The summer of 2011 sent the city of Houston into record heat, hitting 100 degrees for more than 45 days. To handle the cooling load, every building needed to operate its chillers at full blast, pushing the power grid into the red zone. But in the downtown commercial district of Houston — the world’s single largest chiller air-conditioning market, the Houston Police Department Building remained cool, thanks to three recently installed Smardt variable speed centrifugal chillers using Danfoss Turbocor magnetic-bearing compressors.
“On hot days, chillers run at maximum capacity, maximum speed," says Chase Raska, project manager for Schneider Electric Energy Solutions Division, who was responsible for the installation. "That's where the Smardt chillers with Danfoss Turbocor® compressors make a difference. Danfoss Turbocor® compressors can slow down during mornings and evenings, when it’s a little cooler, and save energy. Fortunately, this installation was up and running at the beginning of the summer —just in time for the worst heat wave we’ve ever seen in Houston.”
The Houston Police Department Building is a 28-story facility originally built for the Houston National Gas Company in 1967. “Because a utility owned the building, it was like energy was free,” says Raska. “From today's perspective, the lighting, insulation, and mechanical equipment were pretty marginal.”
An energy performance upgrade pays
The building was using two conventional 1,000-ton constant-speed centrifugal chillers dating from 1994 when the Houston Police Department acquired the building. That year the facility was renovated to incorporate a three-floor crime lab, an emergency tactical command center, a 20,000-square-foot fitness center, and other unique improvements. But it was hardly ideal from an energy standpoint.
In 2009, with money from the American Recovery and Reinvestment Act (ARRA), the city embarked on an energy performance upgrade. Specifications called for replacing the two 1,000-ton constant-speed centrifugal chillers with two newer constant-speed models. But Raska was able to show the City of Houston that the energy savings obtainable from variable speed chillers would significantly reduce energy and ownership costs.
Raska’s firm specified three Smardt WA240 chillers rated at 650 tons each. Each chiller uses five Danfoss Turbocor® TT400 G6 compressors.
“With Turbocor® compressors, you get substantial energy savings compared to conventional constant-speed oil-lubricated compressors,” says Robert Varney, project manager for C-Air-S Mechanical, the Houston commercial HVAC contractor who handled the installation for Raska.
“It’s typically very warm in Houston, and we have high dewpoints in the mid to high sixties,” Varney explains. “Under those conditions, a single constant-speed chiller is consuming 600 kW an hour. The motor is spinning the centrifugal impeller at maximum RPM regardless of the outdoor conditions. When it's cooler outside and full cooling capacity isn't required, mechanical throttling vanes or valves are used to regulate the refrigerant flow and reduce the chiller's capacity. But the motor is drawing amps to run at maximum RPM.”
Varney explains that “with Danfoss Turbocor® compressors, the motor speed can be reduced and the impeller RPM can be turned down to provide the exact level of cooling capacity required. The speed turndown reduces energy consumption to the point that now the whole building can be cooled for the same amount of power it took to run one constant speed chiller.”