Case History No. 1

Industry & Location:

A utility station in the south

Problem:

Low Forced Draft (F.D.) Fan Efficiency

Source of Detection:

Excessive horsepower consumption and extremely loud noise problems.

Cause of Problem:

This boiler, equipped with two 7,000 horsepower F.D. fans of the airfoil design, was originally designed for pressurized service. Each fan is designed for a flow rate of 935,000 ACFM at 41"w.g. When the boiler was converted to balanced draft operation, the required fan pressure was lowered to 13" w.g.

Consequently, the F.D. fan dampers were kept partially closed to reduce pressure development; but, because the original design volumetric flow rate was still required, the inlet air velocity between the damper blades was very high. Extremely high inlet velocities to the fan impeller result in considerable turbulence and subsequent excessive horsepower consumption and low operating efficiency.

Partially Closed Vortex Damper

Solution:

One obvious solution is the purchase of new fans, but this decision has been deemed impractical because of the high capital cost. A more practical solution involves a modification to both F.D. fans which will allow them to be operated with the dampers in a more open position. The opening of the inlet vortex dampers will minimize the turbulence at the fan inlet and restore fan efficiency to near original design levels.

Further, both F.D. fans are installed inside room enclosures for sound attenuation purposes. The room is shown below. The walls divert the airflow entering the fan wheel and worsen the situation. In this case, low fan efficiency is a result of three factors; namely, room effects, poor initial fan design and operation of the fans at the extreme right hand side of the curve where poor efficiency cannot be prevented. Reducing the speed or diameter of these fans results in only marginal savings because an airfoil fan design is not practical for large volume and low pressure applications. Actually, the two fans need to be replaced with three fans but high initial cost discourages such action. The reduced wheel size action is affordable but results in only an insignificant savings.

Fan Room

Fan Scroll Modifications

Benefit:

300 horsepower saved per fan.