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Better Than Oil-Free:
Why Screw Chillers with Pure Speed Capacity Control Save More Energy Than Magnetic Bearing Chillers
Carrier Corporation Syracuse, New York May 2015
2
TABLE OF CONTENTS
INTRODUCTION .................................................... 2 ACHIEVING HIGHER EFFICIENCY ................. 2, 3 TO BUILD A BETTER CHILLER, BUILD A BETTER COMPRESSOR ............................................................ 3, 4 Bearings ........................................................................ 4 OIL-FREE CHILLERS AND THE IMPACT OF OIL ON HEAT TRANSFER................................................ 4, 5 ELIMINATING MECHANICAL UNLOADERS IMPROVES PERFORMANCE .................................... 5, 6 IDEAL FAN LAWS .................................................... 6-10 Warm Condenser Water Operation .............................. 8 Cold Condenser Water Operation ................................. 8
IDEAL FAN LAWS (cont) The Ideal Centrifugal Speed and Less Than Ideal Operating Conditions .................................................... 9 THE MISSING FACTOR IN COMMON INDUSTRY METRICS .................................................................. 10-13 The BIN Method’s Unintended Impact .................... 11 The Counterintuitive Energy Retrofit Case .............. 12 Warm Climates ......................................................... 13 CHILLER PLANT OPERATION: IDEAL vs ACTUAL OPERATION ................................ 13 Performance Degradation Over Time ...................... 13 SUMMARY .................................................................... 14 REFERENCES ................................................................ 15
____________________________________________________________________________________
INTRODUCTION
The proposed benefits of oil-free, magnetic bearing chillers have been widely marketed, and many current trade journals include multiple references to these chillers. Interestingly, the benefits of chillers that offer variable speed screw technology with pure speed capacity control1 have not received the same attention. This paper will compare and contrast the two technologies and illuminate a simple, logical approach to higher energy savings. See Table 1 for a comparison summary.
ACHIEVING HIGHER EFFICIENCY
In the case of oil-free magnetic bearing chillers, eliminating energy losses due to friction and
1 The use of mechanical unloaders of any kind is not required
under normal operating conditions.
reducing resistance to heat transfer are marketed as contributing to higher efficiency. In the case of variable speed screw chillers with pure speed capacity control, the elimination of mechanical unloaders and surge are, in contrast, marketed primarily as reliability and operational flexibility benefits. Equipped with these explanations, and provided only with the chillers’ efficiencies as shown in Figure 1, can you tell which of the chillers shown is oil free?
The answer may surprise you. The chiller on the left, with the higher kW/ton value, is the oil-free chiller; the more efficient chiller on the right is the variable speed screw chiller. This result is consistent across a wide range of capacities in which the two technologies are offered, as shown in Table 2.
Centrifugal Chillers with Oil-Free Magnetic Bearings Screw Chillers with Pure Speed Capacity Control
Frictionless bearings* Improved heat transfer† Less service and maintenance** Low Sound
No losses due to mechanical unloaders No use of hot gas bypass to prevent surge Less service and maintenance Low sound
* Main bearings require power for the levitation system, operating at higher motor speeds may increase in windage losses. Touchdown (back up) bearings are anti-friction bearings. † The impact of the studied benefit is still less than 0.1 – 1% depending on load, ignoring the benefit of oil “wetting” on the upper rows of evaporator tubes. ** Magnetic bearings include multiple sensors and coils as well as the electronics to control and power them.
Table 1 – Comparison of Claims
C(22334
*O 2
T
Capacity Tons)
250 290 360 390 450
Oil-free chiller eff23XRV efficiency
Table 2 – Chil
Oil-Free ChilleClaim (kW/ton0.633 / 0.357 0.634 / 0.328 0.576 / 0.327 0.604 / 0.330 0.590 / 0.346
W
ficiency values ay values are per
ller Efficienc
er n)*
23X(kW0.590.570.570.570.57
Which
FOil-Free M
are presented in pAHRI certified se
cy Values
Figu
XRV Chiller* W/ton)92 / 0.327 72 / 0.313 74 / 0.303 75 / 0.315 78 / 0.307
h chill
Figure 1 – AchMagnetic Beari
public domain, elections.
ure 2 – Chiller
3
TO A B
Givethe bthe moiledundeefficthe chillCondcondloadincre14%
ler is o
hieving Higher ings vs Pure Sp
Losses by Com
BUILD A BETTER CO
en the well-pbenefits of oilmost efficientd chillers. erstand how ciency. The pcontribution
ler at 100%ditioning, Heditions. Thes except the eased while h
% at full load t
oil fre
Efficiency – peed Capacity
mponent
BETTER OMPRESSO
publicized exl-free chillerst chillers in thTo understaneach chiller
pie chart showof losses w
% load, opereating and Re results wercompressor a
heat exchangeto less than 3%
ee?
