Case History No. 10

Industry & Location:

A pulp and paper mill in the south

Problem:

Inadequate primary air flow to a black liquor recovery boiler

System of Detection:

A desire to increase liquor burning capability by increasing primary air flow rate.

Cause of Problem:

A recovery boiler was originally equipped with a single fan to serve both the primary and secondary air wind boxes. Unfortunately, the secondary air wind box pressure requirement is 12" while the primary air wind box pressure requirement is only 3". A single fan providing air to both windboxes results in an obvious excessive pressure drop across the primary air wind box control damper. The present F.D. fan is equipped with a 1,000 horsepower motor which is loaded to 884 horsepower. Because the motor has a service factor of 1.15, an investigation was made to tip out the F.D. fan and utilize all of the service factor of the I.D. fan motor. The cost to tip out an F.D. fan of this type is nominal, and such a tip out procedure could result in an increase in volumetric flow of approximately 7 percent. However, an increase of only 7 percent would not provide the needed results requested by the operating engineers.

Solution:

It was decided to increase the primary air flow by the amount of 50 percent which could not be achieved by a simple tip out procedure of the existing fan. Because of the low pressure at the primary air wind box, it was decided to investigate the purchase of an additional fan for the primary wind box and to isolate the existing fan to serve only the secondary air wind box.

Initially, such a concept of purchasing an additional fan was met with considerable resistance by the operating engineers until it was realized that the horsepower requirements of the new fan would only be 176 horsepower. In fact, a modified impeller in the existing F.D. fan lowered horsepower consumption to 412 horsepower. By utilizing a two fan approach the total horsepower consumption has been reduced to 588 versus the 1,115 horsepower requirement of the proposed tip out procedure. Obviously, this reduction in consumed horsepower is a direct result of the elimination of the pressure drop at the primary air control damper. Although the cost to purchase a secondary air fan far exceeded the cost to tip out the existing F.D. fan, the reduction in horsepower consumption more than offset this capital cost. A project of this type involving the purchase of a new primary air fan is $150,000 and at $350 per horsepower per year, the payback period is rather short.

Benefit:

Increased liquor burning capacity at reduced fan horsepower.

Existing and Proposed Fan Arrangement