Engineering Tomorrow Drives University Pool Pump Efficiency

Western Oregon University has been recognized as one of the top aquatic fitness programs in the United States. The recently enlarged Health and Wellness Center has also been recognized as one of the country’s best-in-class facilities in terms of design and practices — earning LEED Gold certification. Because of its dual commitment to aquatics and sustainability, the Center jumped at the chance to dramatically cut the swimming pool’s electricity consumption with two Danfoss VLT® AQUA Drives, which are specifically designed to meet the challenges of water applications.

“Being located in Monmouth, Ore., our school prides itself on teaching swimming skills for lifelong fitness and recreation,” says Jake Whisenhunt, the university electrician who supervised the drive project. “It’s important to keep pool water clean for swimmers and to meet state regulations. At the same time, we want to reduce electricity costs and the carbon emissions associated with producing electricity. That’s why we were interested to learn that Danfoss variable frequency drives could dramatically cut the pool’s pump energy usage.”

Pumps running at full speed waste energy

Whisenhunt got the energy-saving idea from Neal Hershey, director of technical services for Interlocked Filtration Systems, Tacoma, Wash. During a routine service call, Hershey suggested to Whisenhunt that equipping the pool pump motors with variable speed drives could substantially cut electricity costs.

“The Center’s pool is used more for recreational swimming than competition,” explains Hershey. “The pool measures 25 yards long, five lanes wide, and holds 100,000 gallons of water. To meet state regulations, the water is cycled through a filtration system every six hours using a 15-hp inverter-rated circulation pump. A smaller spa holds 10,000 gallons, which is cycled every 30 minutes using a 10-hp pump. Together, the pumps use about 14,200 kWh per month at an annual cost of $11,200.”

According to Hershey, the original circulation pumps were equipped with soft motor starters only, which reduced the amperage draw during motor startup and motor wear. “But because the pumps were running full speed at 60 Hz all the time, they exceeded the water turnover rate required by the state. They were wasting energy.”

Hershey calculated that the pool could still meet state requirements by running the 15-hp pump at 49 Hz and the 10-hp pump at 47.2 Hz instead of 60 Hz — an average speed reduction of 19 percent. He recommended installing a Danfoss VLT® AQUA Drive for each pump motor.

VLT® AQUA Drives reduce pump motor speed by changing the frequency of electricity supplied to the pump motor. “Reducing pump motor speed (RPMs) decreases power consumption exponentially,” explains Hershey. “Theoretically, according to pump
affinity laws, a 50 percent speed reduction would cut energy consumption by 87.5 percent. Of course, we can’t cut pump speed by 50 percent; minimum water flow has to be maintained. Nevertheless, with Danfoss variable frequency drives, we can slow
the pumps to match the required flow rate. In this case, pump speeds could be reduced an average of 19 percent.

“Energy consumption went down further. In fact, electricity savings were nearly 25
percent. That’s because instead of running at 60 Hz and using 14,270 kWh per month,
we could run an average of 48 Hz and cut electricity consumption 3,490 kWh per
month.”

Saving energy during filter regeneration cycles

Energy is also saved by taking advantage of the pool’s filtration system’s auto-regeneration feature.

“The pool uses a Defender filtration system made by Neptune-Benson,” says Hershey. “As a field technician, I have seen Defender systems deliver unsurpassed pool water quality. The system also has the ability to control pump function.”

The Defender system uses a unique filtration process. Once a day, the system turns on a pump to initiate a 10-minute regeneration process called the “pre-coat” cycle. The pressure induced by the pump causes particles of filter media, typically perlite, to attach to an array of flexible tubes. After 10 minutes, a valve is opened to allow the pool water to enter the filtration chamber, pass through the tubes and return to the pool sparkling clean.

“The Defender system regenerates the media automatically,” Hershey explains. During the filter pre-coat cycle, the controller uses the ‘jog’ speed feature programmed into the Danfoss VLT® Drive to reduce flow. The filter controller sends a signal to decrease pump speed during the daily pre-coat cycle. During those 10 minutes, the motors run at about 40 Hz. It’s not a long time nor a huge speed reduction, but the savings add up.”

By reducing speed to match the general flow rate and the pre-coat cycle  requirements, the VLT® AQUA Drives save the Center a total of $2,750 a year. As a result, the payback period for the variable frequency drive retrofit project was under two years.

Easy energy savings plus improved pump performance

Integrating the Danfoss VLT® AQUA Drives with the filtration system’s controls was easy. Hershey wired the drive to the Defender’s flow meter using the system’s 4-20mA 0-10 VDC analog output. Factory installed fused disconnects eliminated the need for additional disconnect boxes. Both drives were calibrated using the Automatic Motor Adaption (AMA) feature, which was accessible through the VLT® AQUA Drive’s menu.

“The AMA function optimizes the drive settings to the actual motor,” explains Hershey. “It compensates for tolerances in the motor and the motor cable’s electrical characteristics.”

There were operational benefits for Whisenhunt as well. “In addition to ramping pump motor speed up and down, the variable frequency drives gave us a convenient remote start/stop feature,” he says. “For safety, a ‘no flow’ detection feature uses relay contacts to ensure no chemicals are fed into the pool water if the drive goes off line. Plus, everyone noticed that with the VLT® AQUA Drives, the motors are running much quieter than before.”

Whisenhunt also appreciates that Danfoss VLT® AQUA Drives’ circuit boards are sealed against the corrosive effects of chlorine. The internal cooling system separates the electronics from the air flow to protect the system from particles and aerosols. Protected by a rugged NEMA 4X housing, the drives are also slightly smaller than
the original soft start equipment, providing more space in the pump room.

According to Whisenhunt, the Danfoss VLT® AQUA Drive installation is delivering the problem-free, energy-efficient performance he was counting on. “When Neal presented the idea that Danfoss drives could save energy and be easy to operate, I knew we had to dive in. The variable frequency drive retrofit project is a great complement to our Health and Wellness Center’s clean, green reputation.”