How the Environmental Control System Works on Boeing Airplanes
1. Outside air continuously enters the engine where it is compressed.
2. The hot, compressed air then passes to air conditioning units where it is cooled. (On airplanes flown on routes known to encounter ozone, the compressed air passes through an ozone converter before reaching the air conditioning units.)
3. The air passes from the cooling units to a mixing manifold .
4. Outside air entering the mixing manifold is mixed with recirculated air that has been cleaned with high efficiency filters. The makeup of air in the mixing manifold is approximately 50 percent outside and 50 percent filtered recirculated air.
5. Air from the mixing manifold is then supplied to the cabin on a continuous basis from overhead outlets. Although this cutaway only shows the distribution system to the rear of the plane, it is the same throughout the cabin.
6. As outside air enters the airplane, an equal amount of air from the cabin is continuously exhausted from the airplane.