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I. Concept of an expansion tank An expansion tank is an important component in hot water heating systems and central air conditioning water systems; its function is to accommodate and compensate for the expansion and contraction of water within the system. The expansion tank is generally installed at the highest point of the system, usually connected to the return main pipe near the suction inlet of the circulation pump (the chilled water circulation pump in central air conditioning systems). An expansion tank is a container welded from steel plates, available in various sizes. The expansion tank is typically connected to the following pipes: 1. Expansion tube: It channels the increased volume of water in the system due to heating expansion into the expansion tank (connected to the return water pipeline). 2. Overflow pipe: Used to drain excess water in the water tank that exceeds the specified level. 3. Signal tube: Used to monitor the water level in the tank. 4. Circulation pipe: Used to circulate water when freezing may occur in the water tank and expansion tube (located at the center bottom of the water tank, connected to the return water main). 5. Waste pipe: Used for discharging waste. 6. Water supply valve: Connected to the float inside the tank; it opens to supply water when the water level falls below the set value. For safety reasons, no valves are allowed to be installed on the expansion tube and the overflow tube. II. The main functions of an expansion tank in the system: 1. Expansion, providing room for the fresh water in the system to expand when heated. 2. Replenish water by making up for the amount lost in the system due to evaporation and leaks, and ensure that the fresh water pump has sufficient suction head. 3. Exhaust, the air in the emission system. 4. Chemical dosing: Adding chemical agents to carry out chemical treatment of the cooling water. 5. Heating: If a heating device is installed, it can be used to heat the cooling water in order to warm the cylinder. Simply put, the function of an expansion tank is threefold: to replenish water, to maintain pressure, and to hold the expansion water. III. Detailed Explanation of the Function of the Expansion Tank 1. It serves to absorb the excess water resulting from thermal expansion in the system. 2. Set a constant pressure for the system (the lowest water level in the tank should be 1–2 meters higher than the highest point in the system). 3. Leakage in the supplementary system. Heating or air conditioning systems should generally be equipped with a high-level expansion tank whenever possible; if conditions are limited, a pressure regulating tank or a variable-frequency water pump can be used to supply water at a constant pressure. The expansion tank is used in closed water circulation systems to balance the water volume and pressure, thereby preventing the safety valve from opening frequently and the automatic water refill valve from adding water repeatedly. In addition to serving as a container for expanded water, the expansion tank also functions as a water replenishment tank. By filling it with nitrogen, a larger volume can be created to hold the expanded water. High-pressure and low-pressure expansion tanks can utilize their own pressure to supply water to the pressure stabilization system in parallel. The control at each point of this device operates through interlocked reactions, enabling automatic operation with minimal pressure fluctuations. It is safe, reliable, energy-efficient, and offers good economic benefits.