Standard vs. Variable speed pump

[Bhebert449]

I plan to talk with them about it but my primary concern is maximizing the benefit of the VS pump which they don't typically install. Want to know the options to best control pool and spa flow considering the spa spillway.

 

[manoweb]

• One is running the pump at a slower speed that consumes less total power because the work performed is less than at the higher speed. Often the pump is less efficient at this speed but the total work requirement more than offsets the efficiency reduction.

• 
  

Why do you say the "work" is less at a slower speed? Maybe this is the part that I still do not get. At a lower speed, do you run the pump for about the same amount of time compared to full speed?

If the pump is going to have to run mostly flat out to run the solar heater or something else so that you are really at max speed most of the time then it's really hard to pay off the big bucks for a VS. But there certainly are situations that easily pay off the VS... it just depends.

I am interested in solar panels and I thought I read somewhere that they would require a slower pump to operate correctly. Instead you say they need the pump running at full speed? Can you clarify this part?

 

[setsailsoon]

[/LIST]
Why do you say the "work" is less at a slower speed? Maybe this is the part that I still do not get. At a lower speed, do you run the pump for about the same amount of time compared to full speed?




I am interested in solar panels and I thought I read somewhere that they would require a slower pump to operate correctly. Instead you say they need the pump running at full speed? Can you clarify this part?

Work happens when energy is expended. Move one pound one foot and you do a certain amount of work. Move fewer pounds fewer feet and you do less work. Power is just how much work is done in a period of time. Your electric bill charges for power. So if your pool does not require a high flow-rate and you run the pump slower you reduce the number of hours the pump runs then your electric bill is lower. The point I'm making is that your power cost is lowered because you did less work and not because the VS motor is more efficient. It's because you're doing less work and getting fewer turns of your water through the equipment. You can get exactly the same effect by running your single speed pump less time... if the pool doesn't need the number of turns. But there are cases where you can actually adjust the speed of the pump slightly and improve its efficiency. You could also adjust the amount of time so you now have exactly the same number of turns and you will still reduce your power cost because the pump is more efficient. But in this case the efficiency improvement is usually only a couple of percent

Nothing I've said is related to solar power. My guess is the person that said solar powered pumps need to run slower is because solar powered panels don't provide enough power to run the pump at high speeds since there usually isn't room for enough panels to generate the power to run at high speeds. Pools don't care how the pump is powered. So the source of power can be solar, grid power or you can turn a crank but amount of water movement required for a given pool doesn't change.

Chris

 

[JoyfulNoise]

I'd be interested in seeing that picture, too.

On the suction side -

On the return side -

On the suction side there are two three way valves - there’s a valve actuator that selects where the water is drawn from (POOL side or SPA side) and then on the POOL side there’s another three way valve to select between the manual vacuum port I have and the skimmer/MD (sadly they are tied together).

On the return side there are also two three way actuators - one to select spa jet return or pool return. On the pool side is the choice of sending all the water to the pool returns or to split some of it off to the spa for the spillway.

POOL Mode - suction side pulls from pool and returns to the pool.

SPA mode - valves are set to suck water from spa drains and return to spa jets.

Pentair SPILLOVER Mode - suction side to pool and return side to spa jets.

My custom SPILLOVER mode leaves everything in POOL Mode but I use the spa makeup line to draw off water and run the spillway. This way, there’s no aeration in the spa AND I think my system pressure is lower as there’s lower head loss returning to the pool and spa makeup line as opposed to forcing all the water through the spa Venturi jets.

 

[manoweb]

Nothing I've said is related to solar power. My guess is the person that said solar powered pumps need to run slower is because solar powered panels don't provide enough power to run the pump at high speeds since there usually isn't room for enough panels to generate the power to run at high speeds. Pools don't care how the pump is powered. So the source of power can be solar, grid power or you can turn a crank but amount of water movement required for a given pool doesn't change.
Chris

Oh I mean heating water with solar panels. The water will have to be sent up to the roof and then come down. My understanding is that the only moment where I need the full head is when priming the pipes, after that it should be able to go slower as the static head of the ascent is canceled by the one on descent

 

[setsailsoon]

Oh I mean heating water with solar panels. The water will have to be sent up to the roof and then come down. My understanding is that the only moment where I need the full head is when priming the pipes, after that it should be able to go slower as the static head of the ascent is canceled by the one on descent

Mano,

Problem is you need that head to get started. I have this issue with my current installation and this illustrates why the best type of motor/pump depends on your installation. I have a 2 speed pump that was installed initially for my pool (before I added solar panels on the roof). Before the solar I ran the pump on high speed for a while and switched to slow speed to keep the spillover running just for asthetics. This was way cheaper than high speed and I already had plenty of water turnover. When I added the solar heater it adds about 15' of head requirement. The pump won't go that high at low speed so now I have to run at high speed anytime I want solar heat (much of the time during the day). So in my case I'm not getting much benefit from a low speed and wouldn't benefit much from a VS. I'd have to run the calcs to see if a VS could run at a "higher low speed" so I'd develop the head for solar but I doubt the net savings would ever pay for the very large cost increase since the solar panels need pretty high water velocity to maximize heat transfer. I could add a wire from my solar controller to switch to high speed on the pump anytime it runs also and I may do this in the future.

