Case History No. 20



Industry & Location:
A utility station in the northeast

Problem:
Inability to operate two gas recirculation fans in parallel

Source of Detection:
Excessive vibrations at the fan foundations.

Cause of Problem:
Many utility boilers are equipped with gas recirculation fans which serve to recirculate hot gases into the furnace at a lower elevation. This recirculation procedure is used to maintain boiler efficiency at various boiler load conditions. In this particular case history, the boiler was equipped with two gas recirculation fans which were designed to operate in parallel. Unfortunately, because of excessive vibrations, the two fan operation could never be achieved and boiler performance suffered because of the inability to operate more than one fan.


Gas Recirculation Fan

Solution to Problem:
Each gas recirculation fan is of the radial tip design and is controlled with an outlet damper. No inlet dampers were originally utilized and the inlet duct consisted of a 90 degree elbow above each fan inlet box. Further, the volumetric flow requirements from the gas recirculation fans was reduced over the years because of changes in operation procedures. Obviously, three deficiencies were noted and these included: (1) a poor inlet condition, (2) operating the fan at an undesirable point on the performance curve, and (3) operating the fan with outlet dampers instead of inlet dampers.


Inlet Duct Configuration

Turning vanes were installed in the inlet elbow to the I.D. fans to eliminate turbulence and vortex shedding at the fan impeller inlet. Also, inlet box dampers were provided and the outlet dampers were locked in an open position. A further modification included the narrowing of the fan impeller to modify the performance curve so that operation could occur on the right hand side of the peak of the curve. These three combinations of improvement solved the problem totally.

Benefit:
The ability to operate the two gas recirculation fans in parallel and the improvement in boiler efficiency.

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
Return to the Home Page.