Variable Speed/Flow pumps and energy usage

Hi,

A bit of a noob here and I have been reading and searching this forum for an answer. Sorry if this has been addressed already.

Anyway, I understand from reading here the potential inherent problems with the reliability and longevity of using a variable speed/flow submersible pump (like the Grundfos SQE). What i don't understand is the claim that they do not or can not save energy.

Could someone please explain this? Is this based on ROI (and the high initial cost of the pump and the potential for frequent replacements) or are they not energy efficient at baseline? If not, why?

Thanks,
BM99

People who sell variable speed pump systems want you to think they save energy. In reality anytime you vary the speed of the pump, it uses more energy per gallon produced.

Example;
1 HP pump at full speed puts out 10 GPM and pulls a full 1 HP load, which is 10 gallons per horse power.

When slowing down the RPM, the amps will drop by 50%. This is why so many people think, and can make you believe, they are saving energy. However, when slowed to 50% load, the pump is only moving 1 GPM, and using ½ HP load. That is only 2 gallons per horse power, or 500% more energy used when varying the speed of the pump.

So there is NO return on investment. I could write volumes on how much extra it will cost to replace these expensive SQE systems every 5 years or less, compared to a less expensive standard type pump that should last 20 years. People who sell SQE pumps make a lot of money because, people who buy SQE pumps spend a lot of money.

valveman said:

When slowing down the RPM, the amps will drop by 50%. This is why so many people think, and can make you believe, they are saving energy. However, when slowed to 50% load, the pump is only moving 1 GPM, and using ½ HP load. That is only 2 gallons per horse power, or 500% more energy used when varying the speed of the pump.

Click to expand...

But if you have an application where you need 10gpm and also 1gpm would this not be better than a pump that only provides a fixed output? I'm thinking it is like a dimmer switch on a lightbulb. Some times you need the whole 100w, sometimes only 20w. With a dimmer I can control that and save some watts. Please correct me if I have mis-interpreted this.

BM

Oh there are lots of good reasons to be able to do high flow as well as low flow. Just don't expect or believe that you can save energy in the process. Yeah you can get 20 watts worth of light from a 100 watt bulb using a dimmer switch. But you may still be using 80 watts to do it, when a separate 20 watt bulb would save a lot of energy. The dimmer switch is burning the extra energy, the same way a variable speed controller does.

Just like a dimmer switch, a VFD doesn't reduce the energy consumption much more than just putting a dark lamp shade over the 100 watt bulb.

Modern dimmer switches do not use a resistive element, they automatically shus the light bulb circuit off every time the current reverses direction -- that is, whenever there is zero voltage running through the circuit. This happens twice per cycle, or 120 times a second. It turns the light circuit back on when the voltage climbs back up to a certain level.

To bad we can't run a pump on a modern dimmer switch.

Without arguing about the simplicity and longevity of the CSV vs. a variable speed drive pump, when I tested a pump throttled from full flow down to about 10% of flow at the output, the amp draw only fell a minute amount. So is it not true that when you restrict the output of a pump that your cost per gallon pumped also rises in a similar way to a VFD?

When you restrict inlet or outlet pressure of a water pump the motor draws less amps because it is doing less work. With a VFD pump the circuitry in the control board does the same thing as a dimmer. There is not a variable resistor in there so a VFD also uses less electricity at lower discharge rates.

Sorry but the motor in the system is the resistor that uses the electric current. The VFD just changes the frequency of the power so the motor runs at a lower speed. It does not shut off between cycles. As I explained earlier, varying the speed causes 500% more energy used per gallon produced. Not only that, but even at full speed and full flow, the energy used by the VFD device itself, increases the cost per gallon compared to not having a VFD. In other words with a computer operated motor, you have to consider how much extra energy the computer itself is drawing.

As for the amp draw when restricting the pump with a valve, some pumps do much better than others. For instance a 10 GPM Sta-Rite pump will only drop about 10% in amperage, where the same size Grundfos pump will drop about 50%. This has to do with the difference in a pump with a floating stack of impellers, compared to a fixed stack of impellers. And yes this is very similar to the energy used by a VFD. The energy is reduced some what as the flow is reduced but, it is still more efficient to run long term uses of water at the highest flow rate possible. For short term uses such as just taking a shower, the extra energy used is inconsequential compared to the benefits of being able to safely use water at any flow rate required, especially at a constant pressure.

What happened to my post?

Its a pity, Its the most detailed description I saw aboout them anywhere!

A bit technical though. That technology has been around for years. I first ran into it with commercial chillers and air handlers and now the Carling company is using a VFD motor on an oil burner to vary motor speed and the firing rate at the same time. I have however had to pull a crap load of VFD water pumps. The frustrating part is that usually you can not find the reason for failure. Some of it might be manufacturing, some design flaws and some installation error. Either way, I don't have a lot of faith in the system at this point and they are a whole lot of money.