VSD energy savings 18

[mfqd13]

Hi,

I would like to perform some detailed calculations to determine the energy saving in the appliance of a VSD (variable speed drive) in electric motors mainly for pumps.
I searched in many references and there are some explanations, but i didn't find yet one that suits for my intentios. Complete enough...
So, my base point is that i can only measure in site the power consumption of the motor and with this i would like to perform some calculations to estimate the energy saving after apply a VSD.

Can anyone help me?

 

[jonr12]

"Why are you worried about this thread getting deleted? If we discuss this rationally with a view towards seeking real optimum solutions, there shouldn't be any problem. When one goes wild discussing thoughts that make no engineering sense and conflict with well proven methods and test data, that's when threads lose credibility, cross into rant territory... and rightfully get deleted." BI

I am just worried that saying "VFD's burn energy", even though it is true, will be seen as "making no engineering sense and conflicting with well proven methods and test data". I have seen many places where the facts and figures have been deleted, obscured, or altered to keep the VFD from being shown in it's true light. I am seeking real optimum solutions and have been trying to discuss things rationally but, the engineers for the VFD manufacturers I have been talking with, will have none of it. Some even get red face mad, and refuse to discuss it further. They are committed to saying a VFD always saves energy, and are quoting test data which gives the VFD credit for energy savings, even though one can clearly see the VFD is not the real reason for the documented energy savings.

"Caveat emptor." BI

It is extremely hard for the buyer to beware when nearly all the data available is "myth-information", and nearly all the salespeople and engineers refuse to discuss this issue rationally. After the third engineer got red face mad and told me I was stupid for even considering that a VFD would increase energy consumption, they convinced me that the myth was true. This was the beginning of my problems and is the reason I want to get the facts straight here.

Stanier, that link you gave is excellent. I would love to discuss some of the other problems with VFD controls but, I would first like to put this energy savings and energy efficiency thing to rest. Partial load efficiency, cable losses, VFD losses, reactor or filter losses, and many other things will only add to the problem. I would like to discuss these things after I get my head wrapped around the curve issue.

Mauricestoker and ccfowler; I don't work on the air side. I am sure there are applications such as air handlers and conveyor systems where a VSD can save energy. However, now I also wonder if there are better ways than VSD to save energy in these applications. Could it be that since we are so convinced that VSD will save energy, that we stopped looking for other means of control. Could something like multiple motors and fans or reducing the pressure required, save more energy than a VSD, as it does in pumping applications? If 20% of the worlds energy is used in pumping applications, we really need to get the facts straight.

This has gotten long so I will start a new discussion and see if I can paste the curves I have been working on. The new discussion is at this link;

http://www.eng-tips.com/viewthread.cfm?qid=261532&page=1

 

[BigInch]

Sometimes they do, sometimes they don't.
Here we keep it rational with support calculations.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[stanier]

jonr12,

There are some other forms of VSD other than VFD that in the right applications give savings. A motor was developed in the UK with multiple windings. Teh number of windings used determined the speed of the motor. Efficiency of the motor did not change.

Then there are eddy current clutches, DC motors, slip ring motors, scoop controlled fluid couplings, Vee belts and pulleys, cone and disc variators, gearboxes etc etc. All have their place in the engineering world.

The point is selection of a pump in the first place for current conditions is the starting point. Stuff this up and yes, you may consider changing the pump/impeller , adding a control valve or perhaps a VFD. The VFD may save energy compared to the ill applied "over sized pump" however the right sized pump will always be more efficient as it does not have the burden of the VFD's energy waste, cable replacement, larger switch room, special motor etc etc.

The acid test for the VFD sales people is that:- if the pump is "correctly" selected for the current duty point with an adequately sized suction tank, then how does the VFD save energy? Look at your curves from that point of view.

 

[stanier]

Just saved this post to a Word document in case it gets pulled. Too much good stuff to waste here.

 

[MortenA]

OK - just a little input: I just finished an expansion to my house. Theres two new rooms with wooden floors (massive Douglas pine) and here i got floor heating.

The system is from Roth, and this uses a Grundfoss Alpha pump. A cute little thing with - lo and behold - VFD. Its not even hooked up to acontrol loop - you just choose your rpm on a dial. The rooms can be individually controlled so there are valves on each forward flow line. Very neat.

Anywhy - why did they put a pump in with a VFD when its a fixed speed pump for all practival matters?

Its cheaper i guess (for them - but maybe not for me in the long run?). The system can be installed in buildings of many different sizes, and be conncted to up to 6 individually controlled curcuits. So if they wanted to pick and choose the optimum pump they would have to engineer each system seperately. Now they can just put in the little Grundfoss - fits all.

Best regards

 

[BigInch]

No static head, so a valid VFD solution to widely varying flow requirements.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ScottyUK]

Here's their animation justifying the use of a VFD.

I think this is one of the applications where it might be justifiable because there is virtually no static head, just a variable system resistance to overcome (assuming zone valves or TRVs). There is presumably a discharge pressure sensor which causes the pump to speed up or down, although it doesn't explcitly say so. In my opinion even if the application is reasonably sound there are basic problems with mounting consumer-grade electronics on a pump casing which is likely to reach 80C or 90C in service. It just seems to be asking for trouble.

The manual is a disgrace - dumbed down to the utmost.


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If we learn from our mistakes I'm getting a great education!

 

[MortenA]

ah - i didnt think of the discharge sensor. I will go home and check the manual

Oh - and the system is very clever. The forward temperature can be adjusted via an inlet valve with a thermostat that only let sufficent fresh water into the secondary loop to maintain the required temp.

