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TMC-647-(Rev E) - 01/06
MOORE FANS LLC, Marceline, MO 64658 Phone (660 ) 376-3575 FAX (660) 376-2909
4.0 OPERATION
4.1.1 ABOUT THIS SECTION . . . .
It is widely acknowledged that the kinds of mechani-
cal abuse described on the preceding pages are destructive
for all types of rotatingequipment. It is less well recog-
nized that — for fans — aerodynamic stresses are an even
more serious hazard. This section deals with the causes of
destructive aerodynamic stresses and how they can be
avoided.
Although this information is given primarily for the
benefit of operators of Moore equipment, it may be applied
to fans of any manufacture.
Unlike smaller fans, which are typically furnished
complete with their surroundings, the large fan wheel is
supplied as an unprotected component of the system and
is installed in innumerable types of surroundings. Not only
do the types and conditions of the drives for these fan
wheels vary widely, but the entrance and exit conditions
and the enclosure for the wheel assume a myriad of pos-
sible combinations. In designing a product, the manufac-
turer of fan wheels must anticipate the operating condi-
tions based upon his knowledge of what is reasonable and
customary for the industry. One may over-design for ab-
normal stresses only until the practical limit is reached, to
avoid excessive weight, cost and inefficiency.
4.1.2 NORMAL OPERATING CONDITIONS
The fan manufacturer assumes a fairly reasonable
atmosphere for the operation of his product, including the
following:
The fan selection will be reasonably in line with the
performance the unit is expected to maintain, with an
adequate blade area for the pressure required at the
given RPM. Blades will not be loaded beyond their
capacity to maintain air flow.
A fan ring will be provided that is round, rigid and of
a depth at least sufficient to cover the tips of the
blades. Tip clearances will be uniform and controlled.
The approach air will represent a relatively uniform
and axial flow with, of course, some unavoidable
turbulence expected. Adequate open area will be
provided at the inlet of the fan.
Major obstructions will not be present at either the
inlet or discharge of the fan.
The RPM of the fan will be within the design limits.
The relative direction and velocity of approaching air
to the blades will be fairly constant and protection
will be provided from extreme wind conditions.
Under such conditions, the unit stresses in the blades would
not be expected to vary more than plus or minus 50%. Fan
design based on such assumptions is entirely reasonable
and, with proper drives and installation conditions, has
proven highly successful.
4.1.3 ABNORMAL CONDITIONS
Abnormal operating conditions result in destructive
repetitive stresses that can seriously shorten fan life. The
aerodynamic abuses discussed in this section can cause
repeated flexing of the fan blades and hub. Violent dis-
placement of the resiliently mounted Moore fan blades
may occur — a greater displacement than would occur in
rigidly mounted blades. The resilient mounting, of course,
minimizes the structural unit stresses which would be
transmitted to the root of the blade and into the hub and
drive. Although Moore units may be expected to resist
greater stress than units of conventional design, such re-
petitive stresses may exceed the capability of the resilient
mounts to absorb them. If so, fatigue of the mounts and
metal may develop, adjusting linkages may wear, and
ultimate failure becomes a possibility.
Some of the abuses set out in the following text are
far less important than others. All of them may occur in
varying degrees.
Specifically, abuse due to serious repetitive stresses
can lead to mount failure and, if carried to extremes, can
cause clevis breakage and failure in the box section due to
the end of the box section repeatedly striking the clevis as
the blades are violently displaced. In units of other manu-
facture with rigidly mounted blades, repetitive stresses of
this type may lead to blade breakage, probably near the
root or at the point of attachment to the hub where stresses
are highest, or may lead to failure of the hub itself. The
resilient mount design, unique with Moore fans, dampens
these vibrational forces and results in a fan that is far less
vulnerable to failure from these conditions than other units
with rigidly mounted blades. Even so, extreme conditions
can cause damage.
A well-designed fan can be expected to operate for
many years without trouble under normal operation as
described above. The extreme repetitive stresses described
below, however, will certainly reduce the life of the fan,
causing failure many years sooner than would occur if the
fan were operated as intended. Fortunately, these destruc-
tive conditions are readily observable to someone who is
knowledgeable about them, and they can be corrected with
reasonable effort and expense once they are observed.
4.1 AERODYNAMIC ABUSE
OPERATION
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