- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
The concept of VFDs saving energy on centrifugal loads compared to throttling valves is established factual SCIENCE backed up by 20 plus years of empirical data. The only people disputing it are the valve mfrs, who have an axe to grind. Are there circumstances where it doesn’t make sense to use a VFD? Absolutely, and those are the cases cited by the valve promoters as evidence of it being a myth. It’s no myth, it’s just that some people think it’s magic, and it’s not magic either. It doesn’t save energy ANY time you apply a VFD just because you did. One of the biggest misconceptions comes when someone installs a VFD to PERMANENTLY reduce flow, but not flow that CHANGES in response to some signal. If you need to permanently reduce flow in a centrifugal pump, trimming the impeller will actually save you more energy than a VFD OR a throttling valve.
Also, IF your flow profile requires control down BELOW the lower range of VFD efficacy, then yes, you might need a throttling valve for that TOO, I do that all the time when needed. But WITHIN the “sweet spot” of where a VFD works best, which is ABOVE the point where head becomes too low and BELOW about 95% flow, the VFD energy savings is superior to that of a throttling valve. When your flow requirement results in a VFD speed that no longer overcomes system head pressure, then that’s when you need to run the drive at a higher speed and use the valve. In reality, that is something I seldom see compared with the VAST majority of centrifugal pump applications that operate between 60 and 90% flow, right where VFDs perform best.
Proportional throttling valves have their place, VFDs have their place, careful consideration of ALL aspects of a system are necessary to ensure you are doing the right thing, or combination of things, to maximize your total energy efficiency. But blanket dismissal of VFD energy savings is just wrong and in countless applications, properly applied VFDs have saved a lot more money than they have cost. That’s why you used to see power utilities PAY people to install them, their internal justification was the maximize their “avoided cost of delivery” BECAUSE the energy savings was so dramatic in so many applications.
Also, IF your flow profile requires control down BELOW the lower range of VFD efficacy, then yes, you might need a throttling valve for that TOO, I do that all the time when needed. But WITHIN the “sweet spot” of where a VFD works best, which is ABOVE the point where head becomes too low and BELOW about 95% flow, the VFD energy savings is superior to that of a throttling valve. When your flow requirement results in a VFD speed that no longer overcomes system head pressure, then that’s when you need to run the drive at a higher speed and use the valve. In reality, that is something I seldom see compared with the VAST majority of centrifugal pump applications that operate between 60 and 90% flow, right where VFDs perform best.
Proportional throttling valves have their place, VFDs have their place, careful consideration of ALL aspects of a system are necessary to ensure you are doing the right thing, or combination of things, to maximize your total energy efficiency. But blanket dismissal of VFD energy savings is just wrong and in countless applications, properly applied VFDs have saved a lot more money than they have cost. That’s why you used to see power utilities PAY people to install them, their internal justification was the maximize their “avoided cost of delivery” BECAUSE the energy savings was so dramatic in so many applications.
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