5.9 Stacks

An adequate flow of air and combustion gases is required for the complete and effective combustion of fossil fuels. The term draft is defined as the difference between atmospheric pressure and the static pressure of the flue gases in the furnace, a convection pass, a flue, or a stack. Draft loss is the reduction in the static pressure of a flue gas caused by friction and other pressure losses, which are not recoverable, as the flue gas flows through a boiler system.

The flow of gases through a boiler can be achieved by creating draft by four methods: (1) natural draft, (2) forced draft, (3) induced draft, and (4) balanced draft. These are defined as follows:

1. Natural draft. The required flow of air and flue gas through a boiler can be obtained by the stack alone when the system draft loss is low and the stack is tall. In today s modern units, this is not a common method, since mechanical draft is required on boiler systems.

2. Forced draft. Operates with air and flue gas maintained above atmospheric pressure. A forced-draft (FD) fan at the inlet of the boiler system provides pressure to force air and flue gas through the system. Any openings in the boiler setting such as doors and penetrations allow air or flue gas to be released unless that opening is pressurized and thus has air being forced into the opening, thus preventing any release. A boiler designed with an FD...