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Original_Macgyver
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Fan CFM Calculation

08/02/2013 11:23 AM

Hello all,

Here is my question. I have a fan cooling our compressor and it keeps failing. We received a replacement from the manufacturer and that one tripped out after 30 minutes. After lots of calls back and forth the reply from the manufacturer is "We got a batch of bad fans!" and they have no replacements in stock. We are looking at purchasing a fan from an independent source and need to know the CFM of the existing fan ( The manufacturer was little help). I've never had to do this type of calculation before and my Google search got me to a formula and was wondering if their formula is correct. Here it is:

  1. Diameter of fan = 14"
  2. 5 blades
  3. 3" pitch on blade
  4. 1400 RPM

From what I get the formula is this: 3.1416 * fan radius squared in feet * lineal feet of air moved = CFM.

  1. 14" diameter = 1.1666 ft
  2. Radius = .583 ft
  3. 5 blades at 3" pitch = 15" linear air moved
  4. 15" * 1400 RPM = 21000
  5. 21000 / 12 = 1750 linear feet per minute
  6. .583 * .583 = .339
  7. 3.1416 * .339 * 1750 = 1863 CFM

Is this correct? It seems a bit low?

Thank you for your help

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lyn
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#1

Re: Fan CFM Calculation

08/02/2013 12:00 PM

Air Flow formulas

Calculating the CFM of a Fan - YouTube

How to Calculate CFM From RPMs | eHow

CFM Calculator :: Industrial Fans Direct
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Original_Macgyver
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#2

Re: Fan CFM Calculation

08/02/2013 12:05 PM

I used the eHow formula.

However, I took the old fan apart and just found the CFM rating under the fan, it's 2400 CFM.

Thank you

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Tornado
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#3

Re: Fan CFM Calculation

08/02/2013 8:07 PM

The pitch of a fan is not especially easy to see just from looking at it. Thus your calculation could have been imprecise, even though based on correct principles.

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Fredski
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#4
In reply to #3

Re: Fan CFM Calculation

08/02/2013 9:10 PM

this might work
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Mr Fixit
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#5

Re: Fan CFM Calculation

08/03/2013 3:35 AM

Initially I would be wary of your calcs.

The CFM capacity of a fan 'propellor' and the CFM of a specific,swept area opening won't really translate into what your item is designed to do.

So you have a fan cooling your compressor.

Is this an air compressor? An A/C system compressor???

Is the fan OE to the compressor or did you place a fan in front of the compressor because the compressor is tripping off (and it's really really hot)?

Is the fan part of an A/C system condensing unit? Maybe a capacitor for the fan motor is all thats needed.

Not a big money maker of a job so usually ignored and replaced with the 'New Fan' or 'Need A New Unit' pitch.

Hope this helps

Jay
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Original_Macgyver
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#6
In reply to #5

Re: Fan CFM Calculation

08/04/2013 2:31 PM

Is there a good calculation for determining fan blade pitch and what the displacement is. As you can see, the formula on eHow doesn't tell me how to get that parameter. Even though I know the actual CFM I still want to find the correct formula so I can calculate this in the future.

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Original_Macgyver
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#8
In reply to #5

Re: Fan CFM Calculation

08/05/2013 4:16 PM

This is a Screw Type Air Compressor, 25 HP on a VFD and it runs almost 24 /7

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Mr Fixit
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#10
In reply to #8

Re: Fan CFM Calculation

09/21/2013 3:27 AM

Hope you have your fan sorted by now O Mac,

Still though,I'm wondering if your fan is/was 1 or 3 phase (caps or not).

I have heard of large batches of 'bad fan motors' used on a/c systems condensor fans,usually traced to faulty start or run (or both) capacitors.Which means a simple fix 'could' be achieved if an appropiate capacitor could be sourced.

J
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Randall
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#7

Re: Fan CFM Calculation

08/05/2013 5:27 AM

Watch out.

The simplistic calculation only works if there is no pressure difference on either side of the fan. In practice of course as soon as you start to move air you create a pressure differential.

Different configurations of blade shapes and pitches lead to very different pressure/volume curves. You can only really judge a fans suitability for a particular task by looking at the manufacturers curves and comparing them with your application.

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Original_Macgyver
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#9
In reply to #7

Re: Fan CFM Calculation

08/05/2013 4:21 PM

I had a feeling the formula on the eHow site was too simplistic but I needed to start somewhere. I really appreciate the knowledge of the contributors and all the assistance I have received on this post. It feels like the formula is missing another parameter or calculation on the fan pitch.

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Users who posted comments:

Fredski (1); lyn (1); Mr Fixit (2); Original_Macgyver (4); Randall (1); Tornado (1)

Previous in Forum: Process Engineering - Pressure Connections   Next in Forum: Pressure Rating for Flanges as per AWWA and ANSI

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