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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.

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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.

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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 shanson@trane.com.

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 atrgeister@trane.com.

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.

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