1955 is calling and wants it's technology back! 2

[jraef]

Believe it or not, I am being asked to defend the use of VFDs for a large pumping project where the technology being considered is actually Eddy Current Clutches! I have not run into this idea for years. They currently have them on about 1/2 of their systems, the other half are (our) VFDs. The project scope is going to involve replacing everything because of capacity, so since all of the pumps are going to get bigger, they want to go with new speed control systems that are all the same, no more mixed bag. As you might imagine, the MEs are favoring the Eddy Current Clutch concepts, the EEs are wanting the VFDs. The Eddy Current Clutch people were invited in there yesterday for making a pitch, to which the EE manager was invited. He found himself being on the defensive quite a bit, but he doesn't know Eddy Current Clutches well enough to counter some of the claims they were making, chiefly about how they are easier to fix if something breaks and how they are MORE efficient than VFDs!

Mind you I've heard this before, but it was generally being pitched to EEs who knew better. Now the problem is, the MEs are buying it hook line and sinker and they have the power at the moment because the gist of the project is all about the increased flow. As the mfr of the approved standard of VFDs for this entity, it is falling on me to help my Salesmen retain this as an electrical solution. Here's where I need a little help.

My pitch is fine with regard to MTTR etc. We happen to be the PLC vendor as well, so there is no faster fix to get up and running than swapping out our VFD talking to our PLC, because the PLC stores the entire VFD programming and will automatically reconfigure it when replaced; literally making it plug and play (they are in MCCs). But where I'm struggling is on the energy efficiency issue. I find data from the primary ECC vendors that looks suspiciously like smoke and mirrors to me regarding their efficiencies, plus they appear to be comparing themselves to VFDs using performance data from very OLD studies, maybe even based on old Six Step Inverter technology (they discuss how motor power factor decreases the throughput efficiency of the VFD...). But I also found some independent studies, i.e. Department of Energy funded, where they do not appear to have a bias but are showing that at low flows on centrifugal pumps, the throughput efficiencies of ECC system drops precipitously. Fine, I can use that, especially in that the design flow rates will be 33-69% of pump design capacity at this point; in other words it will ALWAYS be low flow. However, looking at the project description I read that the pumps are almost all Axial Flow and a few Mixed Flow pumps! What I'd like from the experiences of this group is whether I am thinking about this correctly.

I'm not an ME but as I have been told, in an Axial Flow Pump the power requirement actually INCREASES as the flow decreases. I believe in fact that I was schooled (scolded) in here at some point years ago in that the Affinity Laws that make the energy efficiency argument so compelling for VFDs do not really apply to Axial Flow pumps. If that's the case, I may be in trouble. But then again from what I know of ECC drives, they may in fact be in MORE trouble in this case because in order to vary speed, they must increase the slip across the clutch. In doing so, they INCREASE the motor-to-driven-load losses. If the pump then needs MORE power as flow decreases, they are going to have to deliver MORE power at the ery time they have MORE losses. VFD's on the other hand, while not providing the same dramatic Affinity Law savings we are used to, will at lease deliver the REQUIRED power to the motor without added losses. That to me would make a case for VFDs being MORE efficient than ECCs in this application. So even though I may have to counter the initial pitch from our salesmen on the Affinity Law savings, I'd like to be the one who tells them the REAL truth about BOTH concepts.

How does that fly with the collective wisdom of this group?

"Will work for (the memory of) salami"

 

[Strong]

Lionel,

Yes, of course the ECC is a torque transfer device with output = input at any operating output speed. The amount of torque transfer varies with the slip speed (input - output speeds). We agree that calling an ECC a constant torque device may be a bad choice of words!

Walt

 

[jraef]

100+ pumps varying in size from 60-100HP, and in the 100HP applicatins, they are considering splitting into two smaller pumps, one fixed speed, one variable speed. That is now apparently the only place they are evaluating the ECC drive option, although that is more than half of the project (new information). They have decided to go with VFDs on all the others, but I still have to make my pitch. Not so much now just to get the whole enchilada, but to defend the technology against some of the disinformation that was spewed out in that meeting.

I've been boning up on Axial flow pumps and the thrubbing I got years ago in here (I think) is looking to be true. As flow decreases with an axial flow pump, power required actually INCREASES! That would be a big error for the ECC guys if they missed that, because if they don't really reduce power with speed, and the load requirement is calling for INCREASED power, then their solution is a double whammy with regard to efficiency. Still, I'm going to be talking to a room full of MEs, probably best that I leave that alone. I've learned that speaking about things you only marginally understand is usually worse than just keeping quiet. A number of those lessons came from this forum...

"Will work for (the memory of) salami"

 

[LionelHutz]

Walt - I don't agree with your statement that the amount of torque transfer varies with the slip speed. The amount of torque transferred is the torque required by the load at the speed the load is operating. The torque transferred could be equal at all speeds if the load requires the same torque at all speeds.

Jeff - any links to this info about the flow decreasing requiring more power? I'm thinking that may apply under valve control but not particularly well with speed control, at least not within a certain speed control range.

 

[Compositepro]

"As flow decreases with an axial flow pump, power required actually INCREASES!" That is true only if the impeller speed stays constant. If you throttle the outlet of an axial flow pump the discharge head will go up and the power used will go up.
However, many applications where these pumps are used are constant head where you are pumping water from one elevation to a higher elevation. It may help if you realize that a boat propeller is an axial flow pump.

It's important to keep straight the independent versus dependent variables. These depend on the application.
This link may be helpful for you

http://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/motor_tip_sheet12.pdf

 

[electricpete]

Here is a FAQ on the subject of BHP variation with flow (assumes constant speed.)
faq237-1543
The link within the FAQ is broken... should point here

http://locator.gouldspumps.com/cpf_0009.html

We have four fixed-speed axial-flow pumps arranged roughly in parallel serving our power plant main circulating water system. When we have three pumps operating and we start the fourth, we have a decrease in flow on each pump but an increase in current reading on each pump motor.


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(2B)+(2B)' ?

 

[jraef]

From the Wiki article on Axial Flow Pumps:

... the power requirement increases as the flow decreases, with the highest power drawn at the zero flow rate. This characteristic is opposite to that of a radial flow centrifugal pump where power requirement increases with an increase in the flow.

I know, I know, Wiki is not a definitive resource. But this isn't the only place I saw this discussed, it's just the one I could still find in my search history (my work PC periodically clears my history automatically per our IT rules).

But thanks, Compositepro, that actually makes more sense. I think I was mistakenly equivocating flow with speed, when they were thinking flow change by restriction.

And the link you supplied is the exact same one I have printed out as a handout for the meeting. The stated flow rates are, as previously mentioned, between 33% and 69%, so per the chart on that DOE document, they are going to be in the range of 29-65% efficiency with the ECC. No way a VFD is going to drop throughput efficiency that far even at speeds that low.

"Will work for (the memory of) salami"

 

[mikekilroy]

Seems like your meeting should go fast and easy now. methinks fig 7 chart is one you want - with vfd & ECC efficiency lines added in different colors. Seems that this one chart would make your case without any further comments or discussion required (other than backup proof).