Variable Speed Pumps

[hAltonJones]

I'm relatively new to the pool world and it seems everyone from my barber to the baker insists a variable speed pump is much more cost effective. However, given my simplistic reasoning, it doesn't make a lick of sense. Please help.

Assuming the purpose of pumping the water through a filter is to "clean" the water, then the volume of water pumped through the filter to clean it to a given level should be essentially the same regardless of how fast you put it through the filter. If you pump 1,000 gallons of water through the filter, it shouldn't matter too much if you pump it through in one hour or two hours (neglecting channeling, etc.) A centrifugal pump has a speed at which it is most efficient from an energy consumption standpoint. Therefore, with a variable speed pump, it - by definition - isn't always running at its peak efficiency. If you cut the volumetric flow in half, you have to run twice as long to get the same volume filtered. However you're now running less efficiently.

Why not have a single speed pump set to run at its most efficient point and keep life simple? What do I say to my barber and baker?

Cheers.

 

[jblizzle]

You are missing the fact that the power consumption is less at lower speeds. For a 2 speed pump, on low it moves half the water, but uses 1/4 the power. So even if you double the run time, you saved 50% on power.

Also, you really may not need to double the run time and still have a clean pool.

Finally, at lower flow rates, filters actually do a better job catching debris.

 

[duraleigh]

Make it even simpler. You often would like a speed for slow filtration and then a high volume speed for vacuuming or other tasks. Why not purchase a 2-speed pump and it will satisfy 98% of your needs and is a LOT cheaper than a VS?

Easily proven that a slow speed will give more effective filtration but you need the high speed for the high volume tasks.

Unless your electric rates exceed about 15 cents, the 2 speed is the most economic.

 

[hAltonJones]

Thanks, but with all due respects, I'm not sure the ¼ power consumption at half the speed is a "fact". I see you're from Tucson. Coincidentally, that's where I did my Ph.D. studies in Chemical Engineering. I think if you're using ¼ power at half speed, the pump isn't sized properly. I would truly like to see the pump curves for the pumps you mention. Are they available somewhere?

 

[JVTrain]

There's quite alot of information in this thread. It's the deep end of pumps and filters. Hydraulics 101...

I don't really think it should be called 101... maybe 201...

 

[mas985]

Thanks, but with all due respects, I'm not sure the ¼ power consumption at half the speed is a "fact".

Actually it is a fact and most two speeds are slightly better than 1/4th. It has been confirmed from measurements done by the manufactures and compiled in a database here:

http://www.apsp.org/resources/energy-efficient-pool-pumps.aspx

This is due to the Pump Affinity Laws. 1/2 speed requires 1/8th of the power to drive the impeller. However, the induction motors used in two speeds lose about 50% in efficiency because the second winding is much higher gauge wire so the resistance loss is twice the main winding. This results in a power level that is about 1/4th of high speed.

 

[hAltonJones]

OK, I see where you're going with this. It does take more energy to pump the water to a higher pressure (head) which is required to increase the flow, however, if we use the approximations presented with the affinity laws and the decrease in motor efficiency and assume you get roughly double the power demand because of a halving of the efficiency, you then have to run the pump twice as long to get the same volume flow. Doesn't that mean it's basically a wash (no pun intended)?

Not trying to be argumentative - just trying to understand the logic.

 

[jblizzle]

Re-read what Mark wrote, you missed a factor of 2.

 

[mas985]

So the impeller uses one 1/8th the energy, the motor loses 50% efficiency:

Relative power = 1/8/50% so 1/4th the power is used.

But you really don't need to filter the same amount of water on high vs low speed. Filtering the water is not what is important. The only reason to run the pump is to add chlorine, skim the surface and clean the bottom if you run a cleaner. So you may only need to run the pump the same amount of time on high as low. It depends on how effective low speed is at these things.

I currently run my pump 1 hour on high speed for the suction cleaner, and 3 hours on low speed for the SWG to get enough chlorine. That is much less than 1 turnover per day and the pool is more than clean enough for my liking.

 

[Desterline]

You're absolutely right. You're also missing an important detail. The pump is part of a system and other parts of the system can have a significant effect on how and when you run your pump.

A full mathematical treatment is... messy, but lets look at a few of the pieces. The classic kinetic energy equation Ke=1/2MV^2 applies here (though the exponent is actually _bigger_) this is the source of the "1/2 the speed = 1/4 the power" that's tossed around. So you can see that velocity is the dominant term.
That's further compounded by all the frictional losses in the rest of the plumbing, you may recall that the drag equation also includes velocity squared. This also applies to filter losses and filter effectiveness - they filter better and with lower head pressures at lower flow rates.

