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TMC-647-(Rev E) - 01/06
MOORE FANS LLC, Marceline, MO 64658 Phone (660 ) 376-3575 FAX (660) 376-2909
4.3 CAUSES OF IMPROPER BLADE LOADING
4.3.2 EXCESSIVE HORSEPOWER
Motors frequently are installed which develop a
greater horsepower than the fan can consume, usually
because a motor of the required horsepower is not avail-
able. For example, a fan might require 31 HP, which
eliminates the use of a 30 HP motor, forcing the selection of
the next larger motor of 40 HP. In the meantime the fan has
been selected to consume only 31 HP. After installation, it
is only logical for the operator to load the motor to the full
40 HP, which automatically places a much higher blade
loading on the fan, and may result in overload.
Actually, in selecting such an assembly, the fan should
be selected to consume the full 40 HP since it is very likely
that eventually the blade angle will be increased to con-
sume the full 40 HP, even though this amount of air was not
originally required.
Should such a situation arise, it is again necessary, in
some manner, to hold the applied horsepower within that
which can be consumed by the fan without blade overload.
4.3.3 EXCESSIVE TIP CLEARANCE
Unless the fan ring is very close to the tip of the blade,
air from the high pressure surface of the blade will flow
around the tip and nullify the negative pressure on the
underside of the blade for some distance in from the tip. For
a fan of, say, 12-ft diameter, the last 12 to 18 inches of the
blade could be producing no pressure whatever and per-
forming no useful function. The balance of the fan blade
toward the hub then must produce a higher pressure to
compensate for the portion near the tip.
Excessive tip clearance also leaves an unswept area
between the tip of the blade and the fan ring. Air that has
been pumped by the fan will return downward through
this unswept area at a velocity greater than that at which it
passed through the fan in an upward direction. This
condition adds even further to the requirements of the
portion of the blade which is doing the work and efficiency
will be greatly reduced.
With the loss of a foot at the tips of the blades, plus
the back flow between the tips and the ring, the 12-ft fan in
this example might be considered an effective 10-ft fan. It
would have to deliver sufficient air to satisfy the perform-
ance requirements of the installation, plus the amount of
air which is returning in the void between the tips and the
throat. Under such circumstances, excessive blade loading
could occur even though the required system pressure is
not achieved.
4.3.4 POOR ENTRANCE CONDITIONS
Air will approach the fan from all possible direc-
tions, increasing in velocity as it nears the opening, then
accelerating rapidly as it enters. The air approaching from
the side must be turned through 90
O
to enter a ring whose
entrance terminates in a flat plate. If the inlet end of the ring
projects some distance out, with approach possible from all
directions, a portion of the air must be turned through 180
O
.
The inertia of the approaching air prevents it from turning
sharply and advancing parallel to the desired flow. It
consequently swoops toward the center, leaving the outer
area of the fan with reduced flow or even reverse flow near
the ring.
The effect of poor entrance conditions is similar to
that previously described for excessive tip clearance in that
the effective diameter has been reduced and excessive
blade loading could occur even though the required sys-
tem pressure is not achieved. Efficiency will be greatly
reduced.
4.3.1 VARIATION FROM PREDICTED
CONDITIONS
Although those who design air coolers and cooling
towers undoubtedly do their best to accurately state the
calculated static resistance of the system, a number of
factors may cause the actual conditions to vary from the
design conditions. When a variation occurs, it may be
found, upon testing, that the static pressure for a given
volume through the system is higher than anticipated. In
this case, the number of blades and/or the motor horse-
power provided may be inadequate to meet the perfor-
mance. On the other hand, the static pressure may have
been overestimated and excess blade area provided, re-
sulting in a fan with unutilized capacity operating at low
efficiency.
Inadequate Blade Area: The blade angle is selected
to move the anticipated volume of air and the number of
blades is selected to maintain the total anticipated pressure
required to move this volume at a given RPM. If the static
pressure turns out do be higher than predicted, the fan
may then be operating in an overload condition. If the RPM
cannot be increased, the only solution to this condition is to
reduce the blade angle until the fan can carry the then
reduced volume at the originally anticipated pressure.
Since reducing the volume, while holding the total pres-
sure as originally anticipated, can only reduce the horse-
power, it is then impossible to consume the horsepower
originally intended without overloading the fan. This is
one of a number of reasons for providing some safety factor
in blade loading at the time of original fan selection.
Excessive Blade Area: Occasionally, an excessive
number of blades may be specified in the interest of mak-
ing a conservative selection. If the static pressure has been
overstated, the theoretical number of blades will be greater
than needed. This theoretical number of blades is usually
a fractional number and the actual number of blades used
must, of course, be the next larger integer, resulting in
some "safety factor" in the selection. If, in addition, a blade
or two is added as a "safety factor" or in anticipation of
increased future requirements, it may be impossible to
meet the original performance requirement efficiently. The
only way to provide the original performance and draw no
more than the original horsepower is to flatten the blade
angle. There is a limit, however, in how far the blade angle
may be reduced before further reduction will decrease
airflow without a further reduction in horsepower. For
belt drive units, the most practical solution to this problem
is to reduce the RPM of the fan.
OPERATION
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