Fan Application Selection
The starting point is to determine the Air Volume required in cubic feet per minute (cfm)
This is calculated from the following:
Heat to be dissipated in watts
Maximum Allowable Temperature Rise above ambient in °C
Formula for calculating is as follows:
Q(cfm) = 1.76 x P
DTc
Where Q = Required Air Volume cfm
P = Internal Heat Rise
DTc = Allowable temp rise °C
Example:
500 watts 10°C maximum temperature rise
Q(cfm) = 1.76 x 500 = 88 cfm
10
From this we can now select a fan that when fitted into the system will deliver 88cfm.
To determine the best fan selection based on the air volume calculated the following additional information is required:
- Maximum size of fan that can be fitted?
- Supply Voltage?
- Maximum power available?
- What is the system resistance?
- Service life required from the fan?
Environmental conditions? (i.e. humidity, dust, vibration shock loads etc.
What is the mounting position of the fan, is the impeller vertical, horizontal, hanging down or on top?
Maximum Acceptable Noise Level
Is a filter required?
Cost restraints
Based on the answers we can select a suitable fan:
- AC or DC
- Physical Fan Size/Type
- Sleeve, Vapo or Ball Bearing
- Select a fan that will meet the duty required.
The Fan Laws
To understand how fans will work in real applications these few basic fans laws should be learnt:
For same size fan Air Volume is directly proportional to speed.
Increase in System Resistance is the square of the Air Volume increase i.e. double the volume 4 times the resistance.
Fans mounted in parallel will give twice the air volume but no increase in pressure development.
Fans mounted in series will give twice the pressure development but no increase in air volume.
Understanding Fan Noise
Fan noise level is expressed in decibels “dB(A)”.
This is a logarithmic scale.
The ‘A’ scale adjusts for human hearing range.
Two fans running together will give double the noise level but will NOT sound twice as loud.
3dB(A) increase is double the noise level
Example:
If one fan is rated @ 45dB(A) then two fans of the same type run together will give 48dB(A)
3dB(A) = Barely noticeable
5dB(A) = Noticeable
10dB(A) = Sounds twice as loud
Noise testing is carried out in an anechoic chamber with the microphone mounted 1 metre from the sound source.
The fan under test is suspended by rubber bands to ensure no influence in the noise level by mounting on a solid surface that could act as a sound amplifier
Fan Curve Generation
Fan air flow curves are obtained using an airflow test rig as shown below. The fan being tested is run at nominal voltage and reading are taken from no resistance,
max. airflow to no airflow max pressure development.
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Bearing Selection
The bearing selection is dependant upon the mounting position, ambient temperature, cost restraints and required service life.
Sleeve Bearing: Lowest cost, lowest life and least able to tolerate harsh environmental conditions. Used in horizontal shaft applications.
Double Ball Bearing: Highest cost, longest life and able to tolerate harsh environmental conditions. Used in horizontal or vertical shaft applications.
Vapo Bearings: Lower cost than double ball, but service life is greater than a double ball.