With its copper-and-granite-covered, geodesic-shaped exterior and 85-ft atrium, the 231,000-sq-ft McNamara Alumni Center at the University of Minnesota in Minneapolis is a campus gateway and showplace, serving as a visitor and conference center, as well as an office building. But despite its ultramodern architecture, the building relied on a single 550-ton chiller for cooling throughout its first six years of operation.
“The mechanical room was built for the single chiller only, leaving little room for chiller failures,” Jon McCombs, the alumni center's operations manager, explained. “We had to wait until chiller technology was developed to the point where smaller machines with adequate capacity would fit in the room.”
The introduction of magnetic-bearing compressor technology was seen by the university as an opportunity not only to add redundancy to the HVAC system, but to reduce energy consumption and improve the life-cycle cost of the building's original chiller plant.
“We did our first designs around a 210-ton stainless-steel-plate heat-exchange chiller, which saved energy, but presented maintenance limitations,” McCombs said. “Cleaning the condenser required completely taking it apart, and the stainless-steel plate added to the unit's weight.”
Albers Mechanical Contractors Inc. of St. Paul, Minn., then introduced the university to McQuay's new 300-ton frictionless chiller.
“The new chiller technology appeared to provide the compact footprint and part-load performance — with the added benefit of extremely quiet operation — that we needed,” McCombs said. “Another benefit of the new frictionless centrifugal chiller is that the shell-and-tube design is easier to service than the steel-plate heat exchanger.”
REDESIGNING TO SAVE ENERGY
With approximately half of the McNamara Alumni Center remaining open for special events during evening hours and on weekends, the university sought to shift after-hours and spring and fall cooling responsibility to the McQuay frictionless chiller, leaving the 550-ton chiller to provide redundancy and carry larger loads.
“Redesigning the HVAC system opened up many new discoveries about how the existing system actually operated,” McCombs said. “For example, I already knew a 300-ton chiller could comfortably carry the building's load without sacrificing temperature or the comfort of the building. That's because, during the summer of 2005, I set the original 550-ton chiller's demand limit to 60 percent, or around 300 tons. I learned that even on an 84-degree-wet-bulb day in July — when the outdoor temperature was 85 degrees with 91-percent humidity at around 9:30 p.m. — we were maintaining a 40.5-degree set point on the chilled-water supply, and all of the air-handling units were running. For me, this proved we could consider our future chiller with 300 tons as a truly redundant system.”
During design, the university installed variable-frequency drives (VFDs) on both the evaporator-pump and condenser-water-pump piping systems and added digital flow meters to monitor both piping systems' gallons per minute.
“This allowed us to regulate flow and regain some flow loss due to triple-duty valves installed in the original design,” McCombs said. “By removing the triple-duty valve from the equation, we increased flows to both the chilled-water and condenser loops and further reduced energy lost to the triple-duty valve.”
McCombs continued: “We also added a dual-refrigerant monitor needed for the high-pressure as well as low-pressure refrigerant-monitoring system. This tied to our existing emergency refrigerant-purge and makeup-air unit. In the event of an emergency, alarm levels within the room trigger an alarm and start the safety system.”
REDUCED SIZE AND WEIGHT
“The first test for the frictionless chiller was actually getting the new machine in the door,” McCombs said.
At approximately 12 ft by less than 4 ft, including the electrical panel, the McQuay chiller helped to meet code requirements. Additionally, it stayed within load limitations, as a magnetic-bearing compressor weighs a fraction of a typical centrifugal compressor. Helping to minimize the chiller's footprint and weight was the chiller's quiet operation — sound-pressure ratings as low as 77 dBA per ARI 575, Method of Measuring Machinery Sound Within an Equipment Space — which allowed the university to avoid putting the chiller on a housekeeping pad.
“The McQuay chiller also was able to pass through our existing steel joists,” McCombs said, “making setting the machine into the room a six-hour process from the time the crane rolled up until the roof was closed back up.”
The university was able to use the existing electrical supply by installing an equipment automatic transfer switch. The switch transfers power between the two chillers, depending on which is running. (They do not operate at the same time.)
“New VFDs adjust flow rates as required for each chiller's design, which are a big help in simplifying the operation and saving energy,” McCombs said. “We installed an amp meter to monitor both chillers' electrical use from the same source. By selecting the chiller that matches the load, we reduce our energy and increase the life cycle of the original chiller. In addition, we have redundancy to around 85-degrees outdoor-air temperature.”
The chiller communicates control and monitoring information to a Johnson Controls Metasys energy-management system using BACnet.
PUTTING IT ALL TOGETHER
During its first season of operation, the McQuay chiller faced a higher number of cooling degree-days than had been the average over the previous three years.
“Factor in the new VFDs and lower gallons per minute in the air-handling units, and the new system has saved about 169,139 kwh,” McCombs said. “At the current local electricity rate (about 7 cents per kilowatt-hour), that's 10-percent less in energy costs compared to operating one non-redundant chiller system.”
The McQuay chiller is in use about 65 percent of the time — whenever part-load performance provides the greatest energy benefit.
“That can be as low as 0.33 kw per ton IPLV,” McCombs said.
The 550-ton chiller's seasonal run-time hours have been reduced by approximately 1,800. Total chiller run-time hours are approximately 3,000 a year.
“The larger chiller operates for just over one-third of the season,” McCombs said. “We've learned that,