Hot compressed air from the air compressor goes first to the supply tank. It’s also called a “wet” tank because as the compressed air cools, moisture tends to condense in the tank. To reduce the moisture, an air dryer may be inserted into the line between the air compressor and the supply tank. From the supply tank, the compressed air goes to the service tank (also known as a “dry” tank). In a dual-circuit air brake system, there are two service tanks: a primary and a secondary.