ognizing the highest efficiency gains
because the chiller making the coldest
water does more lift. By putting the condensers in series counterflow, the lift
of each compressor is nearly the same
(figure 1). The result is a pair of chillers
working together to create high lift without sacrificing efficiency.
Figure 2. Module with dual-circuit chillers in
series provides 8 to 12 stages of compression and uses 0.445 k W/ton at standard ARI
Courtesy Trane Commercial Systems.
Series chillers can be selected in pairs,
or they can come prepackaged and tested
in the factory. One example is the dual-circuited Trane Duplex™ centrifugal
chiller. Dual independent circuits mean
that if one circuit is being serviced, the
other can continue to operate. Series-counterflow design gives all of the previously mentioned thermodynamic staging
benefits of a series pair, and single-pass
water-flow limits pressure drop.
These features are leading some
designers to put two Duplex chillers in
series. Each pair of dual chillers (with
multiple-stage compressors on each circuit) has 8 to 12 stages of compression
equally sharing the load (figure 2). The
chiller plant depicted here created series-pair efficiencies of 0.445 k W/ton ( 7. 8
COP) at standard ARI rating conditions.
Figure 3 shows the component and
system energy use of various parallel and
series chiller configurations using variable
evaporator flow with reduced condenser-water flow. The series-series counterflow
arrangement for the chillers reduces the
chiller energy to compensate for additional pump energy. In the case of this
particular installation, series-series counterflow saves $1.4 million in life-cycle
costs over the parallel-parallel alternative.
Series Chiller Improves
Efficiency, Flexibility in
Smaller Plants, Retrofits
The benefits of low-flow, low-temperature and high-efficiency are universal.
Smaller, non-centrifugal chillers can benefit proportionately more under these
conditions when placed in series. Helical-rotary chillers are sensitive to increased
lift and decreased condenser water flow.
Absorption chillers struggle to make
water colder than 40 F. Both can be put
upstream in the sidestream position for
reduced first cost and higher efficiency.
Reusing existing, older, less-efficient
chillers, again in the sidestream position,
is also a good idea. These sidestream
configurations combine the benefits of
series and parallel chillers while isolating
some chillers from water flow variations.
In smaller plants with fewer chillers,
system analysis may show that condensers
configured in parallel may be more
More Than the Sum
of Its Parts
As chiller efficiencies continue to
improve, district energy designers can
optimize the entire system to achieve
even lower costs of ownership. Owners
can expect more energy savings from
low-flow, low-temperature and highly
efficient chiller configurations. The
unique benefits and flexibility of series
chiller plant designs include lower overall
chilled-water system operating costs,
reduced emissions and improved environmental responsibility.
Susanna Hanson, C.E.M., D.G.C.P., is a
senior product support engineer for Trane
Commercial Systems in La Crosse, Wis. She
specializes in simulated and empirical central
plant analyses and looks for ways to minimize
building energy use. Hanson holds a bachelor
of science degree in Industrial and Systems
Engineering from the University of Florida. Since
2004, she has been a member of ASHRAE 90.1,
whose standard is used the basis for many
commercial building energy codes. She may be
reached at email@example.com.
W. Ryan Geister currently leads the centrifugal and absorption product field sales support
teams in La Crosse, Wis. Geister joined Trane
Commercial Systems in 1995 to support and
design energy and economic software tools.
Geister also has held roles in training as manager of systems training in the Graduate Training
program and as a regional sales manager.
Geister received a bachelor of science in
Engineering from the University of Illinois and a
master's in business from the University of
Wisconsin - La Crosse. He may be reached at
Figure 3. Projected energy use and life-cycle costs for series and parallel chiller configurations.
Arrangement Chillers* Evaporator Condenser System
Power Power Power
Source: Groenke and Schwedler, ASHRAE Journal, June 2002.
Compressor P Number per P Number per Number per Total Life-Cycle
Units/ Efficiency Flow Feetof of Pump Flow Feetof of Pump of Cell Power Cost
Evaporator Condenser Modules kW/ton gpm Water Pumps kW gpm Water Pumps kW Cells kW kW $USD
Parallel Parallel 5/5 0.649 2,800 3. 26 5 2. 18 4,200 3.66 5 3.67 8 60 7324 18,836,302
Parallel Parallel 6/6 0.618 2,333 4. 18 6 2. 33 3,500 3.53 6 2.95 8 60 7001 18,076,391
Series Series- 6/3 0.560 4,667 17.96 3 19.995,250 14. 8 3 18.54 8 48 6379 16,819,167
( 1. 5 gpm/ton)
Series Series- 6/3 0.535 4,667 17.96 3 19.997,000 25. 2 3 42.08 8 60
( 2.0 gpm/ton)
Series Parallel 6/3 0.555 4,667 17.96 3 19.993,500 3.53 6 2.95 8 60 6385 16,888,493
( 2.0 gpm/ton)
* The chillers represented in this table all have dual refrigerant circuits. The full analysis included single refrigerant circuit chillers at various flow rates and efficiencies.