What does the term 'wet tank' mean in an air brake system?

The 'wet tank' (also called the supply tank or supply reservoir) is the first air tank that the compressor pumps into. It collects moisture and oil that are picked up during compression of the atmospheric air. Air contains water vapor, and compression causes some of that vapor to condense into liquid water, which collects in the wet tank along with any oil mist from the compressor. The wet tank protects the rest of the system by trapping these contaminants before air moves on to the service brake tanks. Drivers should drain the wet tank daily (some have automatic drain valves) to prevent water from freezing in winter or corroding the system in any weather. The wet tank is not for cooling water; commercial trucks have separate cooling systems.