newbie question on pump efficiency

[txt4]

Getting lots of useful information from this board. Thanks.

I have some confusion about pool pumps. From what I am reading, the variable speed pumps are much more efficient than the two-speed pumps, which are more efficient than the single-speed pumps, but the typical explanation – that the pump can be operated at lower speed during low demand – is not at all satisfying.

If I used a bank of 3 single speed pumps (for about the same up-front cost as a variable speed) operating in parallel, then I could, in principle, duplicate the multiple 'speeds'. I could run the smallest only for low-level circulation, bump it up during heavy use, maybe run all three when cleaning, etc. But the tables I have seen show that none of the single speed pumps come close to the efficiencies of the variable speed pumps, so I'm obviously missing something.

I can believe that for a given head, a larger pump will pump slightly more gall/kWh than a smaller pump, due to internal efficiencies in the windings. And, for a given internal efficiency and plumbing configuration, a slower pump will be more efficient because the lower flow speed produces less head loss.

Thus, it seems that a given pump could improve its gall/kWh efficiency by running slower, but there must be a limit before residual power losses start to overwhelm the gain from reduced head loss. I find it difficult to believe that a 3HP pool pump stepped down to act like a 10 gallon aquarium pump is going to consume less power than the basic aquarium pump would do. So there must be some flow rate that optimizes efficiency, and I recognize that, if a pump is designed for maximum flow rate, the optimal flow rate may well be significantly below the nominal rate (reading previous posts seems to confirm this)

So do the variable speed pumps achieve better efficiencies simply by operating closer to this optimal efficiency than do single-speed pumps which are designed to maximize flow? If so, are there any single-speed pumps out there that target optimal efficiency rather than maximum flow? It seems that if I only needed x HP of power to my pump for most of the time, then I would be better running a x HP pump optimized for that condition than a much larger and stepped-down pump.

Or, is there something fundamentally different about the construction of the variable speed pumps that is not available as a single-speed?


If anyone has any recommendations on pumps, I would appreciate your thoughts. We are adding a partial in-ground (mostly in-ground, pad will be only slightly below water level) pool, about 17k gallons, with a single-story solar heater. Plumbing will be 2” for most of the run, splitting off to 1.5” lines at the pool.

 

[jblizzle]

Welcome to TFP!!!

There is a lot of information in this Sticky, if you have not seen it:
hydraulics-101-have-you-lost-your-head-t915.html

I am not sure I am clear enough on this to comment too much , so hopefully Mark will come along.

I do recall reading that the VS pumps use the least amount of power (or have the best effeciency?) at around 1000 rpm. A 2-speed pump runs at ~1700 rpm on low speed. So, if your flow requirements are low enough, then the VS can run slower and save more $ on electricity.

It seems that generally, the extra upfront cost of the VS only makes sense if your electricity cost is > ~$0.25 (I can not remember the # we usually quote) OR if you have a large variety of flow requirements which thus allow the VS flexibility to make sense (water features, spa, etc).

You did not mention any water features / spa, so a 2-speed might make the most sense. You could run a small pump like the 1 HP Superflo that I recently installed. I will use it on high speed in the summer for running the solar and currently run it on low speed when I am just circulating / filtering the water.

 

[Melt In The Sun]

A few comments:
The variable speed pumps are indeed less efficient at very very low speed than they are at just very low speed. I don't remember the RPM but I know I've read it on this site...something like 1000-1200 rpm is most efficient, even though the pumps can go lower.

Running multiple pumps off the same suction line does not work well, since they are fighting each other for water. If you wanted to have a "3-speed pump" by having 3 separate pumps, you would need really need 3 separate suction lines, if not 3 entirely separate plumbing loops. Obviously, that's not really practical.

The variable speed pumps use the same wet end as the single- and dual-speed pumps do. The extra efficiency is due to the better motor.

 

[bobodaclown]

Variable speed pumps might be more efficient, but not necessarily cost effective. It depends on the cost of your electricity, pump run time, and the cost of the motor/pump. The cut off point I think is about $0.25 KWH. If your electric cost more than $0.25 KWH then the saving a variable speed pump offer offset the increased price.

But if your strictly looking at the cost to move a given amount of water the variable speed pump wins. There's greater technology in the motor.

 

[mas985]

The efficiency of a pump can be broken up into two pieces, the motor and the wet end. Generally, the wet end efficiency is not much different between manufactures and models. Pump designs have been around for many decades and the designs are fairly advanced.

Motors too are pretty well designed so there is not much difference between manufactures. There can be differences in some motor design types that use larger windings or run capacitors for both speeds. However, the speed of a motor, any motor, can dramatically affect the energy consumption of a pump. This is due to the pump affinity laws which state the following:

GPM ~ RPM * CG
Head ~ RPM^2 * CH
Watts ~ RPM^3 * CW + OW

CG, CH, CW and OW are all constants. OW represents the drive/motor losses for a variable speed drive but it can also represents the losses in a two speed motor that are constant.

Anyway you can see that the watts will decrease much faster with a decrease in RPM than the GPM. Thus the gallons per watt-hr will increase with any pump when the speed is lowers. The difference between a variable speed and two speed is that the variable speed is able to better maintain efficiency when speed is dropped while the two speed is less effective. This is because a two speed uses as second winding for the low speed which uses thinner wire and has higher losses. Usually a two speed will lose 50% of the high speed efficiency at low speed but that is ok because the affinity laws state that the power should decrease by 1/8 on low speed (without accounting for efficiency) but the loss in efficiency brings it back to 1/4 of the energy use. But that is till pretty good. A variable will be around 1/7 of high speed power at half speed so it will be better than the two speed. But as was mentioned, that doesn't always pay for the higher cost of the pump. That depends on a lot of other factors.

Also, having three single speed pumps doesn't really help because the smallest single speed pump will still have a lower energy factor (gallons/watt-hr) than the largest pool pump on low speed.

 

[chem geek]

I took readings from my Pentair IntelliFlo VF pump at various flow rates as described in this post. You can see that power drops dramatically with RPM but that if you multiply out the flow rate times the time for a single turnover, that the pump efficiency flattens out, at least in my pool, at around 1500 RPM and that I can't go any lower than 1185 RPM anyway since that's 15 GPM (the lowest flow rate that the pump lets you program). Obviously, different plumbing will give you different numbers. And as Mark points out in the post following mine in that thread, efficiency bottoms out at around 1000 RPM so gets worse below that most likely due to the fixed losses independent of RPM.

 

[71por914]

Hi, I feel ( or rather had that pain) your pain on the equipment choices. I went through the VS and 2spd pump head banging and filters and ended up getting the 3/4hp 2sped Dynamo for our 13k pool. We have yet to have any water issues and I cant say I've noticed any electric bill jump. We've had a party or two and always get the comment "your pool is so clear".
120sf cart.filter @ 4psi presently 4 months old and the BBB method in SATX