Now, considering run times. If you really only need "one turnover per day" and don't care how long it takes, then find the smallest pump that can do it and run it 24 hours a day. But nobody actually does "one turnover per day".

If you have a salt system, your pump run time is dictated by the time you need the salt cell to be running.
If you have a automatic cleaner, your pump run time is controlled by that.
If you have a heater - solar or other - yet another control of how long the pump runs.
If you have water features - fountains, waterfalls, etc - even more things dictating how you run the pump.

A variable speed pump represents a compromise solution for all of this. The net result is the _system_ has overall lower energy use. Even though on a strict "gallons moved/watts used" comparison, it may not be the most efficient.

-Denny

 

[RobbieH]

Your thought process you mentioned here is why I went the VF route. I have my pump set to run as low as possible to keep the salt system effective and pool clean, which in my case requires 24/7 run time. I did see a drop in my power bill, but with so many other things taking electricity that are variable in nature, it's hard to say what the net savings was. I realize it will take forever to recoup the cost, but since I needed a pump anyway, and wanted to be able to adjust the flow if/when I make additions or changes to the pool, it was what made me happy.

 

[rmturner54]

Not all two speed pumps will fit the needs of all pool situation s. That's why a VS makes the most sense.
Run as slow as possible to keep the pool efficient....Slower the better I say
And you have greater control

- - - Updated - - -

Not all two speed pumps will fit the needs of all pool situation s. That's why a VS makes the most sense.
Run as slow as possible to keep the pool efficient....Slower the better I say
And you have greater control

 

[mas985]

I have been on this forum for 7+ years and I can't remember ever seeing pool where a two speed would not "fit the needs of the pool". In all cases, either pump type would work just fine. It really comes down to economics. Can the VS save enough in energy costs in a reasonable amount of time to justify the extra cost? In most cases, the answer is no but there are areas of the country like mine where energy costs are very high and a VS might save more money than a two speed.

 

[hAltonJones]

Thanks to all who have weighed in on this topic in a perhaps seemingly futile attempt to educate me. I think you have succeeded. To make sure, let me state it simply and see if anyone disagrees.

1. A typical pool calls for two functions that require a pump, vacuuming and filtration. Pump demands for each are different. Therefore, one single-speed pump will not be efficient.

2. The solution is either (a) two pumps, each sized for one of the operations, (b) a variable speed pump that will better accommodate the two separate needs or probably best of all (c) one two-speed pump with suitable controls to allow for automatic operation for each of the two functions.

Assuming I've got it, I'm off to unconfuse myself on some other pool related topic.

Cheers.

 

[RobbieH]

Well, I don't use a pump for vacuuming, I have a robot.

 

[prs]

There may be a third function for the pump: running a SWG, and that will likely require a lower flow than either vacuuming or filtration.

Also depending upon the design of your pool and if you have automated valves, it's possible you'll need enough flow to filter and run a suction vacuum cleaner at the same time.

 

[mas985]

There may be multiple functions for a pump but it doesn't mean you "need" multiple speeds and/or pumps. Having multiple speeds just allows for a reduction in energy use. Remember that the pool industry used single speed pumps for multiple functions for many years. It wasn't until energy got expensive that the use of two speed and VS increased dramatically. That doesn't mean that you should go with a single speed pump. Given the extra cost of a two speed, it doesn't make sense to go with a single speed except for maybe a spa jet pump. I just wanted to draw a distinction between what is actually needed for pool functions and what may be desired (e.g. low energy costs).

 

[RobbieH]

PRS, some SWGs don't like low flow. Some don't mind. Mine falls into the latter category.

Keep in mind, I bought what I bought purely on convenience for my needs. I do agree the two speed pump is the most cost effective method, but for me this was the "best" solution.

 

[prs]

Robbie, my Pentair IC40 only requires about 25 gpm to activate the flow switch.

However I don't seem to generate as much FC at low flows as I do at high flows. I suspect that at low flow some of the chlorine doesn't dissolve in the water and is lost in the bubbles that come out of the return jets.

Sorry for the thread drift.

 

[jblizzle]

That does not make any sense ... with lower flow rates, the bubbles would have more time to "dissolve" in the water before entering the pool and escaping to the atmosphere.