What is the purpose of the supply (wet) tank in an air brake system?

The SUPPLY TANK (also called the 'wet tank') is the FIRST reservoir that receives compressed air from the compressor. Because air from the compressor carries MOISTURE and OIL, the wet tank lets these contaminants COLLECT/settle out before the air passes on to the service (dry) reservoirs that operate the brakes. CDL air brakes. This is why it's called the 'wet' tank — water and oil accumulate there. The wet tank must be DRAINED regularly (it collects the most moisture). Many systems have an air dryer to remove moisture before the wet tank. Knowing the supply/wet tank's role (first reservoir, collects moisture/oil before service tanks) is commonly tested air brake system knowledge.