Single speed pump vs. Variable speed pump

[Randrx2]

Hello. I could not find this topic in the deep end, so please let me know if it has been discussed before.

When most people are changing out an older single or two-speed pump, the recommendation is to use a variable speed pump for the savings on electricity. The thought is that you will run the VS pump at a lower speed, thereby saving on electricity. However, for a centrifugal pump, a motor is a motor, and for pumping the same flow of water, the same amount of power is used if you are pumping the same flow as a single or two-speed pump.

If people want to reduce their flow, why not just close down an outlet valve from the pump? This would reduce the flow of the pump and therefore reduce the power. I understand that not everyone has an outlet valve they can throttle back. Most of the time there are only 3-way valves that just divert the flow from one return to another. But wouldn't a cheaper alternative to a VS pump be a single speed pump or two-speed pump with a valve added to the discharge? Of course, the discharge pressure would have to be monitored and possibly a relief valve would need to be added also.

What do you think?

 

[PoolGate]

I don't think reducing the output volume of a single speed pump would reduce the power usage. The pump turns at one speed regardless of how much restriction is put on it.

Also for those with SWGs running the pump at just enough to keep the flow switch happy on the SWG ensures the SWG runs 24x7 with the minimal electrical usage. I run my pump 24x7.

 

[setsailsoon]

I'll try the simple answer.

• First the motors aren't the same. VS motors are permanent magnet motors, single and two speed are induction motors. PM motors are 3-6% more efficient at any speed.
• If you close a valve on the discharge of the pump you have additional pressure drop in the system that uses horsepower to overcome the higher system pressure drop.
• If you slow the pump down the power consumption is greatly reduced. This is because of the Affinity Laws. Simply stated they are:
  • 1. The flow (GPM) varies proportionally with the change in speed. This means that twice the speed is twice the flow. One-third speed is one-third the flow.
  • 2. The pump head (pressure) varies with the square of the change in the speed. Two times the speed is four times (22) the head generated. Eighty percent speed is 64 percent (.802) the head generated.
  • 3. The power requirement (horsepower or kilowatts) varies by the cube of the change in speed. Two times the speed would burn eight times (23) the power. One half the speed would require one-eighth (.503) the power to drive the pump.

That last one makes a BIG difference if you can reduce speed and most pools can for a lot of the time.

I hope this helps.

Chris

 

[Randrx2]

I'll try and answer both the above responses.

Yes, lowering GPM for a centrifugal pump does reduce the power, even if it is running at the same speed.

The SWG is not a factor for this exercise. Also, the SWG can run on a timer and only runs for the amount of time needed to produce the amount of chlorine.

I will agree there is some small efficiency gained by the more efficient VS motor, but that amount is too small to offset the cost of the pump.

As the head goes higher, the flow decreases (pump curves will show that). The increase in head will not cause a rise in horsepower. The lower flow will cause a decrease in horsepower and amps.

Yes. The affinity laws! That is the whole principle this is based on.

If you look at any pump curve, that shows power and head for a given speed; the horsepower will reduce as the flow is reduced without varying the speed of the pump.

 

[PoolGate]

For mine (and many people's) SWG we run it 24x7 so that we have chlorine continuously being produced. I run mine 24x7 at 53%.

I think you are saying that if you close off a valve (restrict output) of a single speed pump (reducing the GPM), then the pump uses less power. I could see this happening but would love to see an actual tested amount of power usages at various output restrictions. I would think that instead of using power to overcome the suction side resistance it would instead simple use more power to overcome the pressure side resistance. Not so?

Here is a chart showing a variable speed pump output RPMs vs dollar amount to run it. To put in perspective, I run my pump 22 hours a day at 2000 rpms and 2 hours a day at 3000rpms.

 

[Randrx2]

Please see the attached diagram.
Typically, the pool pumps only show the top curve (head and flow).
What they typically don’t show is the bottom curve, that clearly shows that power goes down as flow is reduced.

 

[Randrx2]

For mine (and many people's) SWG we run it 24x7 so that we have chlorine continuously being produced. I run mine 24x7 at 53%.

