Compressed air leaves the compressor hot and saturated with water vapour. The first job of any air treatment train is to cool that air and knock out the liquid water that condenses. That is the role of the aftercooler and the water separator.
How the aftercooler works
The aftercooler lowers the discharge air temperature, typically to within 5–10°C of the cooling medium. As the air cools, water vapour condenses into liquid droplets. Removing this water early reduces the load on the dryer and protects the rest of the system from liquid carryover.
The water separator
A separator uses centrifugal force or coalescing media to remove the condensed droplets and any liquid oil. It must be drained, either manually or automatically, because a full separator sends water downstream where it can damage dryers and tools.
Placement in the train
The correct order is compressor, aftercooler, water separator, then filters and dryer. Skipping or reversing these stages overloads the dryer and shortens its life. Most dryers require the aftercooler and separator to work at all.