VSD, ASD, VFD Energy Savings? 2

[jonr12]

This is a continuation of a discussion from this link;

http://www.eng-tips.com/viewthread.cfm?qid=260203&page=1

BigInch, the curve you pasted is typical. The efficiency is best only between 425 and 375 GPM, even with varying head. This is a very narrow margin to be working with, and anything below 375 GPM is losing efficiency. I made a curve of a similar pump with efficiency curves at the minimum and maximum speed. As you can see the efficiency and horse power curves are very close together. I only see the efficiency track the speed for about the top 10% of the flow rate. So from 0 to 90% flow, a VFD is wasting or burning energy, even when possible to reduce the head required. The green line is my interpretation of the actual horse power as the RPM varies.

 

[jonr12]

This curve shows a pump, as in my application, where the head required does not vary. As you can see the efficiency and horse power curve, for the full speed verses the minimum speed, are nearly on top of each other. Again the green line is my interpretation of the actual horse power as RPM varies. If we start adding back the other losses associated with VFD control, the VFD is actually causing a considerable loss of efficiency and energy.

 

[BigInch]

Ya. Typical curves fit typical cases.

I take it that your efficiency values at the right of the graph are for 100% speed (3450 rpm), so your efficiency at 2900 rpm and 370 gpm is,
adjusting that 370 gpm @ 2900 rpm to a flow at 100% speed (3450 rpm),
Q100 = 370 gpm / 2900 * 3450 = 440 gpm
looking at efficiency for the adjusted flow Q100 = 440 gpm gives about 63%
That's only 1% difference in efficiency when changing from 3450 to 2900 rpm.

Yes, we know that a VFD won't work well with your 85% minimum speed for maintaining minimum head, so I'm not surprized.

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"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/

 

[jonr12]

How do you get curves to paste on this screen without having to be an attachment?

 

[ScottyUK]

Have a look at Keith's FAQ from Power Engineering: faq238-1161.


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