I think you are saying that if you close off a valve (restrict output) of a single speed pump (reducing the GPM), then the pump uses less power. I could see this happening but would love to see an actual tested amount of power usages at various output restrictions. I would think that instead of using power to overcome the suction side resistance it would instead simple use more power to overcome the pressure side resistance. Not so?

Here is a chart showing a variable speed pump output RPMs vs dollar amount to run it. To put in perspective, I run my pump 22 hours a day at 2000 rpms and 2 hours a day at 3000rpms.

Understood about SWG running 24/7. However, you could reduce the flow from the single speed pump and still achieve the same results.

Yes, if you close a valve, and reduce flow, the power goes down (see pump curve in previous post). Standby for video showing this.

I agree with your chart. Lower speed defiantly lowers power use, thereby reducing operating costs. However, if you reduce the flow of the single speed pump, you achieve the same thing, without the upfront cost of the VS pump.

 

[setsailsoon]

I'll try and answer both the above responses.

Yes, lowering GPM for a centrifugal pump does reduce the power, even if it is running at the same speed.

The SWG is not a factor for this exercise. Also, the SWG can run on a timer and only runs for the amount of time needed to produce the amount of chlorine.

I will agree there is some small efficiency gained by the more efficient VS motor, but that amount is too small to offset the cost of the pump.

As the head goes higher, the flow decreases (pump curves will show that). The increase in head will not cause a rise in horsepower. The lower flow will cause a decrease in horsepower and amps.

Yes. The affinity laws! That is the whole principle this is based on.

If you look at any pump curve, that shows power and head for a given speed; the horsepower will reduce as the flow is reduced without varying the speed of the pump.

That's exactly my point. If you reduce flow by restricting flow the pump continues to run at the same speed and consume the same power but move less flow because the discharge pressure is higher. If you reduce impeller speed instead of restricting flow the power is reduced by a cube factor(see law 3).

There's very practical advantage over a 2-speed induction motor. If you have a VS motor with a frequency drive you can almost always set a flow rate that's just what you want and not have to run full speed with no savings and a partially closed valve. Solar on the roof usually needs higher, swg often needs lower.

I hope this helps.

Chris

 

[Randrx2]

That's exactly my point. If you reduce flow by restricting flow the pump continues to run at the same speed and consume the same power but move less flow because the discharge pressure is higher. If you reduce impeller speed instead of restricting flow the power is reduced by a cube factor(see law 3).

There's very practical advantage over a 2-speed induction motor. If you have a VS motor with a frequency drive you can almost always set a flow rate that's just what you want and not have to run full speed with no savings and a partially closed valve. Solar on the roof usually needs higher, swg often needs lower.

I hope this helps.

Chris

Reducing impeller speed reduces power. Agreed. However, reducing the flow of a centrifugal pump causes the pump to work less, therefore less power. The centrifugal pump is not trying to maintain a constant pressure.

See this video that shows by throttling the the discharge of a pump, the power is reduced:

.

Here is a simple demonstration with a vacuum cleaner:

I totally get the purpose of the VS pump applications. I have a VS pump. I am just trying to argue the myth that they save money over a single speed or two-speed pump. My argument is that by throttling the output, you can achieve the same thing.

 

[setsailsoon]

Reducing impeller speed reduces power. Agreed. However, reducing the flow of a centrifugal pump causes the pump to work less, therefore less power. The centrifugal pump is not trying to maintain a constant pressure.

See this video that shows by throttling the the discharge of a pump, the power is reduced:

.

Here is a simple demonstration with a vacuum cleaner:

I totally get the purpose of the VS pump applications. I have a VS pump. I am just trying to argue the myth that they save money over a single speed or two-speed pump. My argument is that by throttling the output, you can achieve the same thing.

Thanks I understand what you are suggesting. I just can't ignore the 3rd law. You may get a little reduction by throttling to almost zero flow which perhaps a more efficient operation point but it's nowhere near the cube root benefit of a slower rotation of the impeller. Look at your flow. You're almost at zero on the vacuum cleaner and you're still drawing 6.8 amps. If you had a vs motor turning at very low rpm you'd have the same flow but your amps would be almost zero...