Control
CHILLER,OR
xplanation su, it is worth n
he industry arnd why, wecomponent i
wn in Figure 2within a watating at AH
Refrigeration re similar atand VFD coner losses drop% at 25% loa
, BUILD
urrounding noting that re actually e need to influences 2 displays ter-cooled
HRI (Air-Institute)
t reduced ntributions pped from d.
Tblt
Aeuiemim
B
Wbc
Ibrmbcprep
Mictbdp
2
3
The outcome better chiller,osses within
the losses in th
A screw chileliminates alunloaders sucmprovement
efficiency momarketed benmpact of oil
minor in mod
Bearings
While markebearings in oconsume pow
In screw cobearings offerrolling elememechanical lobearing lossecompressor published kWrolling elemenelectronic bopotential mod
Many oil-freenstead of m
compressors othe selection bearings are mdo consume positioning s
2 The “other” in t
miscellaneous bypass in the sinefficiency alo
3 When operatedbearing in a 23
suggests tha start with athe compres
he chiller.2
ller with purl losses ass
ch as guide vato the comp
ost. In contrnefit surroundl on heat traern productio
eted as frictioil-free chillwer.
ompressor dr long bearingent bearing oss in a low es further deunloads and
W/Ton of tnt bearings dooards to opdes of failure.
e compressormechanical boperate at ove
of magneticmarketed as “
power. Powensors and
the compressor asmall refrigerant
system piping. Tone was over 50%d at AHRI condit3XRV compresso
NOT IMP
at if you wana better compsor represent
re speed capociated withanes, providinponent that crast, most of
ds the bearingansfer. Theson chillers.
ionless, the mlers actually
designs, rollg life and simlosses represspeed compre
ecline with sd are incluthe chiller. o not require
perate, elimi
rs use magnbearings. (Mer 30,000 rpmc bearings.) frictionless,” wer is requcontrol. Elec
and other categot side pressure d
The compressor m% of the chiller’s ions, the lowest
or is over 50 yea
PACTED BY O
Figure 3 –
nt to build apressor. Thet over half of
pacity controlh mechanicalng significantcontributes tof the oil freeg friction andse losses are
magnetic y do
ling elementmplicity.3 Thesent a smallessor. Thesespeed as theuded in the
Furthermore,capacitors ornating these
netic bearingsMany oil-freem, influencing
While thesethey actuallyired for thectrical losses
ory represented drops and mechanical total kW losses.L10 life of any rs.
OIL
– Impact of Oi
4
a e f
l l t o e d e
t e l e e e , r e
s e g e y e s
assocheat,a fluIn ahighelectwiththe mwill
Evenmechdesigthat or cwithbeari
In slosse
OILOF
The fracequibene
ASHRefrReseconcfromchillfractequivload
4 Me
is gmalow0.5
l on Heat Tran
ciated with th, which does
uid and wouldaddition, the h speed may tric motor. S
h the movememotor as it sincrease wind
n in comprehanical bearign. To ensurthere is an in
component fah a mechaniing.
summary, thes. No additi
L-FREE CHOIL ON HE
e net result ofction of a fraivalent to 0.1efit of oil).
HRAE (Amrigerating, aearch Projeccentration an
m RP 751 to ler, the net retion of a frvalent to 0..4
easurements in agreatest near theay improve heat twer parts of the b5%, too low to im
nsfer is Minima
his function anot advance
d therefore beoperation ofincrease the ince windageent of the cospins, in gendage losses.
ssors that usings are notre the shaft is nability to coailure) the coical bearing
he AHRI ratonal benefit s
HILLERS AEAT TRAN
of oil on chilaction of a fr1% to 1% (ig
merican Soand Air-Concts 601 andd heat transfdetermine t
esult of oil onfraction of a1% to 1%
actual full bundle e liquid level. At transfer by wettin
bundle the oil conmpact heat transfe
al
are generally ethe purpose oe categorized f compressorwindage los
e losses are aooling mediuneral, increasi
se magnetict eliminated supported inntrol levitatio
ompressor is called a to
tings includeshould be assu
AND THE IMNSFER
ller efficiencfraction, esseignoring the
ociety of nditioning Ed 751 pertaifer. Using the impact onn chiller effica fraction, eimpact depe
tests show oil cothe top of the bu
ng the tube surfancentration is leser.
emitted as of chilling as a loss.
rs at such ses in the associated
um around ing speed
bearings, from the
n the event on (power equipped
ouchdown
e bearing umed.
