Some compressed air auto drains work on the float principal. As water and contamination accumulates in the bottom of the bowl, it will ultimately lift a float. The compressed air, under pressure in the bowl, will then vent to atmosphere through the drain opening, blowing the water and debris from the bowl as it does.
A valve used to drain off material that has separated from a fluid or gas stream, or one used to empty a process line, vessel, or storage tank.
Manual: Manual drains are valves that are opened manually to dump condensation. Some operators choose to leave the valve partially open all the time. This method will drain condensation constantly, but also wastes compressed air by allowing a continuous leak.