Case History No. 14

Industry and Location:

A utility station in the east

Source of Detection:

Control difficulties with five parallel operated primary air fans

Cause of Problem:

Improper selection of fan

Solution:

Primary air fans are used to induce hot gases from the Ljungstrom air heater and to discharge these gases into a coal pulverizer system. Ambient air is customarily blended with the hot air to control the temperature level of air to the pulverizers. Primary air fans are sometimes required to develop pressures as high as 60" to 70" w.g. A typical flow rate is approximately 50,000 ACFM.

The designers of utility plants always incorporate considerable safety factors in selecting the initial fans. However, in this particular instance it was determined that a long term coal contract was re-negotiated after the boiler was commissioned and a coal of higher BTU content was purchased. As a result of the improved coal quality, the primary air fans are required to handle considerably less volume but at the original design static pressure capability. Because of this reduced volumetric flow rate to the primary air fans, each primary air fan is operated at an undesirable point on the performance curve. Such operation at the left of the peak of the curve results in unstable control problems and excessive horsepower consumption.

Original Fan Curve

Solution:

Obviously, one solution is the replacement of the fans with new fans. A project of this type would involve a considerable capital expenditure. A more affordable approach is to modify the fan impeller so that the operation results at a more efficient point on the performance curve (right of the peak of the curve). This can be achieved by modifying the impellers during a scheduled outage and reusing the existing housing, bearing pedestals, and bearings.

Benefit:

Improved control and reduced horsepower consumption.

Primary Air Fan System