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.
They took out the existing flow system, installed the bypass and new drive, and had the pump rewired and running within the same day. Although the installation process for all four pumps was scheduled for two weeks, it took only three days.
After installation, maintenance staff set pump speeds using the VFDs' onboard time clocks that initiate preprogrammed, preset speeds. They programmed the pumps to run at 90% power during open pool hours, and to ramp down to 60% overnight. By the end of the pool season, the settings reduced energy consumption by more than 65,000 kilowatt-hours — saving more than $7,000 in operational costs.
BEYOND ENERGY SAVINGS
In addition to lowering electricity bills, the drives also minimize noise. “Now we can actually stand in the pump house and hear each other talk. Before, we were never able to do that,” says Friend.
The drives also enable pool engineers to eliminate hard stops and starts during maintenance. For example, when pool strainers need to be cleaned, engineers ramp the motors down, shut them off, perform the necessary tasks, and then ramp the speed back up — all without across-the-line surges from hard stops and starts.
The drives protect the pumps and motors from over-current and undercurrent by automatically shutting down the system if problems occur, instead of allowing the motors to spin themselves into the ground.
“A major problem occurs when our pumps lose prime,” explains Bruce Lundie, facilities mechanic with the water pollution control division. Prime is lost either when too much air enters the system or when obstructions restrict water flow, causing pumps to stop circulating water. “We are stepping the speed down at night to save power, so if the strainer baskets become clogged with leaves or debris we could essentially not draw enough vacuum from the pump.”
When a pump runs dry, it can overheat and become damaged.
“This happened to us before where we lost prime during the night. We came in the next morning to find the pumps red hot and steam boiling out of the strainer baskets, because we had nothing in place to protect the motor or to shut down the pump,” says Lundie.
But with the new drives in place, an ABB technician used the company's Drives WindowLite software program to develop “user load curves” as a preventive safety measure. These user-defined adjustable load curves detect underload conditions so drives can automatically shut down pumps when prime is lost. To set the load curves, Friend's maintenance staff determined the amperage draw when the pumps lose prime, padded the number a little, and programmed the drives to trip on a fault whenever that low amperage limit is reached.
AN ECONOMICAL SOLUTION
In all, it cost $28,000 to update the pool's pump control system. The project was partially funded by an $11,000 CL&P energy conservation grant, with the division providing its own labor and materials, saving nearly $10,000 in labor and installation fees.
Friend anticipates recouping all or most of the cost of the upgrade by the end of the 2008 operating season, which ends this month. “Between the energy we're saving and the state energy grant, we're thinking the retrofit will have paid for itself. This is by far the best decision we could have made.”
This chart compares energy usage at full pumping speed to usage with variable-frequency drives (VFDs). In 2007, South Windsor paid about $9/kilowatt to power four pumps at its swimming pool complex.Source: Brian Robinson, FlowTech Inc.