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Fan Types – Why Choose a Backward Curved Centrifugal fan?

My background is in Mechanical & Production Engineering however working for ebm-papst that has expanded into electro-mechanical, some electronic and acoustic engineering. When it comes to acquiring and passing on knowledge, I try to keep it as simple and as least painful as possible. I am happy to receive feedback and if there are any questions that arise from anything that gets published. If I don't know the answer to your question immediately, I am sure that I know someone that can help.


April 12, 2021 07:18

Highly informative and useful

January 5, 2021 15:03

Dan, for a backward incline fan with straight vanes, what is the typical angle (from the radial line) of the vanes? Thanks, Paul.

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April 9, 2020 08:09

Hi Dan,
Very usefull article. Thank you.
Are there any restrictions on the ratio H/B and any maximum dimensions.
Best Regards

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April 14, 2020 13:11

@ValioArgirov Hello Valio, In theory at least one of the dimensions could be right next to the impeller circumference and as long as the other was large enough the hydraulic diameter calculation will work. In reality, having one of the dimensions close to where a blade will pass will cause a significant pressure pulse as it is passing. The most significant effect will be an increase in noise at blade passing frequency (I have seen this in one application that made the fan sound like an air powered siren - for an example of this type of sound see As a rule of thumb if you can keep the smaller dimension to a minimum of 1.4 x the fan diameter then you will not go far wrong. Regards Dan

March 26, 2020 12:17

Hello Peter,
Thanks for the question, I agree that the terms low, medium and high are subjective and are unitless. In terms of fan selection these terms are relative. I have given a few examples however there are lots of caveats and conditions that may change the advice below.

An axial fan is designed for high volume low pressure where the duty point ratios could be
100m3/hr @ 15Pa
1000m3/hr @ 60Pa
10,000m3/hr @ 120Pa
30,000m3/hr @ 220Pa.
For an axial fan you need to ensure that the impeller does not operate in aerodynamic stall

A backward curved plug fan being selected at medium flow medium pressure where the duty point ratios could be from
100m3/hr @ 150Pa
1000m3/hr @ 250Pa
10,000m3/hr @ 1500Pa
30,000m3/hr @ 2000Pa.
Dependent on the type of impeller and the opertating speed the pressure developments can reach to 3000Pa

Forward curved fans are best at higher pressures and for delivering high velocity air when a small space envelope is available. Duty point ratios could be from:
100m3/hr @ 250Pa (Single inlet blower)
1000m3/hr @ 500Pa (Single inlet blower)
5000m3/hr @ 500Pa (Double inlet blower)
Forward curved fans are usually limited in size which means the top end flowrate is lower than axial or backward curved

Feel free to browse some of the "Why use a XX fan" articles I have previouusly posted for further information. If you have a specific requirement let me know some details and I will send a few selections across to you.

If there is anything else please let me know

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February 10, 2020 09:07

Hi, thanks for the article you use the idea of medium system pressure demand for forward/backward blade fan selection, could you elaborate - it has a sense of a unitless dimension; what would be the parameters involved to create this and what range of value is used for low/medium/high.

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