MECHANICAL DRAFT

In power-plant engineering the fan plays an important part. Generally, in small-furnace, installations a stack can produce a draft sufficiently high to supply air adequately to the fuel bed and to remove the flue gases. But the present-day capacities of boilers and furnaces require mechanical draft to supplement the natural draft produced by the stack. Mechanical draft is divided into two systems: forced draft and induced draft. In the forced-draft system the fan is located on the air-intake side of the furnace. A positive pressure, a pressure above atmospheric pressure; is produced under the fuel bed and acts to force air through the bed. The forced-draft system is necessary in installations where the pressure drop in the intake system and fuel bed is high. The pressure drop will be high in installations employing air preheaters and/or underfeed stokers. The underfeed stoker has an inherently deep fuel bed and a correspondingly high resistance to air flow.

Generally, the pressure in a furnace should be slightly less than atmospheric pressure. If it is too high, there will be leakage of asphyxiating gases into the boiler room and the tendency for blow-back when furnace inspection doors are opened. If the pressure in the furnace is too low, there will be air leakage to the furnace with a corresponding reduction in the furnace temperature. Because of these restrictions on the desirable pressure within the furnace, the forced-draft system is generally accompanied by a natural-draft system, in order that the removal of the flue gases may be accomplished. However, if the stack draft is inadequate owing to the high resistance created by the furnace passes, economizers, and air preheaters, an induced-draft system is generally added to supplement the stack draft. In the induced-draft system a fan is placed in the duct leading to the stack.

When a forced- and an induced-draft fans are used in combination the system is called balanced draft. The forced-draft fan produces a positive pressure which decreases slightly through the duct work and sharply through the air preheater and fuel bed. If the system is properly controlled, a pressure of a few hundredths of an inch of water less than atmospheric pressure is maintained in the furnace proper. The pressure continues to drop through the boiler passes, economizer, and air preheater until it is raised by the induced-draft fan and by the stack to atmospheric pressure.

The present trend is to construct more furnaces with gas-tight casings in order that they may be operated under pres­sures well above atmospheric pressure. Combustion efficiency is improved at elevated pressures, and the induced-draft fan with its high maintenance cost can be eliminated completely. A number of furnaces using the cyclone burner are now designed to operate at pressures as high as 80 in. of water above atmospheric pressure.