Chillers & Towers / Compressors

College Gives Compressors A-plus for Energy Savings

August 1, 2011
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A first-on, last-off control strategy means the McQuay WMC is the first chiller turned on at the beginning of the day. The chiller is monitored; when it reaches 70-percent capacity, the load is handed off to another chiller.

With top daytime temperatures averaging 80°F and higher from April through October, Florida State College at Jacksonville (FSCJ) needs to be serious about keeping students comfortable. It also needs to take its role as an environmental leader seriously.

The North Campus facility houses classrooms, an auditorium, faculty and administrative offices, nursing labs, a culinary facility, and a bookstore. The four-building, 350,000-square-foot complex is served by three centrifugal chillers that deliver 1,000 tons of cooling, said Mark Gandy, FSCJ North Campus HVAC facilities manager. As part of a 2004 renovation plan, the campus researched a number of upgrade scenarios for its chiller plant.

Initial recommendations for a conventional chiller failed to meet budget and energy-savings requirements. “The best energy efficiency we get from our conventional chillers is 0.58 kW per ton of cooling,” said Gandy. “That’s pretty good - but with all the utility price hikes, we wanted to see if we could do better.”

SEEING IS BELIEVING

During the re-evaluation process, the college team (including project manager Ed Rock, mechanical engineer Geng Liu [now with California State University], and a representative from Brooks Air Systems) visited nearby Flagler College to see a chiller actually using the compressors.

“That installation proved to me that we could run our chiller plant in a new way,” Gandy said. “A conventional chiller is typically most efficient when it’s handling 80 to 100 percent of the cooling load. But starting in October through April, our chillers don’t see those kinds of loads most of the time. We can throttle back a bit.

“There’s a couple of ways a conventional chiller does that,” he continued, “but … the chiller’s impeller is still rotating at full speed, about 3,400 rpm. Consequently, you try to limit a chiller designed to produce 300 tons of cooling down to just 25 to 30 tons needed to supply one or two air handlers. Meanwhile, you’re still using a lot of energy to drive the compressor. All that work is going to waste because you’re not using the full refrigeration effect of the compressors.”

The compressor can throttle back, varying its speed downward to match the reduced load. Gandy could see that the proposed system “could handle those times of the year and parts of the day where we see low cooling loads, far more efficiently.” An integrated variable-frequency drive reduces compressor speed and maximizes energy cost savings as the load decreases.

CONTROL STRATEGIES

The chiller was installed in January 2007, and fully commissioned that June. As the team got more familiar with the machine’s capability, it devised a control strategy that yielded still more energy savings.

A first-on, last-off control strategy means the McQuay WMC is the first chiller turned on at the beginning of the day, when minimal cooling is needed. The chiller is monitored carefully; when it reaches 70-percent capacity, the load is handed off to another chiller. The lead chiller then reduces its speed to handle the remaining load, until it again reaches the handoff point to an additional chiller at 70-percent load. In cooler times of the year, it handles the entire load by itself.

“The high efficiency of this machine at part-load conditions makes it the lead machine when we’re operating at part-load conditions,” said Gandy. “That’s when it turns in some truly amazing energy savings. … The energy consumption of this machine is as low as 0.31 to 0.35 kW/ton.”

UTILITY ATTENTION

Gandy said the utility immediately noticed when the new R-134a chiller came on line. “They saw that the electric consumption was noticeably lower. Plus, they didn’t see a big demand surge when the chiller came on line. When this compressor starts, it’s barely a blip on their radar screen. The soft-start motor pulls just a few amps.”

Reducing high in-rush current at startup is not only an advantage to the utility, it also reduces thermal stress on the motor. The chiller’s energy efficiency also is aided by using two 150-ton compressors to split the load; only one compressor is turned on at startup.

In addition, the magnetic bearing system has one moving part (the motor rotor shaft and impeller assembly), which is levitated during rotation. Because the system eliminates any metal-to-metal contact, there’s no need for lubrication oil.

“We also like the quietness,” said Gandy. “The chiller plant is right next door to some classrooms. At low loads, when conventional chillers are noisy, this chiller hums like a vacuum cleaner at the far end of the room. You almost forget it’s running.”

When the chiller was installed, primary-secondary pumps and variable-frequency drives on secondary pumps were also implemented. The entire project was handled by W.W. Gay Mechanical in Jacksonville.

When all is said and done, the bottom line for the North Campus facility was energy efficiency. “Our energy bills have been cut by close to $90,000 a year,” said Gandy. “At that rate, our payback for this project easily fits our three- to five-year timeframe.”

Publication date: 08/01/2011

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