Environmental Control System - AbsoluteAstronomy.com

The environmental control system (ECS) of an aircraft

Aircraft

An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft 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.Although...

provides air supply, thermal control and cabin pressurization

Cabin pressurization

Cabin pressurization is the pumping of compressed air into an aircraft cabin to maintain a safe and comfortable environment for crew and passengers when flying at altitude.-Need for cabin pressurization:...

for the crew and passengers. Avionics

Avionics

Avionics are 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 meet individual roles...

cooling, smoke detection, and fire suppression are also commonly considered part of an aircraft's environmental control system.

Overview

The systems described below are specific to current production Boeing

Boeing Commercial Airplanes

Boeing Commercial Airplanes designs, assembles, markets and sells large commercial jet aircraft and provides product-related maintenance and training to customers worldwide...

airliners, although the details are essentially identical for passenger jets from Airbus

Airbus

Airbus SAS is an aircraft manufacturing subsidiary of EADS, a European aerospace company. Based in Blagnac, France, surburb of Toulouse, and with significant activity across Europe, the company produces around half of the world's jet airliners....

and other companies. An exception was Concorde

Concorde

Aérospatiale-BAC Concorde was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...

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

On most jetliners, air is supplied to the ECS by being "bled

Bleed air

Bleed air in gas turbine engines is compressed air taken from within the engine, after the compressor stage and before the fuel is injected in the burners. While in theory bleed air could be drawn in any gas turbine engine, its usage is generally restricted to jet engines used in aircraft...

" from a compressor

Gas compressor

A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...

stage of each gas turbine engine, upstream of the combustor

Combustor

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...

. The temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

and pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

of this "bleed air

Bleed air

" varies widely depending upon which compressor stage and the RPM of the engine.

A "Manifold Pressure Regulating Shut-Off Valve" (MPRSOV) 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

Boeing 777

The Boeing 777 is a long-range, wide-body twin-engine jet airliner manufactured by Boeing Commercial Airplanes. It is the world's largest twinjet and is commonly referred to as the "Triple Seven". The aircraft has seating for over 300 passengers and has a range from , depending on model...

).

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 Shut-Off 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

Heat exchanger

A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

called a "pre-cooler." Air from the jet engine fan is blown across the pre-cooler, which is located in the engine strut

Strut

A strut is a structural component designed to resist longitudinal compression. Struts provide outwards-facing support in their lengthwise direction, which can be used to keep two other components separate, performing the opposite function of a tie...

. 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

Boeing 787

The Boeing 787 Dreamliner is a long-range, mid-size wide-body, twin-engine jet airliner developed by Boeing Commercial Airplanes. It seats 210 to 290 passengers, depending on the variant. Boeing states that it is the company's most fuel-efficient airliner and the world's first major airliner to use...

, the bleed air will instead be provided by electrically driven compressors, thereby eliminating the inefficiencies caused by bleed port system.

Cold Air Unit (CAU)

The Cold Air Unit, or "Airconditioning pack" is usually an air cycle machine

Air cycle machine

An air cycle machine is the refrigeration unit of the environmental control system 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...

(ACM) cooling device. Some aircraft, including early 707

Boeing 707

The Boeing 707 is a four-engine narrow-body commercial passenger jet airliner developed by Boeing in the early 1950s. Its name is most commonly pronounced as "Seven Oh Seven". The first airline to operate the 707 was Pan American World Airways, inaugurating the type's first commercial flight on...

jetliners, used vapor-compression refrigeration

Vapor-compression refrigeration

Vapor-compression refrigeration is one of the many refrigeration cycles available for use. It has been and is the most widely used method for air-conditioning of large public buildings, offices, private residences, hotels, hospitals, theaters, restaurants and automobiles...

like that used in home air conditioners.

An ACM uses no Freon: the air itself is the refrigerant

Refrigerant

A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a liquid to a gas. Traditionally, fluorocarbons, especially chlorofluorocarbons, were used as refrigerants, but they are being phased out because of their ozone depletion...

. 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

Fuselage

The fuselage is an aircraft's main body section that holds crew and passengers or cargo. In single-engine aircraft it will usually contain an engine, although in some amphibious aircraft the single engine is mounted on a pylon attached to the fuselage which in turn is used as a floating hull...

. 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

McDonnell Douglas

McDonnell Douglas was a major American aerospace manufacturer and defense contractor, producing a number of famous commercial and military aircraft. It formed from a merger of McDonnell Aircraft and Douglas Aircraft in 1967. McDonnell Douglas was based at Lambert-St. Louis International Airport...

DC-10

McDonnell Douglas DC-10

The McDonnell Douglas DC-10 is a three-engine widebody jet airliner manufactured by McDonnell Douglas. The DC-10 has range for medium- to long-haul flights, capable of carrying a maximum 380 passengers. Its most distinguishing feature is the two turbofan engines mounted on underwing pylons and a...

/MD-11 and Lockheed L-1011 are located in the front of the aircraft beneath the flight deck

Flight deck

The flight deck of an aircraft carrier is the surface from which its aircraft take off and land, essentially a miniature airfield at sea. On smaller naval ships which do not have aviation as a primary mission, the landing area for helicopters and other VTOL aircraft is also referred to as the...

. Nearly all jetliners have two packs, although larger aircraft such as the Boeing 747

Boeing 747

The Boeing 747 is a wide-body commercial airliner and cargo transport, often referred to by its original nickname, Jumbo Jet, or Queen of the Skies. It is among the world's most recognizable aircraft, and was the first wide-body ever produced...

, 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 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 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

HEPA

High-Efficiency Particulate Air or HEPA is a type of air filter. Filters that are awarded the HEPA accolade are used in various locations, whether in medical facilities, automotive vehicles, airplanes, home filters, or wherever very pure air is sought. The filter must satisfy certain standards of...

filters, which trap >99% of all bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

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 A/C Pack upstream of the TCV. Air is also supplied to individual gasper

Gasper

Gasper may refer to:* Gasper , an Indo-Parthian king, one of the three kings that made up the Biblical Magi that attended the birth of Christ.* George Gasper, mathematician* Gasper, a marijuana cigarette....

s (outlet nozzles) over each passenger seat, which may be adjusted individually by passengers. Some aircraft allow gasper supply to be turned on or off from the flight deck.

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 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. The new airliners such as the Airbus A380

Airbus A380

The Airbus A380 is a double-deck, wide-body, four-engine jet airliner manufactured by the European corporation Airbus, a subsidiary of EADS. It is the largest passenger airliner in the world. Due to its size, many airports had to modify and improve facilities to accommodate it...

and Boeing 787

Boeing 787

will have lower maximum cabin altitudes which help in fatigue reduction during flights.

Humidity

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

Relative humidity

Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...

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

Condensation

Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....

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

Fungus

A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

and bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, the low humidity causes drying of the skin, eyes and mucosal membranes and contributes to dehydration

Dehydration

In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...

, 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.

Health concerns

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

Compressor stall

A compressor stall is a situation of abnormal airflow resulting from a stall of the aerofoils within the compressor of a jet engine. Stall is found in dynamic compressors, particularly axial compressors, as used in jet engines and turbochargers for reciprocating engines.Compressor stalls result in...

(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 (carbon seals) can leak oil (containing highly toxic additives) 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.

Nevertheless, oil contamination from this and other sources within the engine bay is leading to serious health concerns, and has triggered research by a number of academic institutions.

External links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.