Just some Memorial Day messing around- RPM vs. GPM vs wattage

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texasmax

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May 24, 2010
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Dallas, TX
All-

Just enjoying our new pool, still trying to find that sweet spot of speed where the heater still has no issue and the sheer descents still look good. Slapped in a shiny new Blue White F300 in a very long straight pipe run after the filter, and took some initial measurements:
RPMReported wattage (with some wiggle room)Flow (as best I could tell)"Efficiency" (watts/GPM) (actually, reverse of efficiency, so, "cost of flow")Notes
60030N/AN/A
120013129gpm4.52sheers dribbling
175032748gpm6.81bumpy sheers
240076865gpm11.822 solid sheers, 1 bumpy
250084868gpm12.47
2750113175gpm15.08
2800117178gpm15.01
3000143782gpm17.52
3250177190gpm19.68
3450209295gpm22.02

So, I'm almost half as efficient at 3450 rpm as I am at 2500 rpm. Will test out 2250 and 2000 tomorrow.

---updated---
 
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I’d expect the sweet spot to be between 1800 and 1900rpm, but you may need more to trigger flow/pressure sensors on your equipment.
 
That is a great experiment! I always appreciate folks who want more of an engineered answer than seat of the pants (although seat of the pants can be faster lol). How did you measure flow? At such high flow rates and with multiple returns, I have never been able to figure out a good method. I also can’t calculate total head to use manufacturer charts because I don’t know exactly what’s underground for plumbing (long story but had a leak and the guys who found and fixed it used an awful lot of trial and error . . .and pvc fittings to make the dreaded quad elbow repair couplings underground before finally being able to find it).
 
SoDel-

I installed this: F-300 - Blue-White Industries. Specifically, on a run of pipe after the filter that all the water goes thru. If installed correctly, should be fairly accurate (and, even if it's off, the relationships between rpm/flow/power will still be there). And I've got not clue what my total head looks like- but it can be backed into plotting flow rates and various pump speeds, so long as you have the pump manufacturer's curves. Pretty sure I've seen charts where the head curves are already drawn in (didn't find one for my pump), and so where the green line is would tell you what the head is. For me, it's probably not important- though if I ever made any major plumbing changes, I'd re-plot to see what changed!
 
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Here's v2, with some additional lower-end data points:
pump_curve_marked_up_v2.png

So the curve really ends up "curving". This was with my sheer descents on; at 2400rpm, I had 2 solid sheers, and the far one was getting "bumpy". At 1750rpm, all 3 were bumpy, and at 1200rpm, all 3 were dribbling down the wall. After 1200rpm was 600rpm, and there, the flowmeter was bottomed out (lowest marking on it is 20gpm). Also, at 600gpm, the salt cell was giving me the "not enough/no flow" error. Didn't try yet to see where the heater sensor cut off is.

If I'm running the sheers, then, I'd probably go 2750, and the difference between 2750 and 3450 isn't noticeable except for staring at them when switching back and forth.

I also ran the test at the lower end with the sheers off, to see if that materially affected the head curve. At both 1750rpm and 1200rpm, both the flow and power usage were both 10% lower. So I'm just as efficient with my electricity, but the lower flow tells me I'm running higher flow resistance? Think that makes sense as there's less parallel pipes handling the water on the pressure side (and the sheers site about 2' higher than the surface of the water so the parallel pipes more than make up for the additional vertical height).
 
texasmax said:
All-

Just enjoying our new pool, still trying to find that sweet spot of speed where the heater still has no issue and the sheer descents still look good. Slapped in a shiny new Blue White F300 in a very long straight pipe run after the filter, and took some initial measurements:
RPMReported wattage (with some wiggle room)Flow (as best I could tell)"Efficiency" (watts/GPM) (actually, reverse of efficiency, so, "cost of flow")Notes
60030N/AN/A
120013129gpm4.52sheers dribbling
175032748gpm6.81bumpy sheers
240076865gpm11.822 solid sheers, 1 bumpy
250084868gpm12.47
2750113175gpm15.08
2800117178gpm15.01
3000143782gpm17.52
3250177190gpm19.68
3450209295gpm22.02

So, I'm almost half as efficient at 3450 rpm as I am at 2500 rpm. Will test out 2250 and 2000 tomorrow.

---updated---
Click to expand...
Your data agrees very, very well with pump affinity law calculations. So physics and geometry still work.
 
I think the reason it flattens more toward the low end is that you will at some point hit a limitation on how much flow you can get given the head of the system. At some point at low speed, the dynamic head isn't contributing much and the flow rate will be dictated largely by the static pressure.

Difference between practical and theoretical & a good illustration of why to do calibrations like this on your actual system.

FYI, you can play with this:

Pump Curve Speed Calculator | Affinity Law Calculator

This centrifugal pump curve calculator is designed to quickly calculate the different operating conditions when a centrifugal pump is sped up or slowed down.
inventory.powerzone.com inventory.powerzone.com

Flow is proportional to shaft speed. Power varies with the cube of the shaft speed for centrifugal pumps. So, if you halve the shaft speed, you should see about 1/2 the flow, but 1/8 the power used. That is why variable speed pumps (and to a lesser extent 2-speed pumps) are able to save lots of power. It also illustrates why to maximize a VSP, it pays to design an efficient plumbing layout so that you can get as much flow as possible at the lower end of the speed range.
 
Here is mine. Also straight line. Right line is pool mode with spillover. Left line is spa mode. My 3 year old drew on it some : )
Measured via vacuum gauge and pressure gauge on port to measure total head.

I keep mine running at 2000 rpm which is 500 watts and should be about 70gpm
225D425E-623F-43A8-A638-5B5CE21166E4.jpeg
 
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