This is quite important especially during the initial heat-up where too hot water could cause the wood to dry out, get cracks and perhaps start to "squeack" (dont know the xact english term).

Best regards

Morten

 

[MortenA]

Theres a principal diagram on page 10/11 in think link. Its in Danish - but you dont need to read to get it:

http://net.grundfos.com/Appl/WebCAP...LPFAM&appcode=HEATING&pdfid=1341&language=DAN

Click next to the adobe icon

Best regards

Morten

 

[BigInch]

Hopefully your floor slats are placed so the outside of the tree, largest tree rings are up, and the heart of the tree, the smallest tree rings, are on the bottom. That will minimize tripping hazards.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ScottyUK]

Thanks Morten. The link to the English version is in my previous post - that's the manual I don't like.


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If we learn from our mistakes I'm getting a great education!

 

[jonr12]

That comparison is certainly designed to make the VFD look good. If the max power needed is only 22 watts as shown on the e-pump, then a standard 22 watt pump should be used. Then if the pump is a good design, a valve closing should drop the 22 watts to about 11 watts, without a VFD.

"In my opinion even if the application is reasonably sound there are basic problems with mounting consumer-grade electronics on a pump casing which is likely to reach 80C or 90C in service. It just seems to be asking for trouble." ScottyUK

Maybe that is why they do it. The high temp on the electronics is like a timed fuse. They can accurately predict and plan the life of the pump. Planning the life of the pump is important because, the pump company only makes money when they sell a pump. If you believe it is saving a lot of energy, you shouldn't mind having to replace the pump every year or two, right? I believe that graphic may lead to a myth-understanding?

 

[ScottyUK]

jonr12,

If you look at how a TRV-equipped heating system behaves as it approaches nominal temperature you'll find that the radiators slowly gag in as they near setpoint. Eventually a fixed-speed circulator will be developing a high head against the few remaining radiators in service, wasting energy in doing so because the radiator doesn't really benefit from the increased flow. So there is merit in what they are saying. Whether it would yield the same energy savings as teaching my wife that light switches are capable of turning lights off as well as on is a different question.

I'd like to think that a market leader like Grundfos wouldn't risk their market position with a product with a latent weakness. Maybe there is enough thermal isolation between the pump casing and the drive components. Personally I'd rather see the drive electronics in a separate enclosure mounted to a nearby structure but that wouldn't suit the the average plumber. Frankly the current brown / blue / green-yellow wiring to the fixed-speed pumps seems to be beyond the majority of the domestic and light commercial plumbers in the UK.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[btrueblood]

"That comparison is certainly designed to make the VFD look good. If the max power needed is only 22 watts as shown on the e-pump, then a standard 22 watt pump should be used. Then if the pump is a good design, a valve closing should drop the 22 watts to about 11 watts, without a VFD. "

There is trouble in that approach, on mild days when the heating is nearly shut down. A typical (commerical) zone heating valve, globe pattern, has trouble throttling below about 10% flow, and will tend to over-flow (or bang shut) as the pump head backs up. Also, continous operation of the pump at full head with hot water ain't much good for the impellers. Again, speaking of commercial grade stuff.

 

[jonr12]

OK we are getting a little off with this last one. I know there are lots of good applications for VFD controls, and I am not bashing all VFD systems. I think this may be part of the problem. VFD's work so well for closed loops, hot water, blood, cement, and a lot of other process controls, that we think they should be just as advantageous for every application.

I have been specifically talking about pumps with centrifugal impellers, moving relatively cool water, with high static head. In these type pumping applications I believe a VFD actually increases the energy consumption, and I know it decreases reliability. Like the one in the curve I am attaching because I still can't figure out how to paste it on the page.

 

[btrueblood]

We agree then, jonr.

 

[rmw]

Back a few posts to the subject of VFD's in power plants, BFW pumps, ID fan drives, and the like. That industry has been using VFD's for decades now. They were called steam turbine drivers and they did what a VFD would do if it were to be an electric drive. If the variable speed function that the ST's achieved was a viable process control, then the question of whether to apply ST's or VFD electric drive is only an economic one. (Assuming of course, similar rangeability.)

rmw

 

[BigInch]

Isn't that more of a drive type evaluation problem, rather than a VFD question, as in you wouldn't need a VFD with a diesel either.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[mauricestoker]

Jonr12,

I'm working on converting a constant primary/constant secondary central chiller plant to variable primary configuration. All load calaculations and life cycle costing has been done, and variable primary was analyzed as best life cycle cost and best life cycle energy usage. This will include relatively high pressure and relatively cold water temperature. If you are looking for an application of VFD to pumps, then variable primary chiller operation is a good one.

Nothing beats right-sizing for energy savings, so VFD's only make energy sense when there is no single right size, and the variation is enough to justify VFD loss (I typically use 0.96 for initial calculations, and variable guide vanes are more accurate down to 90%); the number I use for screening is 85%, same as mentioned above.

VFD's have good aplications outside of energy which should not be overlooked. Differential pressure/volume requirements, such as with labs, hospital, and most HEPA applications, are much easier using a VFD. With preset failover frequencies, they open up doors for easier, safer design (and TAB) than were available 15 years ago. A lot of the VFD's I've had installed had nothing to do with energy.

 

[rmw]

BigInch,

No, my point was along the line with what you have presented repeatedly in this thread. The driven equipment (pumps, fans, etc) would have to be just as carefully chosen with any variable speed driver (Steam turbine, liquid coupling, etc) with respect to its ability to meet the head and flow requirements for the process at speeds less than full speed.

So there is nothing new here under the sun. Just a new device on the scene.

rmw