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The draft (draught in British English) flow rate induced by the stack effect can be calculated with the equation presented below. [12] [13] The equation applies only to buildings where air is both inside and outside the buildings. For buildings with one or two floors, h is the height of the building and A is the flow area of the openings.
Forced draft: When air or flue gases are maintained above atmospheric pressure. Normally it is done with the help of a forced draft fan. [2] Induced draft: When air or flue gases flow under the effect of a gradually decreasing pressure below atmospheric pressure. In this case, the system is said to operate under induced draft.
Modern industrial boilers use fans to provide forced or induced draughting of the boiler. Another major advance in the Rocket was large numbers of small-diameter firetubes (a multi-tubular boiler) instead of a single large flue. This greatly increased the surface area for heat transfer, allowing steam to be produced at a much higher rate.
Induced draft – A mechanical draft tower with a fan at the discharge (at the top) which pulls air up through the tower. The fan induces hot moist air out the discharge. This produces low entering and high exiting air velocities, reducing the possibility of recirculation in which discharged air flows back into the air intake.
The third method is by simply using an induced draught fan (ID fan) which sucks flue gases out of the furnace and up the stack. Almost all induced draught furnaces have a negative pressure. Forced draught: draught is obtained by forcing air into the furnace by means of a fan (FD fan) and duct-work.
Tubular preheaters consist of straight tube bundles which pass through the outlet ducting of the boiler and open at each end outside of the ducting. Inside the ducting, the hot furnace gases pass around the preheater tubes, transferring heat from the exhaust gas to the air inside the preheater.