Originally Posted by
visibility3miles
JRBarret - How much moisture is there in the heated engine bleed air?
Not much at altitude. Even so, with a full ACM that can cool the air using a compressor driving an expansion turbine, there can be a significant amount of water that condenses at the outlet of the turbine, and which is removed by a centrifugal water separator. That is mostly an issue on the ground when the ambient air is very humid.
At altitude, the cabin air supply tends to be extremely dry. Many people tend to have sinus problems when flying for hours in a pressurized cabin. Some aircraft (primarily airliners) have systems to humidify the cabin air.