Comparing Energy Consumption: Different Opinions on Using VFDs

Written by Joe Evans, Ph.D.

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In Figure 2 we see the original fixed speed, 60-Hz curve plus the performance curves for the speeds produced at 55-Hz and 50-Hz. The colored data labels are horsepower required, while hydraulic efficiency, at full speed, is again shown in black.

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Figure 2

The point where the frequency curves cross the system curve is the flow point for that particular frequency at 100-ft of head. It might be more meaningful if we showed each individual frequency curve. But, since the typical VFD has a resolution of 1/100 of 1-Hz, the number of curves (1200) would make the graph unreadable. You will have to visualize those additional curves on your own.

The tables below the system curve show the variable speed results for each major flow point. They include frequency to the nearest whole Hz, horsepower required, hydraulic efficiency, vf/mtr efficiency (includes both drive and motor losses), total efficiency, and power in kW. For flow points where motor loading is greater than 50 percent, vf/mtr efficiency is equal to drive efficiency x motor efficiency (0.98 x 0.89 = 87 percent). At flow points where motor loading is less than 50 percent (200-gpm & 300-gpm), motor efficiency is reduced to 88 percent.

If you compare the power required at 700-gpm, you will notice that the variable speed application uses about 3.5 percent more than the fixed speed example, even though the horsepower required is the same - not exactly what we would expect from a power saving application. Fortunately, this is the only point on the system curve where this situation occurs, and it is a function of the lower vf/mtr efficiency (87 percent compared to 90 percent for fixed speed) and the fact that head is the same for both applications.

But an interesting trend begins just below full flow. Even though vf/mtr efficiency remains lower at every flow point, the power required decreases quickly. At 600-gpm it is reduced by 9.5 percent, compared to fixed speed operation. At 500-gpm and 400-gpm, it is reduced by 21 percent and 31 percent, and at 300-gpm and 200-gpm, it is reduced by 36 percent and 41 percent, respectively.

These rather large power reductions are explained by a slight variation of the equation we used earlier. The kW required at any point on the system curve is equal to 0.746 x ((Q x H) / (3960 x tot eff)). Flow (Q) changes equally in both applications, but, unlike PRV control, a reduction in pump speed also affects the other two variables.

First, head is reduced with a corresponding reduction in speed, and kW is directly proportional to both head and flow at any point. For example, at 300-gpm operating head is reduced by 46-ft, and even at 600-gpm it is reduced by 14-ft. Second, hydraulic efficiency moves to the left with flow as speed decreases, and the kW required is inversely proportional to pump efficiency at those same points. At 300-gpm hydraulic efficiency is 59 percent under PRV control, but under VFD control it increases to 66 percent. (See my November 2006 column "Preservation of Efficiency.") Taken together, these two variables have a sizable impact on the power required at any flow point.

A very important result of this comparison is the fact that a relatively small change in speed can result in a much larger reduction in power. Over the entire range of flow, speed varies by a maximum of 641-rpm (18 percent), but the maximum power reduction over that range is 6-kW (41 percent) compared to PRV control. Even at 500-gpm, a speed reduction of just 291-rpm (8 percent) results in a power savings of 3.7-kW (21 percent).

Other Considerations

The potential energy savings available through variable speed control depend, to a large degree, upon the pump selected for a particular application. BEP efficiency is important, but the "range" of that efficiency is equally important.

In our example BEP efficiency is a healthy 80 percent, but at midpoint on the system curve (350-gpm), it is still 64 percent at rated speed and about 70 percent at its variable speed. Remember that efficiency moves to the left with a reduction in flow, and that movement will result in higher efficiencies at lower flows. Another consideration during pump selection is the curve shape. Our example exhibits a rise in head of about 50-ft from full to minimum flow and allows a control range of 12-Hz.

Although flatter curves can be ideal for circulation applications (vp/vf) where head is primarily a function of friction, they may not offer the control range needed for applications that require higher static heads. There are alternative VFD control techniques that can address these flatter curves, but with hundreds of pump models available from dozens of manufacturers, chances are you can find the proper curve shape for almost any application.

Finally, pumps must be matched to the "actual" conditions of the application. Unfortunately, many systems tend to be oversized and have a margin of safety that is often equal to the expected flow!

The pump used in our example is sized for a maximum flow of 700-gpm and a projected "typical" flow of 400-gpm to 600-gpm. If the maximum flow remains the same but the projected typical flow is reduced to 200-gpm to 400-gpm, then a duplex system (consisting of two smaller pumps) would be a better choice. The control scheme could employ two VFDs for variable speed control of each pump and a PLC to supervise their operation. Another option would be to use a single drive that would engage an across-the-line starter and bring a constant speed pump on-line when flow exceeds 400-gpm.

Joe Evans is the western regional manager for Hydromatic Engineered Waste Water Systems, a division of Pentair Water, 740 East 9th Street Ashland, OH 44805. He can be reached at http://www.pumped101.com/.

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