How the Environmental Control System Works on Boeing Airplanes
1. Outside air continuously enters the engine where it is compressed.
2. The air may then pass through a catalytic ozone converter (used on high altitude polar flights) to the air-conditioning packs.
3. The air passes through cooling packs 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% outside and 50% 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, air is continuously exhausted from the airplane.