Single speed pump vs. Variable speed pump

[setsailsoon]

For those that haven't noticed the link to @mas985 (Mark's) article in my signature has been there for a LONG time. It's one of the best assemblies of fluid mechanics for pools I've seen anywhere. Sort of an pool fluid mechanics extraction from every relevant publication used to teach fluid mechanics in college including Perry's, Crane Manual, and the original Fluid Mechanics textbooks. It's well worth perusing. I think next time we cover this issue I'm just going to refer to that link.

Thanks Mark.

Chris

 

[mas985]

A better approximation is

Power = A + B * RPM^3

 

[Randrx2]

Hmmm...you all are producing good arguments. That is why I posted here, to help think this out.

My logic was for a constant pressure system, which the pool is not. I know on a constant pressure system, the savings of a variable speed pump are not greater than varying the output.

Using your curves, you can see where if the pressure is kept constant, the variable speed pump would have to spin faster to maintain the flow.

 

[Jimrahbe]

While all this is great "fun"... it matters not to 99% of pump owners.

Most people buy a variable speed pump for three reasons..

1. Lower electrical costs.
2. The versatility of having one pump that that can be easily, and automatically, adjusted to handle several different flow demands. Such as spas, waterfalls, blubbers, etc, etc...
3. Noise.. At low RPMs a VS pump does not make enough noise to wake a sleeping baby.

None of these things can easily be done with a Single Speed pump.

That said, I have yet to figure out the whole point of this thread?

Thanks,

Jim R.

 

[Randrx2]

That’s why it’s in the deep end forum, to discuss theory.

I had a basic concept that I was trying to explore (single speed vs. variable speed cost savings). My premise was incorrect, because I was applying a situation that does not occur in pools (constant pressure).

Therefore, I was educated by the responses here.

Isn’t the purpose of this forum to discuss more in depth topics?

”Pondering the deep mysteries of pool chemistry? Plunge into the more technical aspects of pool care. Discussions here may not be applicable to everyday pool care.”

Edit: I just realized I did not post this in the “Deep End”. Sorry for the confusion.

 

[mknauss]

Edit: I just realized I did not post this in the “Deep End”. Sorry for the confusion.

Deep End is for Pool Water Chemistry discussion, that are, well. deep. This is a purely equipment discussion.

 

[mas985]

My logic was for a constant pressure system, which the pool is not. I know on a constant pressure system, the savings of a variable speed pump are not greater than varying the output.

What would be the example of a constant pressure system?

With plumbing, flow rate and head loss are linked together via the system plumbing curve (similar to pump's head curve). You can't change one without changing the other.

 

[Randrx2]

What would be the example of a constant pressure system?

With plumbing, flow rate and head loss are linked together via the system plumbing curve (similar to pump's head curve). You can't change one without changing the other.

A well pump. Or a spray pump application like misters for evaporative cooling.

 

[Brett S]

What would be the example of a constant pressure system?

With plumbing, flow rate and head loss are linked together via the system plumbing curve (similar to pump's head curve). You can't change one without changing the other.

I was curious, so I googled the “Stop Cycle Valve” that was referenced in one of the videos posted above. It looks like it’s designed for a well pump where you want to maintain a constant pressure in the system whether you have one faucet open at a trickle, or two shower heads running full blast or anything in between.

 

[Randrx2]

Deep End is for Pool Water Chemistry discussion, that are, well. deep. This is a purely equipment discussion.

So, possibly in “Agree to Disagree”?

 

[Randrx2]

I was curious, so I googled the “Stop Cycle Valve” that was referenced in one of the videos posted above. It looks like it’s designed for a well pump where you want to maintain a constant pressure in the system whether you have one faucet open at a trickle, or two shower heads running full blast or anything in between.

Yes, we posted at same time. I have also used a constant speed pump with a modulating output valve on an evaporative cooling system.

 

[mas985]

A well pump. Or a spray pump application like misters for evaporative cooling.

The change in RPM should still be more efficient than throttling for those applications as well as long as the pump motor uses a VFD to change speed. Now if you put an inefficient VS motor (non-VFD) on a pump, that is a different story.

[EDIT] Perhaps what you meant here was to have constant head loss with varying flow rates.

Again, using the Intelliflo:

Plumbing Curve 0.0082, 3450 RPM, 82' of Head, 100 GPM, 2827 Watts
Plumbing Curve 0.0167, 3267 RPM, 82' of Head, 70.1 GPM, 2103 Watts

Technically the lower RPM case has a higher loss plumbing curve (i.e. effectively throttled) so for that case it would be more efficient but it is still a lower RPM with throttling.

 

[PoolNewb2020]

Hi, I'm new to pools but not new to electric motors and I have some background with fluid mechanics.

I actually did perform a small experiment on my Tristar VS pump and reducing flow at the discharge side does reduce power. Yes it goes against conventional wisdom, which mostly relates to combustion engines, not electric motors, but it still holds true.
I don't have a flow meter and I only relied on the number my Omnilogic display for watts consumed, but it still proves that reducing flow on a centrifugal pump does reduce power consumed, It's not just a theory.
Since my pool is relatively small and the installer went all out with a 2.7HP pump, I run the pump at 80% max speed, which is 2760RPM. I have three sheer descents with three separate valves and a jandy valve that switches between return and sheer descents. I can only run either sheer descent or returns. Using sheer descent valves I am able to control the discharge flow.

