The bag-type expansion tank is mainly composed of the gas cap, the air charge port, the airbag, the carbon steel tank body, and the flange plate. When it is connected to the water system, it mainly serves as a energy storage device. When the water pressure in the system exceeds the nitrogen pressure between the carbon steel tank body and the airbag of the expansion tank, the water in the system will be squeezed into the airbag under the action of system pressure, which will compress the nitrogen between the tank body and the airbag, reducing its volume and increasing its pressure; at the same time, it will increase the entire water capacity of the system, reducing the system pressure until the water pressure and the nitrogen pressure between the tank body and the airbag reach a new balance. When the water pressure in the system is less than the gas pressure in the expansion tank, the water in the airbag will be squeezed out under the action of the nitrogen pressure between the tank body and the airbag, returning to the system, reducing the system water volume and increasing the pressure, and the volume of the nitrogen between the tank body and the airbag will increase, and the pressure will decrease until both reach a new balance, and the water will stop being squeezed back into the system. The stainless steel expansion tank plays a role in regulating the pressure fluctuations of the system.