Experimenting with actual equipment is the best way to determine the necessary airflow of a fan, however, approximate airflow shall be calculated with following formula;
Components inside the equipment will resist Airflow of a Fan and create pressure loss of air, like voltage drop in electrical circuits through electrical components and wires. This resistance is called "System Impedance", which is calculated with the following formula;
Value of "n" shall be considered as 1 for a straight stream of airflow and 2 for a turbulent stream of airflow. Value of "K" shall vary and there is no way to check it except actual measurement with your fans available.
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.
Examples: As shown below, Q1 Airflow was originally considered suitable with system impedance being unknown: but cooling efficiency becomes 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.
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:
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.
"Airflow-Static Pressure Curve" of fans, when combined in series or parallel, are given below.
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.
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.
Noise:
- Fan blade noise
- Bearing noise
- Motor noise (electro-magnetic noise)
Radius running speed difference
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.
Fan blade is shown below.
Determination of shape and twist of fan blade shall be the most important thing in order to make this speed difference smaller.
Speed difference between the front side and rear side of the fan blade
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.
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.
The number of arms, and the distance and angle between arms and the fan blade are important considerations to prevent air-eddy.
