For centrifugal pumps, energy consumption varies with the cube of the pump speed, so even a modest speed reduction of, say, 10% results in energy savings of almost 27%. This can represent a significant energy cost reduction when the motor is large. In general, larger motors tend to yield large monetary savings, even though the percentage energy savings may appear small. Therefore, you should concentrate on pumps with large motors before addressing equipment with small motors.
In addition, address new installations before existing installations to eliminate not only the costs associated with installing the control valve, but also the costs associated with removing it later. Note that the cost of installation for many VSD systems is often lower than that of a control valve systemeven before energy savings are consideredbecause the cost associated with the control valve (plus manifold) and the motor starter are eliminated. This is not necessarily true for large motors, where the cost of a VSD is high relative to a control valve installation.
Figure 4. Throttling increases hydraulic friction and moves the system curve to the left to maintain flow Q2 at pressure P2.
These energy savings alone are often not sufficient to justify a VSD installation. The real savings come from taking the implementation to the next level. One such application involved an air compressor, where reducing the compressor speed stopped the compressor from unloading, so it was always generating compressed air. Replacing the air compressor starter with a VSD had a simple payback (installed cost/savings) of approximately 2.7 years. Expanding the project to include process changes increased the project cost by 25%, but energy cost savings increased fourfold, and the simple payback was reduced to approximately 0.8 years!
This illustrates that replacing a starter with VSD technology can generate energy savings, but making process changes at the same time is a better investment that can save much larger amounts of energy. This is not possible in most applications, so you should first focus on applications where process changes are possible.
Figure 5. Because it runs close to the system curve, a VSD cuts the need for excess hydraulic energy and saves electricity.
Justifying the air compressor installation involved electrical, mechanical, chemical, combustion, utility, energy and other calculations in the context of plant operation. Aside from technical interactions between plant personnel representing various disciplines, it also involved detailed discussions with the compressor manufacturer regarding mechanical and heat-transfer issues inherent to the equipment itself.
Conceiving, justifying and implementing variable-speed drives often involve multi-disciplinary considerations. Failure to consider all these aspects can quite easily doom your project to failure, and cause you to miss your performance target.
David W. Spitzer is a principal in Spitzer and Boyes, LLC and can be reached at 845.623.1830 or www.spitzerandboyes.com.