Case History No. 9

Industry & Location:

A utility station in the west

Problem:

Inadequate volumetric flow from two primary air fans to five coal pulverizers.

Source of Detection:

Reduced megawatt generating capability.

Cause of Problem:

Two primary air fans (operated in parallel) provide heated air from a Ljungstrom air heater to five individual coal pulverizers. Each fan is equipped with an inlet vortex damper and an outlet butterfly damper. For unknown reasons, the outlet damper was used as the control damper while the inlet damper was maintained in an open position.

Primary Air Fans

Damper Operators

The probable explanation for using outlet damper control is that the fan efficiency is inordinately low because of the angular direction of the inlet duct. The absence of turning vanes in the inlet elbow causes turbulence and subsequently reduced fan efficiency. It was probably decided during start-up procedures that more stable control could be achieved with the modulation of the outlet damper. Because of the motor overload conditions as a result of poor efficiency, the fans were operated in a throttled mode with less than adequate flow rate to the five pulverizers.

Fan Inlet Duct

Solution:

Since the fan motors are operating at the maximum amperage condition, it was obvious that any increased flow rate could only occur by increasing fan efficiency and allowing the dampers to be operated in a more open position. Such an efficiency increase is achievable by installing turning vanes in the upstream elbow and minimizing the turbulence and vortex shedding to the fan impeller inlet. Also, it was decided to begin controlling the fans with the inlet vortex dampers instead of the outlet butterfly dampers. These changes resulted in more ultimate air flow to the fifth pulverizer.

Benefit:

Reduced horsepower consumption and increased volumetric flow to the pulverizer system.