Calculating Actual Pump Speed with VFD?

[sadele89]

How would I calculate actual flow rate when using a VFD? I know the flow [GPM], head [ft], VFD set %, and VFD speed at 100% [RPM].
I know I'm supposed to use the pump affinity laws, just not sure where to start. Any help would be appreciated!

 

[chicopee]

Start by getting a catalog from the manufacturer. That catalog should have all the tables from which you could apply the Affinity law.

 

[chicopee]

Instead of tables some manufacturers will have graphs.

 

[hvacSLOG]

My calculation would be as follows:

CALCULATED GPM AT REDUCED SPEED = FULL SPEED GPM * (REDUCED PUMP RPM / FULL SPEED RPM)

This is based on affinity laws and doesn't take into consideration your particular system curve or any dynamics of your system (valves opening and closing). I believe there would be a minimum speed you have to generate before you can actually start to see any flow...I generally assume this equation is good down to 30% speed or so. But this should be good enough along with some other conservative assumptions to estimate energy savings or troubleshoot the system. Why do you need to know the "actual flow rate?" If very important better off measuring with calibrated flow device (hire a balancer).

You can estimate the reduced pump head and brake horsepower using the square and cube of the proportion respectively.

Does this help?

 

[LittleInch]

Your title is wrong - You say calculate pump speed, but you mean pump flow rate.

All you can actually do is re-calculate the pump curve for different speeds.

The flow you get will depend on your system curve.

So if you have say 10 points on your original curve then your X axis for your new spped (flow) will vary in proportion to pump speed.
Your y axis for the same point will vary by pump speed^2

Rinse and repeat.

So e.g.
at 100GPM at full speed( say 3600 RPM) your head is 200 ft
At say 3000 rpm, your data point is 83 GPM (3000/3600) and your head is 138ft 200 X ((3/3.6)^2)
do the same for 200gpm, 300 .... until you generate a second pump curve for 3000 rpm

The vendor should provide this as the affinity laws are basic and not exact, but you should be pretty close.

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

 

[sadele89]

Thank you so much for your explanations! They were very helpful!