What are the functions of the upper and lower drum of a boiler, and what is the role of the convective tube bundles between them?

The drum is a cylindrical vessel in natural circulation and multi-stage forced circulation boilers that receives the feed water from the economizer, connects the circulation circuit, and delivers saturated steam to the superheater. The drum shell is made of high-quality thick steel plates and is one of the heaviest components in a boiler. The main function of the boiler drum is to store water, separate steam from water, and remove salts and sediment from the boiler water during operation, thereby preventing boiler water containing high concentrations of salts and impurities from entering the superheater and turbine along with the steam. The internal equipment of the boiler drum includes steam-water separation and steam cleaning devices, feedwater distribution pipes, sludge removal and chemical dosing equipment, etc. The function of the steam-water separation device is to separate the saturated steam coming from the water wall from the water, and to minimize the amount of fine water droplets carried in the steam. In medium and low-pressure boilers, baffle plates and slotted baffle plates are commonly used as coarse separation elements ； In boilers above medium pressure, in addition to the widespread use of various types of cyclone separators for preliminary separation, louver screens, wire meshes, or vapor equalizing plates are also employed for further separation. The boiler drum is also equipped with monitoring and protection devices such as a water level gauge and a safety valve.

The main function of the boiler drum is to store water and separate steam from water, removing salts and sediment from the boiler water, thereby preventing boiler water with high concentrations of salts and impurities from entering the superheater and turbine along with the steam. The convective tube bundles between them are primarily used to absorb the convective and radiant heat in the furnace, thereby enabling continuous circulation of steam and water.