Finding Different Speed Ranges for a Fan Using a Fan Curve

[Bilge_Rat]

Good afternoon,

I'm attempting to find a set of different volumetric flow rates for a fan based on different hertz of the VFD to use as inputs for a dispersion modeling program. Based on a technical document for the garage, I know the fan's minimum flow will be 745 CFM and the operating maximum flow will be 11100 CFM. I need to find the CFMs at [minimum + 20% speed range, minimum + 40% speed range, etc.] but I'm tripping up on determining how to go about doing so.

The facility has provided the fan curve (which I've attached) and I used the pump/fan affinity laws in conjunction with the requested operating conditions to find the maximum and minimum rpms. Appling the needed operating percentages (min +20%, min +40%, etc.) in conjunction with the min and max rpm, I found the rpm at each level and used the affinity laws again to get the volumetric flowrate. I realized this achieved the same as just doing [min + ((max - min) * % speed range)] and got me questioning if this was the right approach.

Doing the same above with bhp (with the appropriate affinity law) seems to give me a different set of volumetric flow rates. Should I be using hp and affinity laws to determine flowrate, or does just multiplying the min and max capacities as stated in the paragraph prior suffice?

Appreciate the help.

 

[Snickster]

The attachment you included is actually a printout from Greenheck sizing selection software that is available on their website. I have used it many times and believe it is available for anyone to use. The fan curve you show is only one of the fan curves that can be printed out after you select and size the fan you want. You can also print out multiple speed pressure versus flow curves among other performance curve options, and other data for the fans you select. So all you have to do is go to their website and run the program again for the same model and performance parameters, then print out any curves you want. Here is the link:

https://www.greenheck.com/resources/software

 

[LittleInch]

I don't understand what you've sent us.

From the last page it is pretty clear that the rated duty of the fan is about 14,000 CFM at 694 rpm and the min speed is at 300 rpm which has a flow of about 6,000 CFM

Basically flow will go up as a proportion of speed so say min + 50% = 500 rpm will be about 10000 CFM.

This is a simple non ducted fan so appears to follow the affinity curves quite well.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Lex Mayer]

Your approach of applying the affinity laws to find the RPM at each speed level and then calculating the volumetric flow rate is fundamentally sound. Remember, the affinity laws dictate that flow rate changes linearly with speed, so your method of should theoretically give you a close approximation.

Regarding your confusion with the different results when using brake horsepower, its important to note that BHP calculations involve more variables and might not directly correlate to the volumetric flow rates in a straightforward manner like RPM does.

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