ASHRAE Journal - December 2014 - 66

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66

ASHRAE JOURNAL

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D ECEM BER 2014

FIGURE 1 Centrifugal chiller performance comparison (equivalent price) constant

speed (CS) and variable speed (VS).

Entering-Condenser Water Temperature
65°F CS
85°F CS
65°F VS
85°F VS
55°F CS
75°F CS
55°F VS
75°F VS

1

Chiller Performance (kW/ton)

* Similar price, constant-speed chillers operate more
efficiently than variable-speed chillers throughout their
operating range if the entering condenser water temperature remains near design. In this case the design
entering condenser water temperature was established
at 85°F (29.4°C) with the compressor impeller diameter
selected for this condition.
* Combining these first two observations:
* If the tower water temperature is either controlled to be near design, or if the system is
located in a consistently humid climate, the
constant-speed chiller selection will likely provide better performance.
* For variable-speed chillers a proper cooling
tower control strategy must be used to take
advantage of low outdoor wet-bulb conditions,
when available, to achieve "lower than design"
condenser water temperatures. Care should be
exercised to ensure excessive condenser water
pump and cooling tower fan energy do not occur when the control strategy is implemented.

0
0

20

40

60

80

100

Operating Load as a Percent of Design Load

* If demand charges are significant, the 13% design
power difference will result in a significant increase in
operating cost.
* In multiple chiller plants, one might consider a
combination of constant-speed and variable-speed
chillers to take best advantage of reduced energy consumption as well as electrical demand charges. In these
cases, the best return on investment often occurs when
high efficiency variable-speed chillers are used extensively for all low load and low temperature operation.
Constant-speed and variable-speed chillers are used at
high load conditions when high outdoor temperatures
prevail.
* The variable-speed cost adds for low voltage chillers (<=600 volts) are considerably lower than those
for medium and high voltage drives (>600 volts). The
benefits of variable speed must be evaluated accordingly.
In summary, variable-speed and constant-speed
centrifugal chillers perform differently with respect to
load and entering condenser water temperature. While
both benefit from reduced condenser water temperature a variable-speed chiller benefits more. Designs
should examine similar price constant-speed and
variable-speed chillers, and determine the appropriate
mix depending on the plant location, condenser water
control, load profile, and separate consumption and
demand charges.
ASHRAE Journal - December 2014

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