Case History No. 2

Industry & Location:

A utility station in the south

Problem:

Low F.D. fan efficiency

Source of Detection:

Serious structural cracking of the fan housing, duct and foundation from excessive vibrations.

Cause of Problem:

Two 7,000 horsepower F.D. fans are installed with an inlet duct system which induces warm air from the penthouse area of the boiler. Extreme turbulence at the fan inlet is a result of the absence of turning vanes in the inlet ducts. This boiler was originally designed for forced draft operation and has since been converted to a balanced draft unit. Consequently, each F.D. fan is sized to develop approximately three times the needed static pressure. This case history is almost identical to Case History No. 1 except that the fan inlet turbulence is not caused by room design, but rather, by the turbulence in a 90 degree elbow upstream of the inlet boxes and the absence of turning vanes at the entrance to each inlet box.

Solution:

The most obvious solution is the installation of turning vanes to eliminate air turbulence and the replacement of the F.D. fans with ones which are properly sized to meet the boiler requirements. Obviously, a project of this magnitude involving the replacement of two 7,000 horsepower fans would cost in excess of $1,000,000.

An alternate proposal and one more worthy of consideration (since the payback would be quicker) is the modification of the fan impellers to develop the needed static pressure. Considerable savings can be realized by reusing the fan housings. In this particular case, the fan impellers can be replaced at a cost of less than $200,000 and the replacement impellers can be designed to meet the pressure and volume requirements of the balance draft operation. Because only minor modifications are required at the fan housing, the total installation cost of this proposal becomes very affordable. In fact, existing shafts, bearings and bearing pedestals can also be reused and the equipment replacement would occur during a planned outage when the unit would be out of service for two weeks.

The only disadvantage with this proposal is that the fan operation does not occur at an optimum point on the performance curve. However, while the replacement fan design efficiency would be 85%, this alternate approach yields an efficiency of 70% and a substantial horsepower savings. Existing spare bearings, shafts, and other auxiliary components are still compatible with the modified F.D. fans.

Benefit:

A horsepower savings of 3,000 horsepower per fan and the elimination of housing and duct pulsations.