Expansion tank

An expansion tank is the header tank of a closed cooling-water circuit that takes up the volume change of the coolant between cold and hot operating temperatures and maintains a static head at the circulating pump suction. Mounted at the highest point of the loop, it vents entrained air and combustion gas, carries the chemical dosing and make-up connection, and holds a low-level alarm. On central cooling systems it serves the low-temperature and high-temperature fresh-water loops; the same function on the engine jacket circuit is the coolant expansion tank.