Fan Curves and Fan Laws 1

[Jamieo]

Hello,
I have a question maybe you can help me with. We are upgrading an existing heating ventilating system. I have the original drawings that show the design CFM and SP. Is it possible to plot the fan curve using the affinity laws? I have used the CFM and SP relationship to plot a curve using the orignal design values as constants, varying the CFM from 100 to 10000 in 100 increments, and solving for the static pressure at the different CFM's. A plot of this looks more like a system curve than a fan curve. Am correct in assuming that this would be a system curve? I am assuming that the fan curve and the system curve intersect at the design cfm/sp.

 

[briand2]

You can't plot the fan curve without knowing it. What you've done is indeed plot the system curve (as flow increases, pressure drop increases; the exact opposite shape to a curve of fan flow against pressure developed).

The original fan would have been selected so that it's curve intersected the system curve at around the design flow rate (but with a margin "in hand" to be sure!). Thee fan could have been a constant flow type (vertical line) or constant head type (horizontal line) or (in the real world!) somewhere in between.

Only when you've selected a particular fan will you be able to superimpose the fan curve on your existing graph.

Regards,

Brian

 

[PhilBW]

If you can find the serial number of the fan, the fan manufacturer may be able to provide you with a fan curve. If you don't have the serial number, using the motor speed and sheave diameters, determine the rpm of the fan and provide that and the model number to the manufacturer.