How to Evaluate Chillers for
RESEARCHERS RECORD PERFORMANCE OF DIFFERENT CHILLER
TYPES IN IDENTICAL CONDITIONS
mechanical “unloaders” to release pressure
and provide stable operation. The VSS
meets shifting loads only by regulating
In a previous study published in 2013,
GSA reported that the MBC provided 42%
energy savings compared to the conventional rotary compressor. In the new study,
GSA and researchers from the Oak Ridge
National Laboratory (ORNL) measured similar performance from the VSS and MBC.
Both are considered to be good options for
Researchers concluded that the VSS
consumed an estimated 3.4% less energy
on average than the MBC. The VSS
chiller was also more versatile and able
to perform in ambient conditions not
typically found in Washington, D.C., from
55 degrees F. in spring to more than 95
degrees F. in summer. The VSS and MBC
chillers had similar sound ratings at 77–83
decibels. Both had energy consumption
measured at 35% better than the current
Federal Energy Management Program
(FEMP) minimum standard for water-cooled chillers.
For more information on the research,
see the report, Variable-Speed Direct-Drive
Screw Chiller, at www.gsa.gov/gpg.
If you are facing the costly challenge of replacing a chiller, the government has some free and credible guidance for you based on extensive testing.
The GSA’s Green Proving Ground (GPG)
program evaluates building technologies
under real-world conditions across the
agency’s immense portfolio. The program
provides succinct summaries and recommendations based on GSA’s investments in
new technologies not yet widely deployed
in the buildings industry.
GPG’s evaluation of chillers notes that
efficiency and ease of operation have
increased over the past 15 years but that
there are significant differences in the
design of equipment now available on the
For its test-bed evaluation, GPG used
the Sidney R. Yates Federal Building in
Washington, D.C., a 208,000-square-foot
historic facility completed in 1880 and currently occupied by the U.S. Forest Service.
A 2013 renovation of the building included
a new building automation system.
GSA installed two different chillers to
the same chilled water and condenser
loops to create conditions as identical as
possible to a real-world application. The
equipment types were a variable-speed
magnetic bearing chiller (MBC) and a
recent development, the variable-speed
direct-drive screw chiller (VSS). To accommodate shifting loads, the MBC uses
REPLACING A CHILLER is expensive, but
if your equipment is at the end of its life,
this free advice from the GSA can help
narrow down your options.
STEPS FOR CHILLER SELECTION
■ Consult with a mechanical engineer to do a thorough economic and technical
analysis including control optimization.
■ To size a replacement chiller correctly for peak cooling load, perform a new
heat gain/loss calculation.
■ Evaluate your building’s cooling load profile. If the building spends most of
its time at partial load, then prioritize energy consumption (k W/ton) at part
loads. If the building operates 24/7/365 at comparatively high loads, then
focus on efficiency at peak capacity.
■ Consider condenser water supply temperature during design. Magnetic
bearing centrifugal chillers need to be optimized for water temperature while
VSS compressors are a universal design.
■ Evaluate both your load and demand charges. If your building’s demand
charges are high, thermal storage or load shifting may be cost-effective
additions for a new chiller plant.
■ Consider the service provided in your area by prospective manufacturers.