Pump efficiency 6.7x better at 1000RPM vs 3000 RPM !!

[neuzelaar]

I have a 35000 gallon pool with a Sta-Rite sand filter. I recently replaced the old 3kW single-speed pump with a Pentair Intelliflo XF variable speed pump. Since electric rates here in California are sky-high (I pay 20c/kWh off-peak and 56 c/kWh on peak, highest tier), I am trying to minimize my electric bill without sacrificing pool pump action. Using a rola-chem flow meter, I measured water flow and power usage at various RPM settings of the pump:
[attachment=1:1m8edaay]PumpThroughput.jpg[/attachment:1m8edaay]
The power usage was read from the display on the pump. At 1000 RPM (~36GPM) I still have decent skimmer action, so at first sight there seems to be no downside to running at low speed.

Next, I calculate the efficiency of the pump: the more gallons of water it can pump per kilowatt hour, the better. The result for various RPM settings is shows as the green line in this graph:
[attachment=0:1m8edaay]PumpEfficiency.jpg[/attachment:1m8edaay]
At 3000 RPM, the measured flow is 76 GPM (= 4560 GPH) at a usage of 1840Watt, which yields 2480 gallons per kWh. At 1000 RPM, the flow is 36 GPM at 130W, which means 16615 gallons per kWh.

The blue line shows the number of kWh needed to turn over my 35000 gallon pool once. So at 1000 RPM it costs 2.1 kWh (16.2 hours @130Watt), while at 3000RPM pumping the same 35000 gallons takes 14.1 kWh (7.7 hours @1840Watt). So it is 6.7 times cheaper to operate my pool at 1000 RPM than at 3000 RPM.

That makes a spectacular difference in operating cost, especially since the pool was about 1/3 of my electric bill with the old pump!
It seems logical that 'haste is waste'. There is more lost at high speed due to turbulence at elbow corners, in the filter or in the backwash valve. This flow resistance probably goes up with the cube of the speed, just like wind resistance. So gut feeling sais that pumping faster should always yield poorer efficiency. My measurements seem to confirm that, and then some. So it seems to make sense to run the pump always at the lowest possible RPM setting.

Question: is this correct? and did other pool owners also see this effect? Are other pools run at this very low speeds? My measurement data seem to show the opposite of what TFP expert states in the first hydraulics 101 post, where it is stated that efficiency (energy factor) goes up with flow rate:

The chart also shows that the energy use of a pump is fairly linear with the flow rate. This is true for all residential pool pumps. The higher the flow rate of the pump, the more energy that is used. However, flow rates tend to increase faster than the energy use which is why the energy factor, measured as gallons per Watt-hr, of the pump increases with increasing flow rate effectively reducing the energy consumption.

Did I make a mistake somewhere? I am using a Rola-chem flow meter that I installed in the return plumbing to measure flow, and read out the power from the display on the pump. Both could have errors, but even if the absolute value is off by some, the trend likely is not.

Related posts:
Install picture of my pool equipment: http://www.troublefreepool.com/post494660.html#p494660
Hydraulics 101: http://www.troublefreepool.com/hydraulics-101-have-you-lost-your-head-t915.html
Flow meter: http://www.rola-chem.com/flowmeters.html

 

[mas985]

Using the pump affinity laws, flow rate decreases by a factor of 3 while wattage decreases by 27 (3^3) so the net theoretical result would be an increase in energy factor of 9x. But taking into account power efficiency, 6.7x is not all that unreasonable.

Also, I think you are misreading the sticky. For a constant RPM, the power efficiency goes up with flow rate (i.e. lower head loss) until BEP is reached and the energy factor goes up as well. Power efficiency (Pout/Pin) is different than energy factor (Gallons/Watt-hrs). However, when you reduce RPM on a fixed plumbing sytem, the power efficiency drops but the energy factor increases.

 

[neuzelaar]

Thanks! Yes, indeed I have mis-read the sticky. Power efficiency confused me. I understood that to be the 'bang for the buck' (gallons/kWh), rather than pressure difference across the pump. I would be interested mainly in the first, as that affect my cost.

As a pool owner I have not so many knobs to turn after the pool is in. The pipe resistance (head loss) is more-or-less a given and depends on the plumbing and filters properties. The main thing to tune is deciding when, how long and how fast to run the pump. It appears that running a pump as slow as possible is always best.

Given how much cheaper it is to run at low RPM, I wonder why it is recommended to dimension pool pumps to turn over in 8 hours at a 75 GPM. It saves a lot of money to run 24 hours (non-stop all day) at 25 GPH, so why not design all pools for 24/7 pump operation? That not only ~7x cheaper to operate, its also much quieter, gives less stress on the pump and the constant circulation in the pool likely prevents algae growth. Variable speed pumps make it possible to get the sweet spot by dialing in a low RPM. But it seems the same can be achieved with single or dual speed pumps by dimensioning them smaller for a 24 hour turnover.

Perhaps I am overlooking an important effect that makes a case for a high GPM flow. What could be the advantage at running at high speed? Obviously spa operation and gas heating might require higher flow, but that is only short times. It appears that 99% of the normal pool operation is doable at a low GPM.

 

[jblizzle]

Well, off the top of my head for reasons to need higher flow (beyond the spa):
-some cleaners require higher flow rates,
-some skimmers may not operate as well at too low of a flow rate,
-solar heating is most efficient at higher flow rates.
-water features

You are right that running the Intelliflo around 1000rpm is the most-efficient/cheapest. Even at that speed though, you likely do not need to be running 24/7. Turning the thing off saves 100% of the cost

Read this:
pool-school/pump_run_time

 

[mas985]

Given how much cheaper it is to run at low RPM, I wonder why it is recommended to dimension pool pumps to turn over in 8 hours at a 75 GPM. It saves a lot of money to run 24 hours (non-stop all day) at 25 GPH, so why not design all pools for 24/7 pump operation?

Those recommendations were in the days of single speed pumps. Also, you don't really need 1 turn per day. Again, that is an old rule of thumb that doesn't really seem to be accurate. I can get away with much less that one turn over per day. That is the problem with rules of thumb, they don't always apply this one hardly ever does.

Also, running a pump 24/7 on 1000 RPM is probably overkill and a waste of energy. You may only need 8 hours or perhaps even less.

 

[neuzelaar]

Thanks. Yes, indeed I can do with much less than a full 35000 gallon turnover per day. So running 8 hours at 1000 RPM is enough, which is about 1 kWh/day.