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DistrictEnergy
www.districtenergy.org
T H I R D Q U A R T E R 2 0 0 5
Christine ToddWhitman:
“You have agreat story
to tell.”
Optimizing Chiller Plant Operations
Temporary Boilers, Peace of Mind
Energy Future: Another View
Annual Conference Wrapup
New Chair Takes Helm
and more. . .
DistrictEnergy
Reprinted from Third Quarter 2005 District Energy magazine with permission of IDEA. Third Quarter 2005 7
International CoolingProjects Focus on SeriesChiller Plant Design: Changes in chiller technology make series chillers cost-effective,energy-efficient alternativeSusanna Hanson, C.E.M., D.G.C.P. Senior Product Support Engineer, Trane Commercial Systems; W. Ryan Geister, Manager of Chiller Field Sales Support, Trane Commercial Systems
FeatureStory
It was once unthinkable that an entire
city could be served by one enormous
cooling plant. Over the past five years,
dramatic increases in the number and
size of district cooling projects are chang-
ing how we think. This trend is especially
visible as new infrastructure is construct-
ed in the Middle East and Asia, with
brand-new cooling plants in excess of
50,000 tons.
District cooling providers must find
ways to distribute chilled water over long
distances and to manufacture and deliver
ton-hours as inexpensively as possible.
The challenges of district cooling in
extremely hot and humid weather loca-
tions, equipment capability improvements,
and the economics of super-sized district
cooling projects are creating a resurgence
in series chiller plants.
Challenging Conditions inMiddle East, China
In the past, chiller-plant design was
narrowly focused on the U.S. market,
where engineers designed for a maxi-
mum tower-leaving temperature of 85
degrees F and where standards such as
44 F/54 F evaporators and 85 F/95 F
condensers made it easy on the chiller
(and the system designer). As the U.S.
began to move away from that old para-
digm to minimize system energy use,
international projects are accelerating
change as they discover their own best
practices.
Many parts of China call for 89.6 F
design tower water and parts of the
Middle East design for 95 F. While these
weather extremes challenge any chiller
plant, they force district cooling to con-
sider different approaches, including
low-flow, low-temperature systems proven
to save operating costs. Colder chilled
water is cheaper to distribute and delivers
more effective cooling and dehumidifica-
tion all the way to the last air handler.
But colder chilled water is difficult to
produce with standard single-stage chillers
accustomed to the easier U.S. conditions.
Multiple-stage chillers and/or series
chillers are chosen to provide more rigor-
ous, stable cooling at these conditions.
Larger Projects Lead to SeriesLarger plants have the economies of
scale to explore various combinations of
series and parallel chiller plants; centrifu-
In spite of slower condenser flow-rates, lowerchilled-water temperatures, chiller efficiency isincreasing.
In the past, consulting engineers selectedflows and temperatures to maximize chillerefficiency. Today, an equal price centrifugalchiller can be selected for less condenser flow,with no loss in efficiency. At the same time,chilled-water temperatures are dropping.Pump savings more than overcome chiller-effi-ciency losses, and the larger the chilled-waterdistribution system, the larger the savings. Between 1993 and 2003, data from one manu-facturer (Trane) showsl average condenser flow-rates dropped by 7
percent;l average chilled-water (excluding glycol)
temperatures dropped 1.5 F; l average chiller efficiency improved by 11
percent, in spite of the effects of low-flowcondensers and low-temperature chilledwater; and
l aggregate prices adjusted for inflation areflat, indicating the market has come toexpect better chiller capabilities and per-formance.
As a result, series-chiller plant efficiencies(pumps, towers and chillers) now approach 0.7kW/ton.
adoption in the early 1960s when gov-
ernment buildings in Washington, D.C.,
embraced them for creating cold water
for perimeter induction cooling. Induction
systems supply cold primary air to the
space, requiring colder water from the
chiller. Chillers in those days had a coef-
ficient of performance (COP) of about
4.0, with high-flow (velocity) smooth-bore
tubes, low tube-counts and one-pass
evaporators to reduce pressure drop.
In the 1970s, variable-air-volume (VAV)
systems made the colder chilled water
used for induction systems unnecessary.
Given the chiller’s relatively low efficiency
by today’s standards, it made sense to
raise temperatures.
VAV systems were widely adopted
because they save energy and adapt to
unknown cooling loads. VAV systems are
still the most popular choice for delivering
conditioned air; however, series chillers
offer additional benefits because chillers
have almost doubled in energy efficiency.
RedundancySystem designers are finding that
large chiller plants can be more adaptive
and efficient by installing multiple chillers
rather than one or two large, field-erected
chillers. In plants with more chillers,
redundancy is easily created through
parallel banks of upstream and down-
stream chillers. Different amounts of
upstream and downstream chillers can
meet the load, so if one chiller is being
serviced, its duty can be spread out to
many other chillers.
gal, screw and absorption chillers; and
air-cooled and water-cooled chillers. The
district cooling business model is also
key – overall chiller plant efficiency goes
directly to the bottom line.
More and more chiller plants are
selecting low-flow, low-temperature, yet
highly efficient chillers (see sidebar).
Because pump energy is proportional to
the cube of the flow, even incremental flow
reductions quickly result in net positive
cash flow. But when flow goes down, tem-
perature must also, favoring series chillers.
Why Series? Multiple-stage or series chillers pro-
vide rigorous, stable cooling at extreme
conditions. Series chillers are standard
chillers that are piped or lined up in a
series, which allows the system to use less
energy to cool.
When chillers are lined up in parallel,
each individual chiller must provide the
coldest water required for the entire sys-
tem. In series, each subsequent chiller in
the process can operate more efficiently
and provide colder water. It also uses less
energy for high ambient wet-bulb condi-
tions, which are common in the Middle
East and China.
