Variable-speed screw compressor reduces load efficiency when facilities are in limited use.

As energy prices rise, more businesses and facility owners are under pressure to reduce cost and are evaluating options to reduce wasted energy in their chiller plants.

Most complexes are fully occupied during business hours and unoccupied in the evening and weekends. Under these conditions many chillers are at times lightly loaded and inefficient. Such situations have led the HVAC industry to variable-speed chillers using screw compressors.

One of the first companies to develop such a product was Carrier with its Evergreen® 23XRV variable-speed screw chiller.

“The variable-speed screw gives owners the flexibility to reduce the load efficiently when the facilities are in limited use,” said Charlie Figueroa, Vice President and General Manager, North America Commercial Applied Equipment  “In the case of the Evergreen 23XRV, it delivers full-load energy efficiency ratings as high as 0.53 kW/ton. This significantly reduces peak demand charges as well as power distribution equipment size requirements.”

He added, “The part-load energy efficiency ratings are as high as 0.33 kW/ton, resulting in a low life cycle cost for a water-cooled chiller.”


Even though chillers traditionally have used CFC refrigerants such as R-11 and HCFCs such as R-22, more and more of the latest chiller technology is opting for HFCs that have non-ozone depleting potential. (The unit described in this article uses HFC-134a.)

“With the ozone hole at record proportions, it becomes even more important to accelerate the use of non-ozone depleting technologies,” added Figueroa. “The United Nations has previously reported that ozone depletion causes increased rates of skin cancer, eye disease and eco-system disruptions.  Consequently, the Montreal Protocol and U.S. Clean Air Act require the phaseout of all HCFC-based products.”


As the new technology was being developed to respond to energy efficiencies and ozone issues, researchers also looked at ways to make such units easier to maintain.

For example, “The Evergreen 23XRV compressor design has three moving parts and no slide valves, purge unit, or shaft seals,” commented Tom Franaszek, Carrier product manager for water cooled chillers. “This makes it easy to maintain, which is a critical feature for any facility manager who maintains a large number of buildings.”

The unit described here received favorable response, commented Kelly Romano, president, Carrier Building Systems and Services.

 “In just the first year of production, we have received orders in 15 states and 11 countries.”

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Publication date:05/07/2007