# Q&A forum: Centrifugal and Axial Fan Calculator

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The rule of thumb "one impeller volume per revolution" has been called in to question ... |
As a result of this question, CalQlata successfully carried out an internal verification based upon the energy required to shift such a mass. Whilst it was considered prudent to re-issue the 'Fans' calculator now calculating the impeller speed (RPM) required to generate an entered value for volumetric flow rate based upon the required energy; |

Unfortunately, I am not a fan expert either! The power and pressure seem OK, but the flow rate is the problem. We have measured directly the flow rate and although there are obviously some errors in measurement, we seem to have broadly similar practical results of less than 1m3/s. What I really would like to know is the accuracy of the 'rule of thumb' (effective rotor volume x rpm), which I cannot seem to corroborate on a google search. Does the calculator just produce this value regardless of any other input values? |
The standard theory on centrifugal fans was generated by Charles H Innes in 1916 ("The Fan"), and it appears to have stood the test of time. Twelve blades, however, may not be suitable for very large diameter impellers of high aspect ratios. I (personally) have never seen a fan deliver more than its impeller volume per revolution unless the atmospheric outlet pressure is less than the inlet pressure (an effective vacuum). Therefore, I am unable to refute Innes' theory. I am happy, however, to leave this debate open to anybody that can show Innes' theory to be incorrect. Please let us know if you can do this mathematically and supported with practical evidence. |

Can you comment on the limitations of the equations that were implemented in this software? Specifically with regards to impeller size. I’m investigating design changes on a relatively small impeller (3-4”), and so far this software is predicting an output flow that is much higher than has been empirically captured. Any feedback would be appreciated. |
The theories in the calculator are correct for all sizes, i.e. for fans with impellers smaller than a millimetre to greater than 10m. As your fan gets smaller the ratio of surface area with volume increases, and the smaller it gets the greater this ratio becomes. In this case ‘δε' represents the increase in inefficiency over the previous size Surface friction has a far greater effect on efficiency in a fan than it does in a pipe because the ratio of surface area (contact surface) is greater than in a pipe. Therefore, a similar table to that above for fans would show an even more marked increase at smaller diameters. Centrifugal fans are less suited to smaller diameter for a number of reasons. This is why centrifugal fans tend to be targeted for larger fans and axial configurations for smaller diameters For what it’s worth, if I were designing a very small high-performance fan, I would start with a multistage axial configuration (along with suitable venturies if I was looking for pressure as opposed to flow).
I believe that your input/output data is based upon the following units: With regard to your concerns about flow rate: May I venture a couple of comments on the input data? The recommended angle for the blade inlet should be used where possible as you will see improvements in efficiency, outlet pressure, outlet velocity and power consumption. Whilst I agree that such improvements are very small between 45° to 46.109°, every little helps when attempting to minimise defects (for such small fans). I notice that the outlet angle (120°) has turned the blades from backward facing to forward facing (see http://calqlata.com/productpages/00060-help.html Fig 3). I am not sure if this was intentional (special requirements) but the smaller the outlet angle for a backward facing blade, the better its efficiency. # Note: the efficiency quoted (ε {%}) is for blade design only |

I have a question regarding the power and torque calculation in your fans software. I expected that Power = torque * rotational velocity. So per the below, rotational velocity = 1099.6 rad/s (RPM * 2*pi/60) and power should equal 2.97W. The program is outputting a power of 4.49W. What is causing this discrepancy? Please let me know when you have a moment. |
The torque you get from Fans’ output data is that required to generate the airflow (Q) |