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1) Fan Components: The fan hub is a component to connect
the fan blades and to be mounted to the low speed shaft
of gear reducer. The fan hubs must be enough strong
to overcome the air load imparted onto the fan blades
and the centrifugal force due to the rotation of the
fan blades. The fan blades have a certain neck to be
connected to the fan hub and the effective length of
fan blade is reduced as much as the radius of fan hub
and the length of fan blade neck. Therefore, the air
is not delivered through such fan hub and blade neck
and the air pressure at this area is less than the fan
blade surface. The air delivered through the fan blades
flows back through the area of fan blade neck, where
the air pressure is relatively lower than the fan blade
area. To prevent this back flow (called recirculation)
the aerodynamic seal disk is mounted onto the fan hub.
In general, the diameter
of seal disk is about 20 to 25% to the fan diameter.
If the hub is too large for the required performance,
the result will be an increase in the velocity pressure
due to the smaller net opening, and subsequent waste
of power. If the seal disk is too small, the result
will be deterioration of the flow near the fan hub,
possibly even a reversal of flow in this area.
2) Fan Coverage: For an even
air suction from the drift eliminator section and to
have a smooth entrance of air into the fan, the fan
coverage must not be smaller than 30% of the cross sectional
area of cell. Less fan coverage than 30% will returned
to a poor intake from the entire drift eliminator section.
Therefore, the overall performance of cooling tower
will be dramatically reduced.
The fan diameter affects
the performance of fan primarily because the magnitude
of the velocity pressure depends on the fan diameter.
The pressure capability of the fan could be changed
by changing the number of fan blades, but the fan must
be rated to overcome more static pressure, which is
a cooling tower system resistance, as having less velocity
pressure with keeping a low air velocity through the
fan.
General speaking, the velocity
pressure through the fan should be within 0.14 to 0.25
inch Aq. or the air velocity should be raged within
1600 to 2100 FPM for the optimum rating of fan.
3) Fan Sizing: The major
factors in deciding the number of fan blades are as
below:
(1) Blade Strength
| There
is a limit of blade strength in bearing the torque
or horsepower. In case of Hudson Products Corp.,
the maximum and Trouble Free BHP/Blade by the
fan diameter are as follows;
|
Fan Dia. |
Max. BHP/Blade |
Trouble Free
BHP/Blade |
Fan Dia. |
Max.
BHP/Blade |
Trouble Free
BHP/Blade |
|
12 ft |
8 |
4 |
22 ft |
18 |
14 |
|
13 ft |
9 |
5 |
24 ft |
20 |
16 |
|
14 ft |
10 |
6 |
26 ft |
22 |
18 |
|
16 ft |
12 |
8 |
28 ft |
24 |
20 |
|
18 ft |
14 |
10 |
30 ft |
26 |
22 |
|
20 ft |
16 |
12 |
32 ft |
28 |
24 |
As a general rule,
do not select the fans near to the limit of BHP/Blade
specified like above. The high BHP/Blade will
cause a fatigue in a short period due to the high
blade air loading, and will make a trouble for
the vibration noise. Author's experience is the
less number of fan blades causes the severe vibration
(called Throat Flutter) in the fan stack, unless
a special attention in making the fan stack is
paid.
Any fan that is effectively
moving air at the tips of the blades will develop
a reduced pressure area (or suction) on the fan
throat at the tip of the blade. This suction tends
to draw the throat toward the tip of each blade,
which means that a four blade fan would tend to
draw the throat into something approaching a square
while a six blade fan would draw it into something
resembling a hexagon, etc. Since the fan is rotating,
the effect on the throat is that of continually
drawing it into a rotating polygon. The resulting
throat flutter is frequently mistaken for fan
unbalance.
A substantial throat
will be sufficiently rigid that flutter will not
exist. A weak or flexible throat, particularly
when used with a fan of a low number of blades,
will be greatly affected by this type of vibration.
Throat flutter is easily detected due to the fact
that it is invariably of a frequency of the fan
RPM times the number of blades on the fan. If
in doubt that throat flutter is the cause of vibration,
reduce the angle of the blades until the fan is
doing little or no work. If the vibration ceases
under this condition, it is certain that throat
flutter is present when the blades are loaded.
Throat flutter will cause no damage to the fan
so long as the throat does not disintegrate and
fall into the fan blades. It may be eliminated
by stiffening or bracing the throat. |
(2) Material Constructions
of Tower Structure and Fan Stack
| Common
practice in deciding the number of fan blades
is to maintain the level of vibration below 80
micron at the gear reducer. A general guideline
with Hudson's fans is as below;
|
Structure Material |
Fan Stack Material |
Fan Diameter |
Minimum Blades
No |
|
Concrete |
Concrete or FRP |
7 - 14 ft |
4 each |
|
16 - 20 ft |
5 each |
|
22 - 24 ft |
6 each |
|
26 - 32 ft |
7 each |
|
Wood or Steel |
FRP |
7 - 14 ft |
5 each |
|
16 - 20 ft |
6 each |
|
22 - 24 ft |
7 each |
|
26 - 32 ft |
8 each |
|
Example 9-1. Determine
the axial thrust load produced from the fan using the
above examples.
(Solution)
This is an axial force opposite the airflow direction
and is necessary for engineering the supporting beam
of gear reducer and for checking if the bearing thrust
capacity for the selected size of gear reducer is larger
than this axial load. Ignorance for checking the thrust
capacity will result in an early failure of bearings
of gear reducer.
Axial Thrust Load = 5.202
x Total Pressure in inch Aq. x Net Fan Area in ft2
+ Fan Weight
= 5.202 x 0.6439 x 573.52 + 1639 = 3,561.0 LB
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