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Plant improvements led to an electric demand reduction in the magnitude of two megawatts. At the same time, the new system is quieter, more reliable, and now pushes chilled water into buildings that previously received inadequate service. The improvements included new chiller plant equipment, an improved campus piping system, and improved central dispatch of equipment.
The aggregate result of the system improvements is monthly energy savings of up to $100,000.
BACKGROUNDThe broad campus of the university includes over 10 million sq ft of building and supports an enrollment of more than 35,000. Growth on the campus has mirrored population growth in Arizona. Because of the climate, air conditioning is an absolute requirement for virtually all campus buildings and the university has long relied on a central water system rather than individual building systems.
Bill Wilson, assistant director of facilities management at the campus, indicates that problems with the campus chilled-water system emerged in the early 1990s, as the demand for chilled-water load grew and more chillers were installed at the three major chilled-water plants.
“It became increasingly difficult to supply certain parts of the campus. Even though we thought we had enough capacity, some buildings just weren’t receiving adequate chilled water.”
ON THE CASEPressures were varied widely throughout the campus. In the summer, delivered water temperatures started at 45 degrees F in the morning and reached 58 degrees F by afternoon. There wasn’t a lot of cooling going on and the problem was increasing. There was also concern about the efficiency of the system and the fact that much of the chiller equipment was aging and increasingly unreliable.
Given the size of the campus — over 350 acres — and the heavy air conditioning requirement in Tucson, it was essential that a solution be found. In 1994, the university retained the Tucson firm of GLHN Architects & Engineers to evaluate the situation and make recommendations to solve it. “We were asked to identify the problems with the chilled-water plant and to find solutions,” said Henry Johnston, associate/department manager for the firm. “We started by studying the system in detail.”
GLHN determined that, as the chilled-water system had expanded, not enough attention had been given to the piping network. In some case, major parts of the campus were supplied by single 12-in. radial chilled-water lines. There was a major choke point for chilled water in the center of the campus. One of GLHN’s earliest recommendations was to add large-diameter chilled-water pipe at strategic locations and create a true grid. This would allow looped feed to all of the major areas and promote system flexibility. It would also allow the system to operate efficiently at much lower pressures than the 50 psi currently being seen in many locations.
NEED FOR CHILLED-WATER DISPATCHSecondly, said Johnston, GLHN identified the need to create a chilled-water dispatch system to coordinate operations at the three plants. In the past, the plant operators would receive complaints about inadequate chilled water in certain areas, so they would keep increasing pumping rates. But, because of piping shortcomings, this often didn’t have the desired effect.
“In some cases,” said Johnston, “we had plants operating all of their chilled-water pumps at maximum output, but fighting each other.”
This improvement would allow central monitoring of conditions at each of the plants and at delivery points throughout the campus. A Trane Tracer Summit?building management system was installed, using the campus Ethernet system as the communication link, so minimal additional wiring was required. As the system distribution improvements were put into place beginning in 1995, Gordon Bush was named coordinator for system operations. Wilson indicated that a lot of the credit for improved system operations should go to Bush.
“He’s been patient but persistent in getting these plants to run at optimum levels,” said Wilson. “As we’ve gone along, we’ve learned that coordinating pumping operations is very important.”
Another recommendation of GLHN was to go from a primary-secondary chilled-water system to a primary-only loop. Johnston noted that this reduces pumping costs and allows delivery of lower temperature chilled water to use points
“Lower temperatures mean less pumping volume and more efficient cooling,” he said. “It makes this new chilled-water grid even more effective. With the elimination of the secondary loop, the design delta T of the chilled-water plant went from 45/55 degrees F to 40/60 degrees F. The benefits are obvious.”
BENEFITS OF CHILLER REPLACEMENTThe final improvement recommended to the university was the staged replacement of many of the chillers and constant-speed chilled-water pumps with new, more efficient machines and variable speed pumps. The older chillers were mostly open-drive ranging in age from 15 to 30 years. The decision was made to standardize on Trane CenTraVac?direct-drive hermetic chillers.
Said Wilson, “We chose these after serious consideration. We felt that their low internal pressure drops and their ability to deliver chilled water at 40 degrees F fit in well with our overall scheme.”
The duplex chillers are particularly attractive to the university because of their extremely low internal pressure drops and their ability to deliver chilled water efficiently at temperatures of 40 degrees F or even lower.
“With their high turndown levels, they also give us a lot of flexibility in the winter when chilled water demands drop,” said Wilson.
The three chilled-water plants now include 19 chillers. The university installed six Trane 2,000-ton duplex CenTraVac chillers and five other CenTraVac Model CVHF chillers totaling 5,400 tons of capacity. Another 2,500-ton duplex machine went into operation last summer. The other chillers include two Trane Model CVHB centrifugal chillers, four screw chillers, and one older open-drive centrifugal chiller. The total installed capacity is 27,300 tons.
VARIABLE SPEED CONTRIBUTES TO SAVINGSAt all three of the chiller plants, constant speed chilled water pumps were replaced with variable speed pumps. With the re-configuration of the plant and the change to direct primary cooling, the installed pumping electric demand was reduced by 1,800 kW, system pressures were reduced from as high as 50 psi to a nearly uniform pressure of 12 psi. With the new piping grid and improved valving, chilled water can be pushed out to all parts of the campus efficiently.
Wilson said that, with the installation of the CenTraVac chillers and the new pumps, acoustic levels in the mechanical plants have typically been reduced from 95 dB to 85 dB.
“These are now a lot more pleasant places to work,” he said.
The success of the improvements is measured in dollars and comfort. In the summer months, electric utility billings have dropped nearly $100,000 per month, said Wilson. This is attributed to the improved efficiency of the new chillers as well as the dramatic reduction in pumping cost with the chilled-water grid, variable-speed pumps, and direct primary cooling, he said.
Improvements in comfort are indicated by the dramatic reduction in complaint calls from all around the campus. All of the buildings are now receiving abundant chilled water at 40 degrees F and the temperatures don’t float upward during the day as they did previously.
Wilson indicated that all of the system improvements contributed to resolution of the problem.
“The best part is that now we have better comfort and lower electric bills,” he said. “It took a while, but it was worth it.”
Publication date: 02/04/2002