Bottom line is still the same. I don't think the savings in power consumption from VS motors on pool pumps is due to efficiency improvement. Almost always the majority of the savings comes from reduced flow rate and this can be achieved much more cost effectively by using a multi-speed pump since they are half the cost of VS pumps. But there are definitely exceptions where the VS pump does pay for itself.

I hope this helps.

Chris

 

[manoweb]

Chris - I do not have a pump nor a pool yet so I'm just asking based on what I read.
1) I understand that you need to prime the solar panels and you need a higher head for that - but once they are full of water, Hydraulics 101 - Have you lost your head? says that the static head gets compensated and you should be able to run the pump at a lower speed and still have it circulating.

Static head is the net elevation change of the water which can be either positive or negative and is directly related to the height of the water. So water moving from a higher elevation to a lower elevation has head gain while water moving from a lower elevation to a higher elevation has head loss. For example, in a pool solar system installed on a roof and where the panels/pipes are primed and completely filled with water, there is static head loss when the water rises to the roof but then there is also static head gain when the water falls back to the ground. The static head loss on the way up is directly offset by the static head gain on the way down so there is no net static head change due to the fact that the solar is installed on the roof. However, there can be temporary static head loss or gain during the priming process where there is head loss due to the elevation lift but since the return pipe is not yet full of water, there is no static head gain to offset it.

2) you say that water in the solar panels needs to run at high velocity to maximize heat transfer. I read the opposite, even if I cannot find that specific resource right now.
EDIT yes I have found more resources and you are right; A higher flow will heat *faster* when the water is cold because of the lower temperature gradient.

 

[Bhebert449]

On the suction side -

On the return side -

On the suction side there are two three way valves - there’s a valve actuator that selects where the water is drawn from (POOL side or SPA side) and then on the POOL side there’s another three way valve to select between the manual vacuum port I have and the skimmer/MD (sadly they are tied together).

On the return side there are also two three way actuators - one to select spa jet return or pool return. On the pool side is the choice of sending all the water to the pool returns or to split some of it off to the spa for the spillway.

POOL Mode - suction side pulls from pool and returns to the pool.

SPA mode - valves are set to suck water from spa drains and return to spa jets.

Pentair SPILLOVER Mode - suction side to pool and return side to spa jets.

My custom SPILLOVER mode leaves everything in POOL Mode but I use the spa makeup line to draw off water and run the spillway. This way, there’s no aeration in the spa AND I think my system pressure is lower as there’s lower head loss returning to the pool and spa makeup line as opposed to forcing all the water through the spa Venturi jets.

In your custom spillway mode how much flow do you get at your spillway? Would that be the same as having a pool mode that provides flow to pool and spa? My previous pool did not have a spillway only mode. Just pool and spa and when in pool mode I got flow to pool and spa and spillway.

 

[Litespeed]

Going back to the question of power usage, I set my variable speed pump at 1000 RPMs and it was using about 90 watts (per the screen) and the set it at 3000 RPMS and the electricity usage went to 1900 kwh. So, 3 times the RPMs but about 20 times the electricity usage.

 

[JoyfulNoise]

Still getting the hang of these AR Apps (most of which kinda suck), but the single return in the spa with 2/3rds of the flow going to it -

It’s enough to drive my 18” spillway with an 18” drop.

 

[Bhebert449]

Still getting the hang of these AR Apps (most of which kinda suck), but the single return in the spa with 2/3rds of the flow going to it -

It’s enough to drive my 18” spillway with an 18” drop.

So with the custom spillway mode you can essentially control the level of flow between pool and spa? Do you ever use the full spillway mode? I can’t see ever using that setting.

 

[JoyfulNoise]

So with the custom spillway mode you can essentially control the level of flow between pool and spa? Do you ever use the full spillway mode? I can’t see ever using that setting.

Yes, by adjusting the split between pool returns and spa makeup return, I can have a roaring spillover OR trickle-down-the-walls spillover. I have that three way valve automated and the internal limit cams set to 100% pool or 60/40 spa/pool (automation valves are dumb, they just go one way or the other so you set internal limit switches to control the two stop points).