Common reasons why some design-
ers do not go with series are fear of
something ‘new,’ lack of redundancy and
higher water-pressure drop. All of these
reasons are well-understood, and current
chiller designs compensate appropriately.
History of Series Chillers Series chillers saw widespread
8 District Energy Reprinted from Third Quarter 2005 District Energy magazine with permission of IDEA.
Pressure DropA typical maximum acceptable
chiller pressure drop is 25 ft of water.
Even after adding more tubes to reduce
pressure drop, two chillers in series might
use twice that, because each chiller has
twice the amount of water going through
it. In traditional primary-secondary sys-
tems, constant flow through the chillers
equals constant pressure drop and a con-
stant pump energy.
Today, variable-primary systems
send variable amounts of water flow
through the chillers to limit the effects of
pressure drop at many load conditions.
Variable-primary systems are made pos-
sible by the latest chiller technology and
control advancements. Series evaporators
reduce the need for a bypass in variable-
primary systems because the higher ini-
tial water flow allows for higher flow
reductions before reaching the minimum
flow for the chiller.
Chiller Technology and ItsImpact on Design
The most efficient centrifugal
chillers today have COP of more than 7.0
– which is more than 75 percent higher
than chillers used in the first series
chiller plants.
Chiller technology has had a sizeable
impact on the design of chilled-water
plants. The chiller’s responsive control
and multiple-stage stability allows for
more versatile designs. One example is
variable flow through standard centrifugal
chillers, a design discouraged by manu-
facturers just a few years ago.
Today’s advanced controls offer fea-
tures that incorporate improvements to
chiller capabilities and variable speed
pumps. Improved tube designs and
extensive testing in manufacturers’ testing
labs have cut minimum water velocities in
half, leading to better turndown and higher
pump energy savings.
Chiller efficiency is dependent upon
several variables – two of them are capacity
(tons) and lift (chiller internal differential
temperature). Multiple-stage centrifugal
chillers have the ability to create high lift,
which is roughly equivalent to the differ-
ence between the leaving condenser and
leaving evaporator temperatures.
Series or Parallel? Chiller plants with series evapora-
tors but parallel condensers are not rec-
Figure 1. Series counterflow chiller arrangement equalizes lift performed by each compressor,minimizing the energy needed to create high lift.
Cou
rtes
yTra
ne
Com
mer
cial
Syst
ems.
Reprinted from Third Quarter 2005 District Energy magazine with permission of IDEA. Third Quarter 2005 9
chiller plant depicted here created series-
pair efficiencies of 0.445 kW/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 addi-
tional pump energy. In the case of this
particular installation, series-series coun-
terflow saves $1.4 million in life-cycle
costs over the parallel-parallel alternative.
Series Chiller ImprovesEfficiency, Flexibility inSmaller Plants, Retrofits
The benefits of low-flow, low-tem-
perature and high-efficiency are universal.
Smaller, non-centrifugal chillers can ben-
efit 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
advantageous.
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 envi-
ronmental responsibility.
ognizing the highest efficiency gains
because the chiller making the coldest
water does more lift. By putting the con-
densers 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 with-
out sacrificing efficiency.
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 previ-
ously 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 cir-
cuit) has 8 to 12 stages of compression
equally sharing the load (figure 2). The
Figure 2. Module with dual-circuit chillers inseries provides 8 to 12 stages of compres-sion and uses 0.445 kW/ton at standard ARIrating conditions.
Cou
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Syst
ems.
Susanna Hanson, C.E.M., D.G.C.P., is asenior product support engineer for TraneCommercial Systems in La Crosse, Wis. She specializes in simulated and empirical centralplant analyses and looks for ways to minimizebuilding energy use. Hanson holds a bachelor of science degree in Industrial and SystemsEngineering from the University of Florida. Since2004, she has been a member of ASHRAE 90.1,whose standard is used the basis for many commercial building energy codes. She may bereached at [email protected].
W. Ryan Geister currently leads the centrifu-gal and absorption product field sales supportteams in La Crosse, Wis. Geister joined TraneCommercial Systems in 1995 to support anddesign energy and economic software tools.Geister also has held roles in training as manag-er of systems training in the Graduate Trainingprogram and as a regional sales manager.Geister received a bachelor of science inEngineering from the University of Illinois and amaster's in business from the University ofWisconsin - La Crosse. He may be reached [email protected].
Arrangement Chillers* Evaporator Condenser CoolingTowers System
Evaporator CondenserUnits/
Modules
CompressorEfficiency
kW/tonFlowgpm
PFeet ofWater
Numberof
Pumps
Powerper
PumpkW
Flowgpm
PFeet ofWater
Numberof
Pumps
Powerper
PumpkW
Numberof
Cells
PowerperCellkW
TotalPower
kW
Life-CycleCost$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-Counterflow(1.5 gpm/ton)
6/3 0.560 4,667 17.96 3 19.99 5,250 14.8 3 18.54 8 48 6379 16,819,167
Series Series-Counterflow(2.0 gpm/ton)
6/3 0.535 4,667 17.96 3 19.99 7,000 25.2 3 42.08 8 60 6284 16,656,947
Series Parallel(2.0 gpm/ton)
6/3 0.555 4,667 17.96 3 19.99 3,500 3.53 6 2.95 8 60 6385 16,888,493
* 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.
Figure 3. Projected energy use and life-cycle costs for series and parallel chiller configurations.
Sou
rce:
Gro
enke
and
Sch
wed
ler,
ASH
RA
EJo
urn
al,Ju
ne
2002.