MOORE FANS LLC, Marceline, MO 64658 Phone (660 ) 376-3575 FAX (660) 376-2909
Page 15
TMC-647-(Rev E) - 01/06
4.4 CHECKING BLADE LOAD
One method of checking blade load is to run a
complete field test on the fan. Although laborious, this
method will provide ample proof so long as neither exces-
sive tip clearance nor poor entrance conditions are present.
If either are present, however, the conditions set out above
under Section 4.3.3 would apply and the fan could be
overloaded even though the total pressure indicated by
the test was within the allowable blade loading.
A better, more convenient and simpler method of
detecting blade overload, or determining maximum al-
lowable blade angle, is set out below. The equipment
needed is a wrench, a torque wrench, a protractor, an
ammeter and a draft gauge (or manometer).
All fans are shipped with the blade angle set for the
anticipated performance requirements furnished to The
Moore Company by the purchaser. This blade angle is
called out on Section 1.1 Fan Specifications. This angle
refers to the angle of the top or bottom of the clevis with the
horizontal. Hubs are shipped with the clevises set at this
angle.
To start the test, adjust the blades to an angle of
approximately half that called out on the specifications or
measured on the units. Connect the draft gauge to as
quiescent a spot in the plenum as possible, preferably in
the corner of the plenum and either ahead of or following
the fan, depending upon whether the application is in-
duced or forced draft. Since the figures obtained are
purely relative, it is not necessary that accurate static
pressure readings be obtained, but rather that the readings
taken represent a consistent series of pressures at the point
of reading chosen.
Start the fan and record on the chart provided the
blade angle, the static pressure indicated, and the ampere
input to the motor. Advance the blade angle by one or two
degrees and repeat the measurement, recording again
these readings. Keep increasing the angle and following
this procedure until the motor is fully loaded, in which
case the fan is able to consume full motor horsepower
without overload OR until the curve which will have started
on a definite slope begins to approach the horizontal. It will
be noted that the static pressure will be consistently increas-
ing with increased blade angle until the blade loading
reaches maximum, at which point it will level off.
Subsequent increases in blade angle may have quite
different effects, depending on the individual installation.
The static pressure curve may merely stay level or may drop
off sharply. In rare cases, it may level off and again start
rising as the fan begins operating as a centrifugal blower.
Typical examples are shown in dotted lines on the
chart opposite. Operation beyond the first point of level-
ling, or in the area of the dotted lines, is indicative of blade
overload. Note that motor load will continue to increase
even though the fan has passed into overload condition. The
maximum blade angle allowable is that which produces a
static pressure about 5% below the point where the curve
becomes level. This represents a safe loading, and the
blades may be set and left at this angle regardless of the
location on the chart, assuming the motor is not overloaded.
(See note below.)
The point so selected will also approximate the point
of the most efficient operation of the fan. Due to possible
error in static pressure predictions, or in readings which are
intended only to be relative, as well as other variables, the
final blade setting chosen may fall below or above the
specified static pressure.
A typical performance chart is shown opposite for a
fan that was undersized for the motor. A blank chart is also
provided for your use.
Caution: Horsepower will vary with air density. If blade
angles are set to fully load the motor on a very hot day,
the motor may be overloaded when operating on a very
cold day. For this reason, it is good practice to set angles
when the temperature is moderate or adjust the setting
for the lowest anticipated temperature.
4.3.5 EXCESSIVE DEFLECTION
The pressure which the fan can achieve is depend-
ent upon the square of the velocity of the blades relative
to the air. If the air could be moved into the fan in an axial
direction and passed through the fan into the discharge
without changing direction, the relative velocity of the
blades to the air stream would be the true velocity of the
blades at any point. This, of course, is not the case. For the
blades to accomplish work upon the air, they must also
deflect the air in the direction of rotation of the fan. The
air when rotated with the fan is moving velocity in the
same direction as the rotation of blades and the air by som
portion of this rotational velocity.
Moore fans are designed in contemplation of a
maximum deflection of 50 degrees at the hub, decreasing
to a very small value at the tip. This deflection is consid-
ered in the determination of the pressure which may be
provided by each blade over its full length. If fans are
selected, or if conditions exist, which cause the deflection to
exceed 50
degrees at the hub, the velocity of the blades
relative to the air is less than anticipated and the blades will
not provide the rated pressure. The test below, however,
will show the full allowable pressure capability of the fan,
even though it does not reach the full rated pressure.
4.3.6 CONCLUSION
As can be seen by the various points discussed in this
section, there are a number of complex factors which tend to
cause fans to be operated in a condition of improper blade
loading which can shorten fan life or lower efficiency. When
blade angles are set to consume the specified horsepower (at
the fan shaft), the resulting performance should be very
close to the specified performance. If this is not the case and
the problem cannot be identified or conrrected, please con-
tact Moore for assistance.
OPERATION
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