Ugh!! Water Well contractors...

[npalen]

Oh I am afraid using inverters, better knows as VFD's for down well pumps is not only recommended these days, but heavily pushed on every unsuspecting homeowner who is out of water and at the mercy of the installer. They use that phrase..."Adjusting the compressor motor speed gives exactly the right amount of cooling at the minimum amount of energy". Only a well pump is centrifugal, not positive displacement like a compressor, and reducing the speed actually increases the amount of energy used for the gallons of water produced. Yet millions are falling for the "energy savings lie" and think the problems inverter's or VFD's cause, as you just touched the tip of the ice burg on, are worth the added expense and short life of a "variable speed pump".

It is true that reducing the speed of a pump also reduces the energy needed to spin the pump and motor. However, using half the energy a variable speed pump has to run ten times as long to pump the same amount of water. The variable speed pump can use up to five times (500%) more energy than a standard full speed, inexpensive, and long lived well pump.

However, "home water use is (NOT) too intermittent to benefit much from a just it time water flow". The idea of the variable speed pump is good for many reasons. Working with a small pressure tank the pump will cycle for small amounts of water. Although, the benefit of "constant pressure" means the pump will never cycle during a long term use of water. A standard full speed pump with a large pressure tank will cycle less when small amounts of water are being used. But even with a large pressure tank system, cycling during long term uses of water can add up to destroy pumps much more quickly than necessary.

Along with being able to use a small pressure tank and "Things like soft start/stop, lower starting current, avoiding startup and shutdown water hammer, and reducing the torque on plumbing", there are many other benefits to a constant pressure well pump system. Out of sheer luck over thirty years ago I discovered a simple control valve can deliver "constant pressure" similar to or even better than a VFD. The difference is the valve is simple, inexpensive, and doesn't have all the problems that go with "electronic pulse width modulation", which is variable speed or VFD.

It is also an amazing fact of physics that when restricting a full speed centrifugal pump with a valve will the amp draw or energy used will reduce almost exactly as much as if you were reducing the pump speed with a VFD. I will try to find a pump horsepower curve to post. It is counter intuitive and hard to understand without a curve.

when restricting a full speed centrifugal pump with a valve will the amp draw or energy used will reduce almost exactly as much as if you were reducing the pump speed with a VFD

It seems like the above statement contradicts what you are saying in the second paragraph above about the inefficiency of a vfd driving a pump, but perhaps I'm reading it incorrectly.

 

[ponytug]

a tiny pressure chamber that adjusts flow to meet demand, without a big pressure tank, and with a more even pressure level.
So that sounds like a built-in CSV, right?

I've noticed, on our Lennox 1.0 ton mini-split, that it takes a long time to shut off when approaching the set temp. Is this a combination of sensing outdoor temp as well as near set point temp and inverter slowing the compressor to keep it running as long as possible? I've read that mini-splits work best (most economical) when running as much as possible so that is what "it" is trying to achieve?

I will let a CSV expert comment on that, but the Grundfos SCALA system runs whenever there is water usage in the house and the system was designed to be variable demand. It supposedly will even detect leaks.

What you describe on your Lennox is the exact behavior you get when the inverter is tapering the compressor output to not overshoot your set indoor temperature. The effect is that, yes, the compressor runs longer, but not at full HP, and using less energy. I would point out that besides the electrical design changes, in an ideal world, the manufacturer also changes how the lubrication and internal cooling are set up, so that lower speed operation doesn't affect the compressor life. So, slapping an inverter on any old compressor is not a recipe for success. The general add-on VFD "rule of thumb" is plus or minus 50% in an AC motor speed will generally work. By design, many systems are able to do well at a factor of 20 of motor speed up/slow down, more if it is a DC motor.

All the best,

Peter

 

[Nutcracker]

As to those constant pressure pumps, few if any of the drilling companies in our areas use them. We always felt they was turning the motors too fast & just more gadgets to worry about. We encouraged our customers not to use them. We wanted to drill not pull pumps. Whatever lasted the longest is what we sought after. We liked fat tanks & mostly used well x troll 250. We found the wider the tank the diaphragm/bladder seemed to last longer.

 

[Valveman]

when restricting a full speed centrifugal pump with a valve will the amp draw or energy used will reduce almost exactly as much as if you were reducing the pump speed with a VFD

It seems like the above statement contradicts what you are saying in the second paragraph above about the inefficiency of a vfd driving a pump, but perhaps I'm reading it incorrectly.

Maybe I should say it like this...
Everybody knows when restricting a pump a valve "burns energy". If you understand a pump curve you will see that varying the pump speed with a VFD burns almost exactly as much energy as a valve. So, if because of the drop in amps to spin the pump/motor they can say a VFD saves energy, I can also say a valve saves almost as much energy as a VFD. However, anytime you reduce the pump speed or restrict the flow with a valve you are getting fewer gallons per Kw. Again, there is noting more efficient than a full speed pump working at its best efficiency point. Although there are many good reasons for a constant pressure system, saving energy is not one of them.

