Slow speed operation of fan systems within the air handling
industry is generally performed due to two reasons: a coast
down operation is required for hot (induced draft) fans to
cool down before shutdown (often by using a turning gear),
and operational efficiency improvements can be achieved
during non-peak periods by slow speed operation using a
VFD. In either case, when these fans are supported by hydrodynamic
bearings, it is the oil film thickness developed from
the bearing-shaft interaction that limits the minimum speed
that can be maintained without causing premature wear and
bearing failure. This paper will present a brief overview of
lubrication theory and critical design parameters to achieve
slow speed operation.
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