With the pump running at 2760 RPM and sheer decent valves fully open I am consuming 1200W according to omnilogic and my filter shows 9psi.
With two out of three valves fully closed, the wattage drops to around 900W and pressure goes up to around 15psi.
Gradually closing the third valve the power consumed drops as well. I was able to get it down to about 580W, with water just trickling and 20 psi on the filter.

Now, in comparison, if I run my pump at 30%, which is 1035RPM, the power consumed is at 90W and all three sheer descents trickle, not just one and the psi is so low that my filter shows 0psi. So clearly in my particular setup, with an over sized pump, slowing down the RPM is the way to go to save power.

So what does this really mean? Well, in my opinion, it means that there is no "right" answer. But the more information we have, the more informed decision we can make to fit our individual needs.

For example, on a brand new install or a major reno where the old equipment needs to be changed anyways, going with a VS pump makes the most sense, plus it's the only way if you want to integrate automation.
But for those with existing single speed pumps, or those that need to get a new pump because the old one failed, installing a flow control valve on the discharge may be a much better option than upgrading to VS pump and all the associated controls that go with it.

RPM is not the only way to control power consumption for electric motors coupled to centrifugal impellers.

 

[AndyTN]

You could also save 100% of your pool's electricity usage by substituting your pool pump for one of these...

 

[mas985]

So clearly in my particular setup, with an over sized pump, slowing down the RPM is the way to go to save power.

Lower RPM will always reduce power more than throttling.

RPM is not the only way to control power consumption for electric motors coupled to centrifugal impellers.

Perhaps not but it is always the most efficient.

 

[PoolNewb2020]

RPM is not the only way to control power consumption for electric motors coupled to centrifugal impellers.

Perhaps not but it is always the most efficient.

It is not "perhaps", it is a proven fact.

Efficiency is just part of the equation, initial cost also has to be factored in. This is especially true for existing installations. That's why I said in my initial post that VS pumps make the most sense in new installs.
You have to remember that going from a single speed pump to a VS pump is not just a simple pump change. You need to install the controls as well. So the initial cost is quite high.
Installing a ball valve, on the other hand, is dead simple and cheap. And as I demonstrated, it can easily achieve a 50% reduction in power consumption. It may not be as efficient, but will save the owner a lot of money without a large initial cost.

The premise behind my post was to inform people that there are other options to lower operating costs besides going VS.
Isn't this the whole point of this website, to arm people with knowledge so they can make informed decisions?

 

[mas985]

But you are totally missing half of the economics. There are on going savings with a VS vs Throttling.

For example, in the case I gave earlier:

Plumbing Curve 0.00820 @ 1375 RPM = 40 GPM, 13' of Head, 269 Watts
Plumbing Curve 0.06100 @ 3450 RPM = 40 GPM, 94' of Head, 1929 Watts

Assuming 8 hours of run time per day, and $0.15/kwh, it would only take about 17 months of operation to break even for an initial cost of $1000. After that, the VS would be saving even more.

 

[PoolNewb2020]

Well, that's just one example and for a VS pump compared to itself. And your $1000 cost seems to be on the very conservative side.
Plus, looking at the power consumption of that Intelliflo pump, it is probably around 3HP. A would imagine most pools with single speed pumps fall between 1.5hp-2.0hp, so the return period will be longer when compared to more powerful pumps.
It's like going to a hybrid car. It sure saves money, but makes far less sense if you already have a fairly efficient car to begin with. It makes much more sense if you have a gas hog.

Not trying to dispute that VS pumps save more than flow control, they do, but the upgrade may not make sense in some situations, looking just from cost perspective.

 

[mas985]

Ok, so lets compare the Intelliflo to a 1.5 HP Whisperflo single speed pump.

Intelliflo: Plumbing Curve 0.0082 @ 1375 RPM = 40 GPM, 13' of Head, 269 Watts
Whisperflo: Plumbing Curve 0.053 @ 3450 RPM = 40 GPM, 84' of Head, 1686 Watts

Plus using Amazon pricing:

Amazon.com: Pentair 011028 IntelliFlo VS Pool Pump, Almond: Garden & Outdoor

Amazon.com: Pentair 011028 IntelliFlo VS Pool Pump, Almond: Garden & Outdoor

www.amazon.com

Pump cost: $1130
Savings per month: $51
Break even: 22 months
Total savings for 120 months: $4990

But 40 GPM is kind of high for a VS pump in normal operation so the economics will get even better the lower the flow rate target.

Also, not sure where you live but where I am at, I pay $0.30/kwh most months and occasionally $0.40/kwh in the worst months. So the savings are twice as much and payback half as long for folks like me.

 

[Randrx2]

But you are totally missing half of the economics. There are on going savings with a VS vs Throttling.

For example, in the case I gave earlier:

Assuming 8 hours of run time per day, and $0.15/kwh, it would only take about 17 months of operation to break even for an initial cost of $1000. After that, the VS would be saving even more.

You are assuming that the 13’ of head the pump is pumping out is sufficient to overcome the head loss in the piping to achieve the flow rate. In some cases it may be. However, in cases where it is not, the user may not be able to lower the RPMs enough for a cost savings because they cannot achieve the flow they need.