Convective Heating Surfaces

Abstract

After leaving the furnace the flue gas enters the convective section of the boiler, where it cools further by transferring heat to water, steam, and in some cases to the combustion air. The principal mode of heat transfer in this section is forced convection. Hence this section is called theconvective section.Also, since it is located at the back of the boiler, it is calledback-pass.Here the gas enters at about the furnace exit temperature and leaves at slightly above the stack temperature. In most utility boilers, the heat exchangers located in the convective section include the superheater, reheater, economizer, and air preheater. These are generally arranged in series. In some designs, the convective section may be split in two parallel paths with parts of the superheater and reheater located in each section. Here, the gas flow rate through individual sections can be controlled by means of dampers to adjust the superheat or reheat steam temperatures. This chapter will discuss design methods and operating issues related to the convective section.