 Description
of The Electronic Cooling Fan |
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| Determination
of Airflow |
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Experimenting
with actual equipment is the best way to determine the
necessary air flow of a fan, however, approximate airflow shall
be
calculated with following formula; |
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| System
Impedance |
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Components
inside the equipment will resist Airflow of a Fan and create
pressure loss of air, like a voltage drop in electrical circuits
through
electrical components and wires.This resistance is called "System
Impedance", which is calculated with the following formula;
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Value
of "n" shall be considered as 1 for a straight stream
of airflow and
2 for a turbulent stream of air flow. Value of "K"
shall vary and there is no
way to check it except actual measurement with your fans available.
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| Temperature
Test |
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Actual
temperature tests should be made on the cabinet to test the
selected fan according to the calculation of the graphic chart
below.
The selected fan has to be reconsidered when its cooling efficiency
is not adequate in the test. |
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Example:
As shown below, Q1 Airflow was originally considered
suitable with system impedance being unknown; but cooling
efficiency became the level of Q2 in the actual test. This means
that system impedance was higher than originally estimated and
a
higher capacity fan is recommended for use in this application.
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| Measuring
Method |
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Ps(Static
Pressure) and Pn (Different Pressure of Airflow) are
measured with the following measuring instrument, and airflow
shall be calculated with the following calculation formula: |
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| Performance
Point |
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Performance
point is defined as the cross point of the System
impedance curve and the Airflow-Static pressure curve.
The performance point indicates the Airflow of a Fan when the
Fan is applied to the intended equipment. |
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| Series
and Parallel Operation |
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"Airflow-Static
Pressure Curve" of fans, when combined in series
or parallel, are given below. |
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Generally,
the performance of two fans in parallel will result in
doubling the airflow volume while the performance of two fans
in
series will result roughly in doubling the static pressure.
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| Fan
Noise: Designing Point of Fans |
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The
noise of a fan in its operation are classified into fan blade
noise, bearing noise and motor noise (electro-magnetic noise).
Except for a fan that is damaged with irregular impact, bearing
and motor noise are relatively lower than fan blade noise.
Accordingly, special attention has to be paid to fan blades
in
their design. |
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Noise:
Radius
running speed difference |
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The
difference between inner circumference and outer circumference
results in running speed difference. This running speed difference
causes disturbance in airflow that ultimately becomes fan blade
noise. |
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| Fan
blade is shown below. |
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Determination
of shape and twist of fan blade shall be the most
important thing in order to make this speed difference smaller.
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Speed
difference between the front side and rear side of the
fan blade. |
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Cross
section of the fan blade is shown below. Air speed is
different between the front side and the rear side of the blade
and this difference causes air-eddy and results in noise.
To minimize this difference, the shape of the fan is designed
to be streamlined. |
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Noise
generated between the fan arm and the blade: Fan arms
are installed on the exhaust side and the generated airflow
of
the fan strikes the arms and creates air-eddy. |
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The
number of arms, and the distance and angle between arms
and the fan blade are important considerations to prevent air-eddy. |
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| Material
Comparison Chart |
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| 1.
Metal Frame vs. Plastic Frame |
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Radiation |
Strength |
Weight |
Cost |
Insula-
tion |
Static
Electricity |
Heat
Resistance |
Magnetic
Field Leak |
| Metal |
Good |
Good |
Not
Good |
Not
Bad |
Bad |
Good |
Good |
Good |
| Plastic |
Bad |
Not
Bad |
Good |
Good |
Good |
Bad |
Not
Bad |
Bad |
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| 2.
Ball Bearing vs. Sleeve Bearing |
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Life |
Loss |
Noise |
Cost |
Impact
Strength |
Mounting
Flexibility |
Heat
Resistance |
Cold
Temp Resistance |
| Ball |
Good |
Good |
Not
Bad |
Not
Bad |
Not
Good |
Good |
Good |
Good |
| Sleeve |
Not
Bad |
Not
Bad |
Good |
Good |
Good |
Not
Good |
Not
Bad |
Not
Bad |
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| 3.
Metal Blade vs. Plastic Blade |
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Radiation |
Balance |
Cost |
Heat
Resistance |
Starting
Time |
Cold
Temp. Resistance |
Magnetic
Field leak |
| Metal |
Good |
Not
Good |
Not
Good |
Good |
Not
Bad |
Good |
Good |
| Plastic |
Bad |
Not
Bad |
Good |
Not
Bad |
Good |
Not
Bad |
Bad |
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