Merging New and Existing Systems

April 7, 2008
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Danfoss VLT® 6000 Series variable-frequency drives greatly increase the efficiency and cost-effectiveness of the system.


In 2003, the M. D. Anderson Cancer Center, Orlando, opened a new $63 million facility, combining five locations and bringing inpatient and outpatient services under a single roof. The center, which alone serves 4,000 people per year, is part of the sprawling downtown campus of Orlando Regional Healthcare (ORH) system, the largest medical center in central Florida and one of the top cancer care centers in the nation.

Of course, the design for the center called for the latest technology in cancer diagnostics and care. But it also required creating a comfortable environment for patients and staff while being energy smart. This demanded a highly efficient indoor comfort system. After all, the center was to become part of a 1,576-bed health care complex that annually serves 545,000 patients. Add in workers and visitors, and it’s obvious that a great many people depend on the cooling system.

THE BIG CHILL

The ORH central energy plant houses five centrifugal chillers with a gross total capacity of 5,900 tons of chilled water, which is pumped to multiple buildings across the campus. Prior to project design, the plant was operating four of the five chillers at maximum capacity, with the fifth out of rotation for redundancy. Several system anomalies prevented the plant from operating at optimum efficiency, and electrical utility costs were resulting in a premium of $200,000 per year.

The challenge was to retrofit the central plant HVAC system and correct the deficiencies so that the new cancer center could be served without adding chillers. The system had to deliver additional cooling capacity at a sustainable rate through the existing underground pipes. At the same time, it had to comply with current American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standards in energy efficiency.

The problem was that the existing primary/secondary pumping system was improperly mixing the chilled water primary supply and secondary system chilled return water in the decoupler/bypass. The mixing had resulted in a net plant capacity loss of approximately 20 to 25 percent, and the chilled water generated was significantly greater than the amount sent to the terminal loads.

To compensate for the loss, plant operators routinely increased secondary water flow, creating a temperature differential lower than design, and expending tremendous amounts of energy.

The system is comprised of five chillers, each on a loop like this one.

VARIABLE PUMPING DOES THE TRICK

The solution was an engineered conversion to a variable volume primary water management sys- tem, called Systecon VariPrime®, provided by Carroll Air Systems, Inc. The constant volume primary pumps serving each of the chillers were outfitted with Danfoss VLT® 6000 HVAC Series drives for pump speed control, along with Systecon VariPrime software. The software adjusts the flow of chilled water according to the buildings’ cooling demands and virtually eliminates waterflow through the decoupler/bypass.

The secondary distribution loop is programmed by wire-to-water efficiency control, which operates the most efficient number of pumps at all loads on the main chilled-water loop. As part of the construction project, the new building was outfitted with a Systecon tertiary variable volume pumping system utilizing Danfoss variable-frequency drives for pump control.

With this system, there is no flow in the chiller bypass (except to meet chiller evaporator minimum flow requirements at low loads) with variable system flow ranging from 1,000 gallons per minute to the maximum flow of 11,600 gallons per minute.

Closing the existing bypass and tying the variable primary and variable secondary pumps together added about 45 feet of head pressure capability to the secondary pump system. Both the primary and secondary pump systems can then operate at lower speeds.

Historically, engineers have been wary of systems that vary flow through water chillers for fear of equipment failure and evaporator bundle freezing. But the Systecon VariPrime software avoids such issues by reacting to exact load and flow requirements monitored by differential pressure sensors in the campus loop. The system also includes sensors across each evaporator, direct digital control (DDC) chiller controls and a control valve in the decoupler line to ensure proper flow at minimum loads, eliminating the potential for an evaporator freeze.

COOL RESULTS

“The pumping system now effectively responds to variable volume control,” said Mark Yankech, mechanical designer at Rogers, Lovelock and Fritz Inc., the architecture firm that designed the M. D. Anderson Orlando facility and other large structures on the Orlando Health System campus.

“Last summer, we were running four machines during peak periods at near 100 percent, and now four machines operate at 80-85 percent of full load.” The effect of the modifications is most evident in the secondary water temperature differential. Prior to construction, the temperature differential was approximately 8°F. That number now exceeds 13°F, which translates directly into energy savings.

The VLT 6000 HVAC Series drives with Systecon Variprime software make this system highly energy efficient. The software sequences the chillers and pumps so that they operate at their minimum kilowatt points, resulting in optimal efficiency at part-load and full-load operation.

The addition of the drives also reduces start-stop operations, and increases system temperature differential by maximizing the cooling tons circulated per gallon of water.

So far, the owner has realized an immediate and significant reduction in electrical energy consumed, increased system temperature differential and improved chiller capacity output. The system is able to run fewer chillers even with the additional cooling load of the new building. Chris Mills, manager of Carroll Air Systems Inc., said, “Closing the bypass and increasing the water temperature differential equated to a significant return on investment for the owner - increasing temperature differential is return on investment.”

“This client likes the results,” says Yankech. “They added a quarter-of-a-million square feet, didn’t have to add any chillers, and yet they’re using less electricity.”

Current campus expansion plans include a new Hospital for Women and Infant Care, and an additional chilled-water plant to supplement the current system.

As the campus continues to expand its health care facilities, it will build upon this smart model for indoor comfort control.

For more information, visit www.danfossdrives.com.

Publication date: 04/07/2008

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