Page 229 - Improving Machinery Reliability

200 Improving Machinery Reliability

C+D=F
A + B = F tan 9
A-B=-TA
2A = Ibs A= lbs
B= lbs
C= lbs
D= Ibs
F'
=
Rated tangential load per helix = * - Ibs
2
F'
Service factor with maximum thrust = - largest force =
2

Evaluating Cooling Tower Fans and Their Drive Systems

Over the past decades, cooling tower fans have been designed and put into service
with diameters exceeding 30 ft. More often than not, the vendor is simply extrapolat-
ing his past design by going from 26 ft to 28 ft, 30 ft, or even larger diameters.
Extrapolation is generally synonymous with simple scaleup of representative dimen-
sions. At other times, only the blade length is increased and the blade hub or blade
internals are left untouched.
Many of these extrapolations have resulted in costly failures risking extended
downtime or injury to personnel. Detailed design reviews are appropriate and the
following items represent a cross section of topics.

1. The dynamic natural frequency of cooling fan blades should be at least 20%
away from the fan rpm and its multiples.
Background: Force amplification resulting from coincidence or near-coinci-
dence of blade natural frequency and forcing frequency has caused catastrophic
blade failures in many cooling tower installations. Figure 3-81 shows one such
event.
2. If urethane-foam filter material is used in constructing fan blades, the vendor
should submit data showing dynamic natural frequency of blades after urethane
filler material loses intimate bonding, Le., delaminates or separates from blade-
skin interior surfaces. Alternatively, the vendor should submit proof that delam-
ination or separation between filler and blade-skin interior will not occur with
his design.
Background: Loss of bonding has been experienced on a large number of
blades. The resultant lowering of the blade dynamic natural frequency may
cause coincidence or near-coincidence of blade natural frequency and forcing
frequency.

-.
*F' = actual tangential driving load times lowest calculated service factor (service factor powbly per
manufacturer's nameplate).