Once a spring-fed mud pond, the South Windsor, Conn., Public Works Department completed the transformation of the town's landmark swimming hole into a modern-day aquatics center in 2002. Featuring three outdoor swimming pools — a beach-entry training pool for children; a 25-meter, eight-lane lap pool; and a main pool that includes a 50-meter, eight-lane lap pool with attached beach-entry free-form swim area — the redesigned complex attracts an average of 600 to 800 visitors every day during the summer.
Though the new construction brought the aging facility up to date, and up to state code, it also introduced a dilemma: how to keep 1 million gallons of recreational chlorinated water up to state health regulations without driving energy costs through the roof.
THE HIGH COST OF ‘CLEAN'
In addition to treating South Windsor's wastewater, the public works department's Division of Water Pollution Control maintains the filtration systems and water quality at Veterans Memorial Pool Park. The park is open June through August, but pool maintenance begins mid-April and doesn't end until late October.
Keeping three pools clean is a demanding, round-the-clock task. Minimum chlorine and pH levels have to be maintained, as well as alkalinity and hardness levels.
Water flows through a system of purification filters; four high-power pumps pull the water from the bottom of the pools, through a series of gutter systems, and into surge tanks (one tank per pool). From each tank, the water is pumped through a series of chemical-purification filters and back into the corresponding pool. To meet the high demands of this constant flow, the 152,000-gallon lap pool draws water through the system with a 10-hp, 200 V, three-phase pump. The 156,000-gallon training pool is equipped with a 20-hp pump, and the 755,000-gallon main pool uses two 30-hp pumps.
Each season South Windsor spends tens of thousands of dollars — by far the facility's largest operating expense — to power the four three-phase pumps. In 2006, Tim Friend, plant supervisor with the water pollution control division, began looking for ways to reduce the pumps' energy usage.
“We knew we were wasting energy running the pumps at full power, especially after hours,” says Friend. “Fewer impurities are in the water when nobody is swimming, so by slowing the flow at night we knew we could still maintain adequate filtration in the pool with the same levels of water quality.”
Friend's maintenance team tried several strategies with limited success.
The first attempt was to shut off the pumps at night. This saved in electricity costs, but made it difficult to maintain water quality. Another attempt involved throttling the valves to achieve the proper flow rate (2200 gallons/minute) to maintain water quality, but because the pumps ran on single-speed motors the tactic drove up the motor amps and back-pressured the equipment.
Friend finally turned to a local distributor he'd worked with while maintaining the town's wastewater treatment and purification plant. FlowTech Inc. had suggested using ABB Inc. variable-frequency drives (VFDs) to vary the speed, torque, and direction of alternating current induction motors by safely controlling the frequency of the electrical current supplied to the motor.
“By installing the drives at the pool, we could reduce the motor speed while still running our system with the valves completely open, achieving the full-flow rate we need to run the filtration systems. The drives were the perfect solution in that regard,” explains Friend.
He selected an ABB ACH550 VFD for the 20-hp motor and similar variable-frequency drives for each of the 10-hp and two 30-hp motors. FlowTech provided bypass panels for the pumps so pool engineers could bypass the drives as needed. While in bypass, the motors operate at full speed. Friend's maintenance staff installed the equipment, beginning with the smallest pump first, before the start of the 2007 pool season.
“The lap pool was our guinea pig,” says Friend.