I've been researching fans/blowers and their use in pneumatic systems (particularly conveying systems), and I'm confused by the application of density correction factors. Let me describe a sample situation:

I have a wood processing plant in which I will be using a centrifugal fan to move material through a pneumatic system. My pneumatic system is characterized by the system curve $$P_s = K_{sys} Q^2$$ where Q is the volumetric flow (CFM) and $P_s$ is the static pressure. In order to move the material through my system, I need a minimum CFM of (say) 10,000. Via my system curve, I find that this corresponds to a static pressure of (say) 7 inWC.

So, I need a fan that can supply 10,000 CFM at 7 inWC. I will do this by superimposing my system curve on the fan curves for the various fans I might select to determine the HP and RMP requirements. Here is where the density correction comes in:

My Understanding: Ultimately, what I want is material to be moved through my system. The fan gives energy (i.e. does work) to the air, and the air in turn gives energy to the material it moves. At a given volumetric flow rate, denser air will have more energy and thus be able to do more work. Now, the 10,000 CFM I need is the flow rate needed under actual conditions (ACFM). Fan curves are calculated at standard conditions, so I should convert my ACFM to SCFM before comparing to the charts. I do this by simply multiplying by the standard/actual air density ratio.

My Question

  1. Do I also need to correct static pressure? Physically speaking, I don't think the pressure generated by the fan is a function of the air density. Particularly because this pressure is just measured relative to ambient air pressure.
  2. Is it wrong of me to correct flow rate? Some pages I found online said that CFM should not be adjusted, as centrifugal fans are "constant volume devices" (volumetric flow at a particular RPM does not depend on air density). However, I believe that this reasoning neglects the work/energy considerations I made above.

Any references to pages/papers explaining the underlying physics would be greatly appreciated!

  • $\begingroup$ Is "WC" a typo for "WG", i.e. "Water gauge"? $\endgroup$ – alephzero Sep 24 at 19:15
  • 1
    $\begingroup$ "Water column"; just a different name for the same unit. $\endgroup$ – Sean Ericson Sep 25 at 14:59

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