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The role of pressure vessels and expansion vessels in a heating or cooling system is to act as a method of compensating for fluid volumes. With water systems, fluids flowing through the circuit will change in volume as the temperature changes. The fluids expand with higher temperatures and contract with lower temperatures. The pressure of the entire system, however, has to be maintained within strict upper and lower limits for optimal operation. Both expansion and pressure vessels are usually made of drawn and welded steel. The exterior surface is treated with an anti-corrosion compound, while a diaphragm is installed within the tank interior and effectively divides the interior space into two. Water on one side of the diaphragm is connected via pipework to the heating and cooling system. When the heating system is operational, the water within the system expands. Too great an expansion of water volumes could damage the system’s pipework, so excess volumes are directed to the tank. Water cannot be compressed, and older heating systems could double pressures within the pipework, causing some catastrophic failures. With an expansion tank, the water will accumulate by compressing the air present on the other side of the diaphragm. If a tap in the system withdraws the water, or the temperature falls and the water contracts, the air will push the water out of the tank to resume circulating around the heating or cooling system. Such vessels are a vital part of relieving excess pressures in domestic, commercial and industrial heating and cooling systems.