If I were to use the canned Pentair SPILLWAY mode, then all of the pool water is routed through the jets and the spillway looks like a roaring waterfall. I much prefer my custom spillway because I get finer control over the return flow without any aeration effects.

 

[Jimrahbe]

Yes, by adjusting the split between pool returns and spa makeup return, I can have a roaring spillover OR trickle-down-the-walls spillover. I have that three way valve automated and the internal limit cams set to 100% pool or 60/40 spa/pool (automation valves are dumb, they just go one way or the other so you set internal limit switches to control the two stop points).

If I were to use the canned Pentair SPILLWAY mode, then all of the pool water is routed through the jets and the spillway looks like a roaring waterfall. I much prefer my custom spillway because I get finer control over the return flow without any aeration effects.

Matt,

If it works for you that is all that really matters, but I do have a question...

If you did not have the spa make up valve and you used the canned Pentair Spillover mode, what prevents you from just running it very slowly since you have the best VS pump known to man...

Seems to me you would have the same effect of supplying just enough water to barely spillover into the pool. What am I missing?

Thanks,

Jim R.

 

[JoyfulNoise]

Matt,

If it works for you that is all that really matters, but I do have a question...

If you did not have the spa make up valve and you used the canned Pentair Spillover mode, what prevents you from just running it very slowly since you have the best VS pump known to man...

Seems to me you would have the same effect of supplying just enough water to barely spillover into the pool. What am I missing?

Thanks,

Jim R.

Sure. My "homebrew" spillover mode and the canned Pentair spillover mode achieve practically the same result - running the spillway. I think the differences in my "homebrew" spillover is that I can return water to BOTH the pool and the spa and thus get better overall circulation as opposed to having all of the water emanate from the spa and thereby lead to dead-spots. The operating pressure is also lower since the 5 spa jets present a larger head loss than the returns do. My way also avoids additional aeration from the jets. You can stop aeration in the Pentair method too, but it requires walking over to the equipment pad and putting a cap on the air intake pipe or otherwise rigging up an additional valve to do it for you.

This was purely setup according to my PB's way of doing things as he has built pools for 25+ years and all of this automation is fairly new. So, in the days before automation, this method of using a spa return on the pool side of the return plumbing allowed him to run spillways and pool returns at the same time in a "set-it & forget-it" mode. When he setup my pool, he adjusted the speed so that everything basically ran all the time - suction side cleaner, pool returns and spa spillway. So, in his head, he was still stuck in the mindset of having one single speed, high horsepower pump and then using valves to fine-tune everything. When VSPs came along, he just saw that as an opportunity to run pump at a slightly lower speed than 3450RPM and thus get some savings. He had no idea how to setup Feature Circuits to create customized speed and valve options. I think a lot of pool builders really don't understand how these automation systems work and it is most often the case that the pool owner learns it over time. A few of the smarter PBs will leverage local Pentair reps to come out and program the systems for them but many just try to hack through it on their own and miss A LOT of good features. Couple that lack of automation knowledge with very little plumbing knowledge and you find a majority of the pools built have really poorly thought-out plumbing setups. I only wish more people would utilize the vast knowledge here on TFP to help with spec'ing the plumbing on pool builds - the net result would be much better pools for the pool owner (and more headaches for the pool builder have to deal with persnickety customers )

So, even though my pool has some redundant features, I like having the added flexibility of different water directions and speed choices.

 

[setsailsoon]

Chris - I do not have a pump nor a pool yet so I'm just asking based on what I read.
1) I understand that you need to prime the solar panels and you need a higher head for that - but once they are full of water, Hydraulics 101 - Have you lost your head? says that the static head gets compensated and you should be able to run the pump at a lower speed and still have it circulating.




2) you say that water in the solar panels needs to run at high velocity to maximize heat transfer. I read the opposite, even if I cannot find that specific resource right now.
EDIT yes I have found more resources and you are right; A higher flow will heat *faster* when the water is cold because of the lower temperature gradient.

Mano,

If your system has a perfect seal you could just prime once. In practice you'd have to have a leak proof valve on the discharge of the solar panel and a completely leak proof valve on the supply to the panel... in my experience this never happens. Water flows back when the system is off and water on the discharge line slowly leaks down.

On the velocity issue slow speed will heat the water to a higher temperature but the total heat is a function of the temperature and the throughput. This usually requires a much higher velocity and is limited by maximum frictional losses and pressure drop.

Good luck with your new pool and I hope this helps.

Chris

- - - Updated - - -

Going back to the question of power usage, I set my variable speed pump at 1000 RPMs and it was using about 90 watts (per the screen) and the set it at 3000 RPMS and the electricity usage went to 1900 kwh. So, 3 times the RPMs but about 20 times the electricity usage.