MPACT
cy is a entially e wetting
Heating, Engineers) in to oil the value n the full
ciency is a essentially ending on
oncentration undle, oil ace. In the ss than
TtprhTrdttfwpcn
AbfcterisaT5
ARcacmumpucacThcmrRd
5
These works that referencproperly. Freferences a heat transfer This should nreduction in cdeals only withe evaporatoto create the lfeed and drawere coveredpossible in ancomparisons not translate r
As there was be a water filfouling and mchiller has oilthe overall evaporator tubrefrigerant filmmpact evapo
shown in the accounts for The impact o5% of the loss
Another comResearch Proconcentrationa college camcurrent desigmanufacturedused R-11 management, pressure in understood tcommonly haare not issucentrifugal anThe inherent higher differechillers is onmodern desigresults from R-134a positidecades of im
5 The total kW lo
comparing a mchilled water teof a chiller withtemperature eqa perfect theor
are quite tecces to theseFor example“22-25% redcoefficient dunot be misintchiller efficienith the refriger. Electric caoad. The te
ain system thd in liquid rn actual chilleof oil to oil-
reliably from
no water in lm resistance,
metal resistancl in it or not.resistance t
be. The remm and is the rator heat trapie chart in
only 5% of tf oil is therefses in the chil
mmonly citedoject 601. s in samples
mpus. These wgn. Half od prior to 197
refrigerant. oil retards
low pressurthat old nead oil recoveues with pond screw chil
improvemenential pressuree of the signgn. It woul this study
ive pressure mprovements.
osses in an evap
model of a chiller emperature less h a zero degree aqual to saturatedretical evaporato
chnical and ite publication, Research
duction in refue to averageterpreted as ncy. Researcerant side heartridge heateest bundle righereby assurirefrigerant, a
er. In general-free in real studies using
the tubes, the, or fouling. ce are the sam. These repreto heat tran
maining 27% ronly opportunansfer. (See Fn Figure 2, ththe losses in fore 22 to 25ller.
d reference This paptaken from 1were older ch
of the ten c70 and all o Without the refrigera
re chillers. egative pressery challengesositive pressllers in produ
nt in oil recoes associated nificant advanld be incorreare applicab
chillers built
porator were detewith a normal apsaturated suctionapproach (leavind suction temperar).
t is importantns are cited
Project 751frigerant sidee oil effects.”a 22 to 25%h Project 751
eat transfer iners were used used an overing all tubesa set up notl, quantitativebundles maybundle rigs.
ere could not Water film,
me whether aesent 73% ofnsfer in anresides in thenity for oil toFigure 3.) Ashe evaporator
the system.5
% of 27% of
is ASHRAEer cites oil10 chillers onhillers, not ofchillers weref the chillerseffective oilant-oil vapor It is wellsure chillerss. But thesesure R-134auction today.overy due towith R-134a
ntages of theect to imply
ble to modernt after 3 to 4
ermined by pproach (leaving n) with a model g chilled water ature, signifying
5
t d 1 e ”
% 1 n d r s t e y
t , a f n e o s r 5 f
E l n f e s l r l s e a .
o a e y n 4
Givefree overdata.smalchillelimperfothe d
ELIUNLPER
To bspeethe aA diprovmech
RefrcompAlonturneto rerecircondcond
Imagthroureduunit usedguidcreat
en the well-pdesign impro
restimate the . As outlinedll. In the clers, it may b
minating meormance. It dramatic impa
IMINATINGLOADERS RFORMAN
better illustrated capacity coadvantages ofiscussion of svide ample ophanical unloa
rigerant flowspressor and ng this path, ted and acceleestrict, recircurculation aredenser) or hdenser, evapo
gine a pipe wugh it. Now
uce flow and oof throughpu
d to reduce flode vanes in ate a pressure
Figure 4Losses
publicized expoves efficienc
impact in td above, the icase of the be obvious thechanical may also be act this has on
G MECHAIMPROVE
NCE
te the compreontrol, it mayf eliminating speed and refpportunity to aders used to r
s from the eis dischargethe refrigeran
erated. Mechulate or bypa
e hot gas bhot gas rec
orator, and firs
with a large vow place a resobserve what
ut. In a chilleow. While re
a centrifugal drop of 1% to
4 – Inlet Guide s Increase as th
planations ofcy, it would bthe absence impact is actuvariable spe
hat reducing unloaders easy to unde
n performanc
ANICAL ES
ession benefiy be helpfulmechanical u
frigerant gas observe the
restrict flow.
evaporator thrd into the cnt may be comhanical unloadass flow. Exaypass (bypa
circulation (bst stage impel
olume of fluistriction in tht happens to pr, inlet guide educing flowcompressor (o 13%.
