Kluver contacted an equipment supplier near St. Paul with a VFD specification, and the supplier recommended a VFD that was dedicated to water and wastewater applications. This allowed for simplified implementation. The VFD had many built-in features—including a cascade controller to radio frequency interface filter to input the choke to real-time clock.
On this plant’s basic positive-displacement blower project, the equipment supplier and Kluver took advantage of the built-in water/wastewater intelligence. They were able to input the DO value into the on-board, closed-loop control of the drive. The VFD display then showed the actual DO value, DO set point, motor speed and kilowatt consumption. As a result, an integrator was not needed, which helped with cost savings.
“Another cool feature is the VFD’s payback time,” added Kluver. “We entered the price of electricity, which is about seven cents per kilowatt hour, the equipment investment, and the load profile for the application. The VFD comes with software that continuously calculates the remaining payback time right on the display. It’s a great way to show how energy savings add up and when the investment is paid back.”
Kluver also appreciated that, when considering the implementation, a VFD was lent as a loaner.
“The supplier was really easy to work with. The loaner let us verify that the drive would work with our blowers and that we could get the energy efficiency we were hoping for,” said Kluver.
The drives are enclosed in National Electrical Manufacturers Association/Underwriters Laboratories (NEMA/UL) Type 1 cabinets in the blower room. No special ventilation was required or used. The VFD was able to handle high temperatures well. It is rated up to 50 C (122 F), because of a heatsink built into the chassis.
In calculating the return on investment, Kluver explains, “We calculated the potential electric savings assuming that kilowatt reductions would decrease in linear proportion as horsepower decreased. Based on those assumptions, we calculated that air flow reductions would cut kilowatt costs by $6,342 per year.
“Regarding equipment cost, we could subtract a hefty VFD rebate from the utility. For all three blowers, we received $8,341. That lowered our cost to give us an estimated return on investment of about 10 months.”
Kluver discovered that a restriction was designed into the original blower system to reduce excess air volume. Removing that restriction decreased the load on the motors, which would have improved the payback, except more flow was required than originally planned to maintain the necessary level of turbulence and aeration. In the end, the payback period was extended to about 15 months.
“That’s still very respectable,” said Kluver. “Especially with today’s higher energy prices, we’re saving the city of Mayer about $8,738 in annual electricity costs. By providing a robust product that’s easy to maintain and operate, we can count on the VFD technology to keep rewarding us with energy savings.”