Elements Of Healthy Cabin Air". Science and Technology - Fifth Report (Report). House of Lords. Retrieved 2010-07-05.
↑ "Aircraft fumes: The secret life of BAe", "In the back" column, Private Eye magazine, issue 1193, 14–27 September 2007, pages 26–27; Pressdram Ltd., London.
HVAC Applications volume of the ASHRAE Handbook, American Society of Heating, Ventilating and Air-Conditioning Engineers, Inc. (ASHRAE), Atlanta, GA, 1999.
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