 

[Valveman]

I will let a CSV expert comment on that, but the Grundfos SCALA system runs whenever there is water usage in the house and the system was designed to be variable demand. It supposedly will even detect leaks.

What you describe on your Lennox is the exact behavior you get when the inverter is tapering the compressor output to not overshoot your set indoor temperature. The effect is that, yes, the compressor runs longer, but not at full HP, and using less energy. I would point out that besides the electrical design changes, in an ideal world, the manufacturer also changes how the lubrication and internal cooling are set up, so that lower speed operation doesn't affect the compressor life. So, slapping an inverter on any old compressor is not a recipe for success. The general add-on VFD "rule of thumb" is plus or minus 50% in an AC motor speed will generally work. By design, many systems are able to do well at a factor of 20 of motor speed up/slow down, more if it is a DC motor.

All the best,

Peter

They spend a lot of money advertising those variable speed pumps like the SCALA. This is not because they are good for the consumer, but rather because the SCALA s the perfect "fluid" product, as it costs a lot and doesn't last long. On the false pretense of "saving energy" they trick you into the least reliable, shortest lived pump they make. Because of the complicated electronic controls in a VFD like the SCALA, they have an unbelievable failure rate. I hear from the supply houses that they are filling dumpsters with SCALA pumps that have failed. The SCALA has a worse failure rate than the MQ, which has been filling dumpsters for years. I will post a picture.

Let me say this again. Varying the speed of a compressor or any positive displacement pump can save energy. However, varying the speed of a pump with a centrifugal impeller always
causes more energy use per gallon pumped.

 

[npalen]

Maybe I should say it like this...
Everybody knows when restricting a pump a valve "burns energy". If you understand a pump curve you will see that varying the pump speed with a VFD burns almost exactly as much energy as a valve. So, if because of the drop in amps to spin the pump/motor they can say a VFD saves energy, I can also say a valve saves almost as much energy as a VFD. However, anytime you reduce the pump speed or restrict the flow with a valve you are getting fewer gallons per Kw. Again, there is noting more efficient than a full speed pump working at its best efficiency point. Although there are many good reasons for a constant pressure system, saving energy is not one of them.

Thanks for expounding on this.

So I guess we could say that a 1/2 HP centrifugal pump running with no head and no restricted output is more efficient than a 1.0 HP pump throttled with a valve to the exact same GPM as the 1/2. Right? A graph where the curves reflect that would be interesting.

 

[Valveman]

As to those constant pressure pumps, few if any of the drilling companies in our areas use them. We always felt they was turning the motors too fast & just more gadgets to worry about. We encouraged our customers not to use them. We wanted to drill not pull pumps. Whatever lasted the longest is what we sought after. We liked fat tanks & mostly used well x troll 250. We found the wider the tank the diaphragm/bladder seemed to last longer.

Good for you! Most drillers/pump guys are just the opposite, preferring to push the VFD as they make a LOT more money that way.

The WX250 is good, but that 44 gallon tank only holds 10 gallons of water. It is not that the larger diameter tanks last longer, but cycling on and off destroys the diaphragm in any pressure tank. The bladder or diaphragm in a tank goes up and down with each pump cycle. This is like bending a wire back and forth until it breaks. Eliminate the cycling by adding a Cycle Stop Valve to a traditional pressure tank system and the pump, tank, and everything else will last much longer.

 

[Valveman]

Thanks for expounding on this.

So I guess we could say that a 1/2 HP centrifugal pump running with no head and no restricted output is more efficient than a 1.0 HP pump throttled with a valve to the exact same GPM as the 1/2. Right? A graph where the curves reflect that would be interesting.

That is absolutely correct! But it is the same thing with a VFD. Restricting or slowing a 1HP pump down until it only uses 0.5HP energy would only be producing about 0.1HP worth of water. Varying the pump speed of a 1HP down to 1/2HP uses at least 500% more energy than a 1/2HP pump running at full speed. But they will show you amp meter drop from 9 amps to 4.5 amps and say you are saving 50% energy. However, running the pump slow enough to use 50% energy means it will have to run 10 times as long to produce the gallons needed. A centrifugal pump acts completely different than a compressor when varying the speed, and that is what confuses people into purchasing a VFD well pump when it is the worst thing they can do.

 

[Egon]

I was kinda thinking the energy input should equal energy output with the corresponding losses factored in.

 

[ponytug]

I was kinda thinking the energy input should equal energy output with the corresponding losses factored in.

Yes, they will. It is a fundamental law of physics. (Conservation of energy)

Not everything that is written is accurate...

Pump efficiencies can have rather high non-linearities in energy/volume water pumped (work), centrifugal pumps especially so. (E.g. centrifugal pump at 1 rpm is never going to pump water into any sort of head or resistance, but a positive displacement pump will meter the flow.)

All the best,

Peter