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Fan and Pump Curves vs. Affinity Laws

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vesterholt

vesterholt

Mechanical
Sep 12, 2013
1
I have always wondered why fan curves don't appear to be consistent with the affinity laws that we are taught. For example. We are taught that:

P2/P1 = (RPM2/RPM1)^2 = (CFM2/CFM1)^2

This would mean that fan curves should be perfect quadratic squares and all look like parabolas. However, we know this is not the case. They are often very flat at low CFM or even have a dip.

For example: [URL unfurl="true"]http://www.fumetech.com.au/images/300perfcurve.gif[/url]

or


or


Does this mean the affinity laws are not correct or are an approximation? What is the explanation for the discrepancy between the curves and the laws? Do the laws not apply under certain conditions? Does this have to do with laminar/turbulent flow?

Thanks!
 
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Cant say I am familiar with Affinity laws.... must be something new in engineering ciriculae or were called something totally different back in the day , but I suspect the answer is related to the fact that a fan runs at anywhere from about 10% efficiency upto around 82% effiociency at different spots on the curve.
 
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Are you confusing the SYSTEM curve with the published fan/pump curve provided by the mfgr? The affinity laws describe the SYSTEM curve, which when placed on the published fan/pump curve determines where you device will operate. Once set you can use them to determine head at different flow, impeller diameter or speed etc.... at different conditions...





 
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miningman, Affinity laws, probably older than Adam :)

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
 
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The affinity laws are an estimation rather than an absolution. If you cut speed by half, airflow should be about half. I haven't had much luck with the cubic power part of the estimation though. A cubed deduction in power would be great if I go to half speed, but reality usually shows lesser results. There are different results for different fan curves/types, e.g., backwards inclined vs. forward, plug fans vs. centrifugals with shrouds, etc.
 
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I will say, after many years of Engineering and testing, that the affinity laws are pretty good approximations.
 
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Agreed, good approximation that becomes less accurate the more you reduce diameter or speed when applied to centrifugal pumps - can't speak for fans.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
 
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Most literature that I see alludes to the decreasing accuracy of the application of affinity laws that occurs when the range of operation between two states is increased.
 
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I agree with 11241 comment and it sounds like you're mixing the curves up.

The page has some good images on it you may find helpful. There are many other great webpages and books available as well though. That was just my first hit.

 
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ChaseBean1: Curious to see if the minimum control head or static pressure was affecting your estimations of energy savings? Say for a pump at 1,000 gpm & 100 feet w/ a min control head of 20 feet, the variable flow head would be 80 feet. This would shift your system curve up 20 feet at O flow and impact your actual savings. I figure you know this just had to bring up as it puzzled me until I plotted it out one time.



 
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11241, who said anything about estimations? My prior post was about what I'd really seen rather than calculated - I'm not sure about your question...
 
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