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What is an Environmental Control System?

By J.M. Densing
Updated May 23, 2024
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An environmental control system uses various components to manipulate the environment in a closed area, which is particularly useful when trying to provide high air quality. These systems are typically used on airplanes to help provide comfortable, safe surroundings for both the crew and the passengers. It can control the temperature, filter it to ensure high air quality, control the humidity, and maintain a safe cabin pressure.

The environmental control system constantly circulates air inside a cabin of an airplane. The air being breathed by passengers and crew is a mixture of treated outside air, and recirculated, or re-used cabin air. Recirculating some of the air helps to save energy, and allows it to keep moving through the cabin at a faster rate. Air is brought into the airplane through intakes located on the plane. It is then pressurized and treated, then mixed with an equal amount of recirculated air, and blown through the cabin. After traveling through the cabin, about half of the air is exhausted to the outside, the other half is reserved to be re-used again, and the cycle continues.

One of the processes the cabin air undergoes is pressurization. This is extremely important in order for the people in the plane to be able to breathe properly since the air pressure at a typical flight altitude is much different from the pressure on the ground. Without systems in place, lungs do not function fully. To compensate for this, the environmental control system pressurizes the air in compressor stages located in the jet engines so that the air inside the airplane is approximately the average pressure that it is on the ground.

The pressurization process leaves the air very hot, so the environmental control system needs to cool the air. The air is initially cooled in heat exchange units that are located in the jet engines. Then, it is cooled even further by the cold outside temperatures as it travels through ductwork on the wings. Finally, if needed, it is cooled in air conditioning units before entering the airplane cabin.

After leaving the air conditioning units, the air is mixed with filtered recirculated air from the cabin. Humidity is adjusted. The air is normally very dry at high altitudes, which is preferable to humid air in an airplane for several reasons. Reasons include the fact that humidity can cause damage and corrosion to the plane's delicate instrumentation, and it can also encourage the growth of bacteria. The environmental control system maintains humidity at about ten percent, which is better for the plane's systems, and healthier for occupants.

Any air that is going to be recirculated is passed through High Efficiency Particulate Air (HEPA) filters as it leaves the cabin. The HEPA filters remove virtually all particles from the air that will be re-used by the environmental control system. Bacteria and viruses are also removed and trapped by these filters, which is helpful to prevent the spread of illness between passengers. The filtration helps to maintain a healthy and safe environment for all of the plane's occupants.

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