Boiler Principles (I)
A boiler is a heat exchange device that generates steam or hot water. Its external manifestation is the conversion of heat into steam, while its intrinsic essence lies in transforming the reaction energy contained in fuel into usable thermal energy. To gain a comprehensive understanding of boiler equipment, boilers can be classified from multiple dimensions.
In terms of evaporation capacity, boilers are categorized into small, medium and large types. Classified by steam pressure grade, low-pressure boilers operate at a pressure of no more than 2.5 MPa; medium-pressure boilers work at approximately 3.9 MPa; high-pressure boilers have a pressure of no less than 10.8 MPa; and superhigh-pressure boilers run at a pressure of no less than 14.7 MPa. Based on fuel types, boilers can be divided into coal-fired, oil-fired and gas-fired boilers.
According to combustion modes, boilers are classified as grate-fired furnaces, suspension-fired furnaces, fluidized bed furnaces and cyclone furnaces. Grate-fired furnaces are commonly seen in laboratory settings, where fuel burns on the furnace grate. In suspension-fired furnaces, fuel burns in a suspended state inside the furnace chamber, which is further split into dry-bottom pulverized coal furnaces and wet-bottom pulverized coal furnaces. Fluidized bed furnaces realize fuel combustion within a fluidized bed.
Circulation mode serves as a vital classification criterion for boilers, which are mainly divided into natural circulation boilers and forced-flow boilers. Equipped with a steam drum, natural circulation boilers generate circulating driving force through the density difference of working fluid in downcomers and risers, making them the mainstream structure for boilers operating under subcritical pressures. Forced-flow boilers rely on water pumps to drive the circulation of working fluid, including once-through boilers without steam drums that operate fully dependent on water pumps, as well as combined circulation boilers and controlled circulation boilers, which are applicable to subcritical and supercritical working conditions.
The complete production and operation process of boilers falls into two major categories: essential processes and harmful processes. The core objective of studying boiler principles is to suppress or even eliminate all kinds of harmful processes, ensuring that essential processes can proceed continuously in an economical and efficient manner.
Essential processes are subdivided into primary processes and auxiliary processes. Combustion, evaporation, heat transfer and fluid flow constitute the four core primary processes of boiler operation. Coal pulverization, ash and slag removal, operation regulation, chemical water treatment and steam purification are auxiliary processes that guarantee stable boiler operation.
Harmful processes are grouped by their occurrence areas into working fluid side and flue gas side. Problems such as salt entrainment, sedimentation and corrosion occur on the working fluid side. On the flue gas side, adverse phenomena including equipment abrasion, flue dust accumulation and furnace slagging tend to take place. Both types of harmful processes will damage equipment and reduce boiler operating efficiency, hence they are key prevention and control targets in boiler design, operation and maintenance.
