Posted by james w hanna on November 07, 2001 at 13:11:44:
A valued customer of ours has a large wood fired steam boiler . We were having repeat bearing failures on the OFA ( over fire air fan ) I had suggested that they insulate under the frame work to reduce some of the heat comming from the boiler . This reduced the OFA bearing temperature from about 350 degrees to under 200 degrees. I had also suggested the OFA fan speed be reduced , because the shaft speed was 2080 rpm's which was over the recommended rpm for this bearing when grease lubricated. My thoughts were the reduced speed would reduce the bearing temperature. We had noticed belt squeel on several ocasions . We checked the belt sheaves and found excessive wear on both belt sheaves.
The belt sheaves and belts were replaced. Through human error the new sheaves were put on the wrong location , They had the right sheaves ,but put wrong one on motor and wrong one on fan . Well this reduced the rpm from 2080 rpm's down to 1563 rpm's , which is about 33% speed reduction.
They changed the OFA damper setting to compensate . The problem I'm concerned with now is there is a resonance vibration on the FD ( Forced Draft Fan ) . Finally my question , could the reduced rpm on the OFA fan cause this resonance vibration on the FD fan
james w hanna
Certified Vibration Analyst
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