be armored with specially treated alloys that offer previously unheard of extended life in fly ash laden airstreams. Internal structural parts in these induced draft axial fans can be supplied in the abrasion resistant alloys previously mentioned. The single-stage and multi-stage axial fans have straightening vane sections that are exposed to the rigors of fly ash abrasion and erosion that must be armored for a long life.

A second major concern for operators is the reliability and maintenance of fan bearings. Bearing failures can be caused by contaminated lubrication, lube system failure, excess temperature, fan vibration, aerodynamic pulsations and foundation resonances. Large in-flight adjustable blade axial fans have both main shaft and adjustable blade bearings within the internal fan mechanism. Regular bearing maintenance is critical and is assisted by new predictive maintenance improvements. Additional bearing monitoring systems can be installed that will report on vibration

levels and bearing temperatures. External lubrication systems can be installed that will cool, heat, filter and add lubricant to fan bearings. Almost all older fans systems use friction-type sleeved bearings.

Tilting pad and anti-friction bearings have also been installed on some newer high-pressure fans. Analysis and monitoring of these bearing types is critical to reliable operation. Occasionally, a utility plant operator requires a bearing upgrade that will bring the fan manufacturer’s engineers into the design discussion. Because of the critical nature of these bearings in plant availability, plan to consult with the fan engineers at every opportunity.

Complex Control Systems

Coal-fired boiler fans were previously designed with constant speed operation and volume control that were affected by various dampers. The demand for higher efficiency fans along with peak load cycles have produced complex systems of fan control.

Variable speed electric motors and standby steam turbines are common in most new plant designs. The combination of daily peak load starts and the variable speed controls has drastically changed the number of load cycles for these once-single-speed fans. New calculations must be applied to these fan rotating elements so that premature stress cracks and bearing failures can be avoided.

Here, too, the combination of modifications to existing fan equipment over the years affects cycle fatigue life. The combinations of fan speed, fan control dampers and variable pitch fan blades are all used to keep fans at peak efficiency during variable load cycles. Variable speed motors can be retrofit to existing fans to extend their life span and position fans on maximum efficiency points for lowest cost.

 

Author: John Gray is National Sales Manager with TLT-Babcock Inc., a business unit of New York Blower..