APU

When air is compressed, the compressor takes in water vapor with the air, compresses the water vapor & puts it into the system as liquid water.
Any water that accumulates in the vehicles air lines damages seals, valves & removes lubricants. In cold weather it can freeze blocking air lines & damaging air system components.

The Air Dryer protects the vehicles air systems & components from this by drying the air passing through the air tanks and into the system via the desiccant cartridge. A coalescing cartridge has additional material to
trap oil & aerosol particles. Water is then removed from the desiccant via a backflow of dried expanded air.