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.

FANS

Fans are used extensively in the heating and ventilating industry and in most power plants. Their basic design principles fall into two classes: axial-flow fans and centrifugal- or radial-flow fans. Axial-flow fans are basically rotating air-foil sections similar to the propeller of an airplane.

The simplest axial-flow fan is the small electric fan used for circulating air in rooms against very little resistance. Axial-flow fans for industrial purposes are the two-blade or multiblade propeller type, and the multiblade air-foil type. Air enters the fan suction from the left and flows over the rotor with a minimum of turbulence owing to the streamline form of the rotor and drive mechanism. The air stream is straightened by guide vanes located on the discharge side, thus decreasing the rotational energy of the air by converting it to energy of translation.

The axial-flow fan operates best under conditions where the resistance of the system is low, as in the ventilating field. The axial-flow fan occupies a small space, is light in weight, is easy to install, and handles large volumes of air

Centrifugal fans may be divided into two major classes: 1) the long-blade or plate-type fan, and 2) the short-blade multi-blade fan. The blades of either type may be pitched toward the direction of motion of the fan, radially, or away from the direction of motion of the fan.

A plate-type radial-blade rotor with double inlet is best suited for handling dirty gases, since there are no pockets in the blades to catch and collect the dirt. The rotor has wearing strips welded to the blades to increase their life. The fan is designed for induced-draft service. The housing of such a fan may have catch plates in the scroll face to collect the fly ash.