ESET-212 Lecture Note PDF

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Draf Definition: Draft is the difference in pressure between two points that causes air or gases to flow. The primary purpose of a draft system on a boiler is to supply sufficient air to complete the combustion process and remove products of combustion. To burn fuel completely, sufficient quantities of air with pressures high enough to overcome the resistance caused by breeching, dampers, baffles, tube passes, economizers, and superheaters are required. Approximately 15 lb of air is required to burn 1 lb of fuel.

Natural draft is produced by a chimney alone, without mechanical means. It is caused by the difference in weight between the column of hot gas inside the chimney and a column of cold outside air of the same height and cross section.

Mechanical draft is produced by power-driven fans. There are two types of mechanical draft: Forced draft is used when the fan or blower located in front of the boiler setting forces (pushes) air into the furnace. Induced draft is used when the fan or blower located between the boiler and chimney removes (pulls) gases of combustion from the boiler and discharges them into the chimney by applying negative pressure. Combination forced and induced draft use both forced and induced draft fans on large boiler installations. To maintain correct air-fuel ratios for optimum combustion efficiency, draft must be controlled at all times, especially during changing load conditions. Draft is controlled by the speed of the induced or forced draft fans, adjustment of inlet and outlet dampers, use of inlet vanes on the fan...