 

[Randrx2]

Thanks I understand what you are suggesting. I just can't ignore the 3rd law. You may get a little reduction by throttling to almost zero flow which perhaps a more efficient operation point but it's nowhere near the cube root benefit of a slower rotation of the impeller. Look at your flow. You're almost at zero on the vacuum cleaner and you're still drawing 6.8 amps. If you had a vs motor turning at very low rpm you'd have the same flow but your amps would be almost zero...

It would be almost zero, but there is a minimum operating speed that it can actually spin at. So you can only reduce the speed so much.

Also, look at the pump curve. If you reduce the speed to the desired flow, it is the same power as when you reduce the flow with a valve.

 

[BowserB]

A lot of science here, but all I know came from Pentair. They say simply that reducing the speed to 1/2 reduces power consumption to 1/8. So my pocket calculator says if I run twice as long at half the speed, I get the same filtration for 1/4 the kwh. If the values on the pump display are accurate, here are some numbers I've seen for speed vs. power consumption with my Pentair Intelliflo VSF (3HP): full speed 3450rpm-2,530w; 1725rpm-298w; 1300rpm-111w (111 watts--that's a light bulb!) That 1/2 speed is not 1/8 power--actually a tiny lower. I wouldn't consider the savings enough to replace a working pump, but I'd definitely go for VS pump to replace a broken pump. Other benefit of a variable speed pump: noise. At 1725 rpm, with normal backgound noise in our neighborhood, I have to walk over to the pump and can see that it's running almost before I can hear it. Also, while I have no hard evidence, it stands to reason that a slower running pump and motor should last longer.

 

[mas985]

Power draw from a centrifugal pump is proportional to hydraulic HP (head & GPM) not just GPM:

Hydraulic HP = GPM * Head / 3960

Wire to Water:

Electrical Power (Watts) = GPM * Head / 3960 * 746 / Motor Efficiency / Pump Efficiency

Motor efficiency is fairly constant with a constant RPM and varying head loss but pump efficiency decreases with increasing head loss.

So by increasing head loss to decrease flow rate, it is not as much a power decrease as would accomplished by RPM alone. It is very inefficient to throttle a pump while changing RPM is much more efficient.

For example,

The Intelliflo at 40 GPM using RPM and head loss:

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

 

[MarkTX]

I work with hydraulics so my two cents
hydraulic power = flow rate x pressure
Throttling a valve will decrease the flow rate but increase the pressure. I agree that slowing the impeller (less rate, less pressure) will net much larger power savings.

/edit oh well pretty much an exact restatement of the post above. I agree with the other Mark and I type slow

 

[Randrx2]

We can show in a real world situation where power draw is reduced by throttling the output of the pump. This is how many fluid control systems operate in various industries.

Is what you are saying is that the power reduction is greater with the VF motor operating at a lower speed than throttling the single speed pump?

 

[mas985]

Is what you are saying is that the power reduction is greater with the VF motor operating at a lower speed than throttling the single speed pump?

Yes, Again real world results:

The Intelliflo at 40 GPM using RPM and head loss:

Plumbing Curve 0.00820 @ 1375 RPM = 40 GPM, 13' of Head, 269 Watts
Plumbing Curve 0.02100 @ 2070 RPM = 40 GPM, 34' of Head, 582 Watts
Plumbing Curve 0.06100 @ 3450 RPM = 40 GPM, 94' of Head, 1929 Watts

You can pull values off this chart:

 

[setsailsoon]

Is what you are saying is that the power reduction is greater with the VF motor operating at a lower speed than throttling the single speed pump?

Yes, absolutely. By the cube root. Savings are huge by slowing down the impeller.

 

[mas985]

Actually savings is a little less than the cubed root. This is because the drive itself consumes power and is constant over all RPM. Technically affinity laws only apply to the pump wet end.

 

[setsailsoon]

OK a little less than cube root. I agree - more of a practical field engineer here. But closer to cube than square right?

 

[JamesW]

Probably close to x^2.6.