Case History No. 13

Industry & Location:

A pulp and paper mill in the midwest

Problem:

Poor control of a lime kiln operation

Source of Detection:

Inability to control temperature along the longitudinal length of the kiln

Cause of Problem:

Lime kiln I.D. fans are used to draw flue gases from the kiln and discharge these gases through a venturi scrubber system. System resistance in this case is a relatively constant factor since the system is comprised of a venturi scrubber (which operates at a fixed pressure drop), a plenum and very short duct section. As a result, regardless of lime production rate, the I.D. fan is required to develop a relatively constant static pressure which is impossible.

It is not uncommon to find dampers with some blades locked shut. Shown below are two mud lime kilns in the pulp and paper industry which are equipped with fans that almost always operate at a speed of 1180 RPM . For some reason or other, the industry has specified fans with considerably excess volumetric capability but with marginal static pressure capability. As a result, a universal problem exits throughout the pulp and paper industry wherein these fans are operated at a point on the left hand side of the peak of their curve. This represents an unstable point to operate a fan and also a relatively inefficient point.

Inlet Damper with Blades Locked Shut

Lime Kilns

Solution:

The first and most obvious solution is the total replacement of the lime kiln I.D. fan. A project of this type would require new foundations and considerable modifications to existing ductwork at a cost of more than $100,000. A more affordable and pragmatic approach is the replacement of the fan impeller with one which would operate more efficiently and would provide an operation on the right hand side of the fan performance curve. The modification to replace the fan impeller is quite practical and inexpensive.

In this particular case, the spare fan impeller was completely redesigned and rebuilt to provide the needed pressure requirement. Actually, the fan wheel was made narrower so that the volumetric capability was reduced and at the same time the wheel diameter was increased to give added pressure capability and improve control. The cost of this project was less than a third of the cost of a new fan.

Benefits:

The horsepower requirements were reduced from 950 horsepower to less than 600. Lime production was increased and improved control realized.