I am being asked to compare a fan which is going obsolete with its proposed replacement. From a mechanical and electrical point of view, they are almost completely identical so I have no concerns from that point of view. However, when I look at their respective performance curves, they are somewhat different, especially in the low air flow region. This is outside of my area of competence, and although I have done some reading about it, I am not sure what to make of it. Can somebody explain to me which one is best and why (I have purposefully not put any legend)?

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  • 3
    $\begingroup$ Most important question: where on the curve will you be using it? $\endgroup$
    – Solar Mike
    May 6, 2021 at 8:56
  • $\begingroup$ @SolarMike good question. In truth, I don't fully know. I expect it will be in the high air flow, low pressure region as it's being used to cool an enclosure with PSUs, etc... using just ambient air, so I'm guessing there won't be much pressure to fight against. $\endgroup$
    – am304
    May 6, 2021 at 9:25
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    $\begingroup$ "Inch H2O"? WHY? $\endgroup$
    – SF.
    May 6, 2021 at 10:28
  • 2
    $\begingroup$ @SF. Because the units match with CFM - if it was pressure in bar or Pa then it may be m^3/s... $\endgroup$
    – Solar Mike
    May 6, 2021 at 10:38
  • 1
    $\begingroup$ @SF. that's how the data is provided in the datasheet, not my choice :-) $\endgroup$
    – am304
    May 6, 2021 at 13:40

1 Answer 1


The yellow curve corresponds to a fan that will provide mostly a set pressure differential, say, keeping fumes from escaping a tank through elsewhere than dedicated vents. If there is a circumstance that reduces the pressure differential, it will speed up and increase the flow, to compensate.

The blue curve is a fan that doesn't expect any significant pressure differential and will provide a constant air flow, e.g. for cooling a machine or providing air circulation through a room, maintaining near-constant flow regardless of factors that may restrict the pressure - likely higher RPM would result in extra noise, so it peaks at a certain threshold and doesn't attempt to pump more air than necessary.


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