Pressure Drop in Gas and Air Ducts

Abstract

The purpose of a boiler draft system is to provide the air needed for the combustion of fuel and to discharge the flue gas produced in the furnace. In early days, gas passages in boilers were large. As a result the pressure drop owing to the flow of fluids through these passages was small. So most boilers did not need any external means for moving the fluid through the system. As with an open fire, air was driven through the fuel grate and the boiler furnace simply by the buoyancy force of the hot flue gas. This is called natural draft. Modern boilers are much more compact. So the resistance through the system is too large for the natural draft to overcome. A fan is required to force the gas through the system. This type of system is, therefore, called farced or mechanical draft. Figure 16-1 shows the schematic of a typical draft system of a modern power boiler. Air from the atmosphere is drawn by forced draft fans and delivered to the air heater. This air passes through the air heater and coal mill. It then enters the furnace, where the fuel burns to produce the flue gas. The flue gas passes through back-pass heat transferring sections, air heater, and air pollution equipment. Finally, an induced draft fan draws the gas and releases it to the atmosphere through a tall chimney. A reasonable assessment of the magnitude of the pressure drop through the system is essential to specify the fans. Furthermore, in most fans the discharge volume varies with the imposed resistance. Thus a knowledge of the system resistance is also important for the assessment of the flow through the entire boiler draft system. The present chapter describes different draft systems and presents methods for calculation of the system resistance. 16-1 Draft Systems In a modern boiler a large number of heating surfaces are located in a very compact space. As a result a mechanical means, i.e., fan, is required to overcome the flow resistance in its compact flow passages. In natural draft systems the buoyancy force carries the gas through the system. The mechanical draft uses a fan to draw the air and gas through the system. Mechanical draft can be one of three types: negative, positive, and balanced draft. A forced P. Basu et al., Boilers and Burners