The drum is the main components in natural circulation evaporation equipment, it and  water-cooling wall, downcomer,tube and header from the evaporation equipment. Its main functions are: 

1. Connecting water-cooled pipe, down comer as well as the header to form natural circuit and at the same time receiving the water supply from the economizer to feed saturated steam to the superheater. Therefore, the drum is the connection point of three procedures of heating, evaporation and overheating.

2. In the drum a certain amount of water is stored, it has a certain heat storage capacity. When the working conditions change, a part of heat energy can be released and absorbed, thus reducing the changing speed of steam pressure. 

3. Inside the drum, the steam-water separation device is provided to purify the steam.

4. The dosing device is used for water treatment inside the boiler and for improving the quality of the steam turbine. 

5. The blowdown device can reduce the saltness of boiler water. Drum is the cylindrical pressure vessel for collecting the boiler water and saturated steam, the cylinder is made of welded and connected steel plates. The oval manhole door is provided at the sealing head, in order to facilitate installation and maintenance. Inside the drum, the water supply, steam-water separating device, dosing and blowdown device are provided, and the accessories of pressure gauge, water level gauge and relief valve are equipped. Outside of the drum there are many different sockets for connecting all kinds of pipes, such as the water supply pipe, water-cooling pipe or the inlet pipe of water-cooling pipe, down comer , outlet pipe of the saturated steam and continuous blowdown pipe, emergency water draining pipe and recirculation pipe, and other pipes connecting some instruments and automatic devices. The size, material and wall thickness of the drum differ depending on the differences of boiler capacity and parameters.