Case History No. 8



Industry & Location:
A pulp and paper mill in the south

Problem:
A requirement to increase liquor burning capability of a black liquor recovery boiler by increasing volumetric air flow to the secondary air windbox.

Source of Detection:
A desire to increase black liquor burning capability of a black liquor recovery boiler

Cause of Problem:
An existing 300 horsepower secondary air fan produces 12" w.g. pressure and discharges air through a steam coil air heater to the secondary air wind box. The steam coil air heater is used to preheat the air to 300 degrees F and because of its physical size, the operating pressure drop is 6 inches. In order to improve liquor burning capacity, it was decided to increase the volumetric flow rate to the secondary wind box from 125,000 ACFM to 175,000 ACFM. This represents an increase in volumetric flow capacity of 40 percent. It was also decided to increase the wind box pressure by 8" w.g. Since the pressure drop across the steam coil air heater is 6" w.g., the new pressure drop under the increased flow condition would become 12" since pressure drop is proportional to volumetric flow rate to the second power. The horsepower required for a single new secondary air fan is prohibitive.


Fan Proposals

Solution:
Initially, the operating engineers requested an engineering study to evaluate the replacement of the secondary air fan with one having increased capability to develop the needed 175,000 ACFM at a pressure of 28" w.g. However, this approach was evaluated as impractical because of the significantly increased size of the fan and the subsequent increased cost of associated electrical components. The horsepower increase would have more than tripled because of the excessive system pressure losses and the inefficiency of operating a steam coil air heater at an excessive pressure drop of 12" w.g.

A much more practical and economical approach to achieving the objective of increased flow to the secondary wind box is the purchase of an additional secondary air fan and steam coil air heater for installation on the opposite side of the boiler. The fan impeller in the existing secondary air fan was replaced with one developing more static pressure and slightly less volume since the housing was large enough to accommodate an increase in wheel diameter. The parallel fan approach results in a total horsepower requirement of only 600 horsepower compared to the 1000 horsepower requirement of the single fan proposal. This alternate proposal was offered at a capital cost of $350,000 compared to a $1,000,000 cost associated with the single fan proposal. Also, the down time required to install the second fan was less than eighteen hours compared to a down time of almost three days for the installation of the 1000 horsepower secondary air fan.

Benefit:
Increased liquor burning with two, 300 horsepower secondary air fans instead of one 1000 horsepower fan.


New Secondary Air Fan

Process Equipment, Inc.
P.O. Box 1607
2770 Welborn Street
Pelham, AL 35124
U.S.A.
Phone: (205) 663-5330
Fax: (205) 663-6037
E-mail to: information@process-equip.com
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