Induced-Draft Fans
Sometimes a chimney is too small to provide enough updraft to con- vey the products of combustion from the boiler to the outside atmosphere. When it is evident that natural venting will be inadequate, a mechanical draft inducer can be used to increase the capacity of the chimney. These devices are used with both gas- and oil-fired boilers.
The two principal types of induced-draft fans are (1) direct-fan draft inducer and (2) induced-flow draft inducer. When a direct-fan draft inducer is used, the gases are drawn through an inlet located on the side, top, or bottom of the unit and discharged as shown in Figure 15-80. The moving parts of an indirect fan draft inducer (Figure 15-81) are located outside the path of the hot flue gases. Cool outside air is introduced through the blower section of the unit. This creates suction at the throat of the inducer body, which, in turn, increases the flow rate of the flue gases.
The draft created by the induced-draft fan should closely match the demand. If the size of the draft inducer is correctly estimated, any dilution and excessive cooling of the flue gases (a condition that may result in condensation) will be greatly reduced. Many boiler manufacturers recommend the size draft inducer to be used with specific boiler models and provide data for making the appropriate calculations. Manufacturers of draft inducers are similarly helpful.