RPM's are not directly proportional to flow rate... not even close.

Chris

 

[Teald024]

The pump affinity equations can be used to determine how a pump's characteristics will change with speed. So if the flow rate, head or hydraulic HP is known for any one speed A, it can be calculated for another speed B using the following formulas.

GPM B = GPM A * (RPM B / RPM A)
. flow is directly proportional to rpm. half flow at half rpm

Head B = Head A * (RPM B / RPM A) ^ 2
. head is proportional to square of rpm. half rpm gives a quarter of head

Hydraulic HP B = Hydraulic HP A * (RPM B / RPM A) ^ 3
. power is proportional to cube of rpm. half rpm gives 1/8 of hydraulic power. but since motors lose significant efficiency % at half rated rpm, the realized power reduction is closer to 1/4 instead of 1/8.

This power savings is the primary reason a two speed or variable speed pump can save so much energy at lower speeds. So even though you have to run twice as long to pump the same amount of water, overall energy usage is cut in half. It has nothing to do with piping, friction losses, difference in pressure. It is purely the pump affinity laws of a centrifugal pump



For the solar heating portion, higher flow rates with a lower delta T between inlet & outlet are better (within reason). The heat transfer from the poly tubing into the water is faster/more efficient as the difference in temp between the two increases. Colder water / Hotter tubing is better than warmer water / warm tuning.
Also, you want to keep the water flow inside the tubing high enough to have turbulent flow / prevent laminar flow. This also helps to more evenly heat the water inside the tubing instead of creating hot water near the tubing wall and cooler water in the center of the tubing (large temp gradient).

 

[setsailsoon]

The pump affinity equations can be used to determine how a pump's characteristics will change with speed. So if the flow rate, head or hydraulic HP is known for any one speed A, it can be calculated for another speed B using the following formulas.

GPM B = GPM A * (RPM B / RPM A)
. flow is directly proportional to rpm. half flow at half rpm

Head B = Head A * (RPM B / RPM A) ^ 2
. head is proportional to square of rpm. half rpm gives a quarter of head

Hydraulic HP B = Hydraulic HP A * (RPM B / RPM A) ^ 3
. power is proportional to cube of rpm. half rpm gives 1/8 of hydraulic power. but since motors lose significant efficiency % at half rated rpm, the realized power reduction is closer to 1/4 instead of 1/8.

This power savings is the primary reason a two speed or variable speed pump can save so much energy at lower speeds. So even though you have to run twice as long to pump the same amount of water, overall energy usage is cut in half. It has nothing to do with piping, friction losses, difference in pressure. It is purely the pump affinity laws of a centrifugal pump



For the solar heating portion, higher flow rates with a lower delta T between inlet & outlet are better (within reason). The heat transfer from the poly tubing into the water is faster/more efficient as the difference in temp between the two increases. Colder water / Hotter tubing is better than warmer water / warm tuning.
Also, you want to keep the water flow inside the tubing high enough to have turbulent flow / prevent laminar flow. This also helps to more evenly heat the water inside the tubing instead of creating hot water near the tubing wall and cooler water in the center of the tubing (large temp gradient).

If flow is directly proportional to RPM the performance curve would be a straight line. It's not for a centrifugal pump. It's a curve. For positive displacement pumps the performance curve is a straight line (or at least virtually straight). Centrifugal pump performance curves can be close to a straight line only for narrow ranges. But not for larger ranges. Please see the curve below to verify:

 

[Teald024]

If flow is directly proportional to RPM the performance curve would be a straight line. It's not for a centrifugal pump. It's a curve. For positive displacement pumps the performance curve is a straight line (or at least virtually straight). Centrifugal pump performance curves can be close to a straight line only for narrow ranges. But not for larger ranges. Please see the curve below to verify:

View attachment 69671

For a centrifugal pump, the curves you show are the right shape and are not a straight line. If you look at the axis, the x-axis has flow and the y-axis has head. This is why they are curves and not straight lines. The curves themselves are at a specific speed.
The flow is directly proportional to RPM statement means that the speed is cut in half then the flow is cut in half. But also, you need to cut the head by 1/4 to get the new point. Your curve shows this. The lines are basically parallel to each other.
The formulas I posted are accurate and are basic pump affinity equations. I didn't discover them and they are well known in the industry and proven.

OP, sorry to sidetrack your questions.

 

[setsailsoon]

I completely agree with your comment we shouldn't hijack this thread... Also very familiar with the affinity equations. I learned all about them when I bought my "Perry's Handbook for Chemical Engineers" l back in the early '70's. Thanks for engaging in this thread.

Best regards,

Chris