Vane Mechanihe Vanes Close
f how oil- be easy to of actual
ually quite eed screw
losses by improves
erestimate ce.
its of pure to realize
unloaders. flow will impact of
rough the condenser. mpressed, ders serve amples of
assing the bypassing ller).
id moving he pipe to power per vanes are
w, the inlet (Figure 4)
ical e
Tcitt
I
Tauti
6
This increascompressor thmpact observ
to 50%.6 Notthe <1% from
IDEAL FAN
This section wapproach (deuses only speethe ways in wmpact efficie
6 In the paper “A
Centrifugal Coof inlet guide vcompressor, isaccounts for. Taerodynamic eon the compre
1s
2n
3r
*It
es the heahereby increaved in comprte, the impact
m the oil free e
N LAWS
will comparescribed aboved to control
which eliminaency, we will
Aerodynamics of
mpressors” 1996vanes on efficiencs far higher than tThis observation
efficiency due to cssor map are als
st Law: Mass
nd Law: Lift (a
rd Law: Power
t may be less tha
d requiremeasing power.ressor performt is significanteffect discusse
e the mechanve) to a comflow. By coting mechaniunderstand th
Rotatable Inlet G6, JJ Brasz notescy, 4% to 50% inthe increase in h suggests that chchanges in the oso a factor.
flow (capacit
ability to gene
r is the cube o
an cubic due to th
ents on the The actualmance is 4%tly more thaned earlier.
nical unloadermpressor thatnceptualizingcal unloadershe magnitude
Guide Vanes for s that the impact n a centrifugal head alone hanges in
operating point
ty) is linear w
rate head) is
of speed*
he proximity of g
Figure 5 – Id
6
e l
% n
r t g s e
of themplof scentrrefrigadhe
Signinduand pa cruabiliomit
ith speed
the square of
uide vanes to the
deal Fan Laws
he savings. loy limited spspeed controrifugal comprgerant type,
ere to the idea
nificant markeustry on the 3r
power. Unfoucial role in eity for the chitted.
f speed
e impeller inlet
s
Oil-free cepeed control,ol where poressors (regarmotor type, o
al fan laws, ill
eting has beenrd law, relation
ortunately, theequipment effller to stay on
entrifugal chi leveraging th
ossible. Howrdless of beaoiled or oil flustrated in F
n done to edunship betweene 2nd law, whificiency (and in line), is com
illers also he benefit
wever, all aring type, free) must igure 5.
ucate the n speed ich plays in the
mmonly
Ics8od
Telmibhafss
Tc
7
In simple tercentrifugal cospeed. At 8080% x 80% operating condesign head, t
To avoid sureither speed uoad balance
magnetic bearnclined to be
based on capahead requiredat 50% load afirst law wouspeed, but thspeed. (See F
This raises ancompressor m
7 Centrifugal com
between the locondenser. If treversal will oc
rms, the abiliompressor is r0% speed, a = 64% of
ndition requthe centrifuga
rge, the centup or engage h
valves or rering machinelieve that comacity, it is ac
d. For exampand 75 F enteuld suggest whe 2nd law reFigure 6.)
n interesting qmust operate
mpressors moveow pressure evapthe pressure diffeccur (surge).
Figure 6 –
ity to generarelated to the
centrifugal wits design hires more th
al compressor
trifugal comphot gas bypascirculation fl
es). So whilempressor speectually often ple, if a chilleering condenswe could opeequires us to
question: If that 84% spee
gas through an
porator to the higerence (head) is
– Determining
ate head in asquare of thewill generate
head. If thehan 64% ofr will surge.7
pressor mustss (also calledlow on manye we may beed is typicallybased on the
er is operatingser water, theerate at 50% run at 84%
he centrifugaled to develop
open path gher pressure
too large, flow
Compressor Sp
7
a e e e f
t d y e y e g e
% %
l p
suffi50%inletbalanof dour inletconsor vabypagas bto mmeet(hot first The morewhiccooli
Speed Required
icient head, %? This requt guide vanesnce valves (h
diverting or rsimple pipe
t guide vasumption andariable geomass or recircubypass or rec
move throught the load. Tgas bypass) stage of thenet result i
e refrigerant ch representsing at the ope
d to Develop Su
how do we uires an unlos, discharge
hot gas bypassrestricting floexample, it
anes will id reduce effic
metry diffusersulation flow wcirculation flo
h the compresThis flow by or the conde compressois the comprthan is requ
s an increase erating point.
ufficient Head
limit the caoading deviceflow restrictis) or some othow. Thinkingbecomes obvincrease theiency. If gus are removedwill be requiow allows hissor than is nypasses the edenser, evapor (recirculatiressor is com
uired to meet in power pe
apacity to e such as ions, load her means g back to vious that e energy uide vanes d, hot gas ired. Hot gher flow needed to evaporator orator and ion flow). mpressing the load,
er unit of
W
NchlcepcsrFcwTcewccsmdromwca
8
Warm Cond
Now considerchiller. Screwhead at any spaws. So if th
capacity at 7essentially rupoints of spechiller. To uscrew comprreduction is gFigure 7 illucentrifugal anwith constant The variable capacity contreven with conwater temperchiller is less chiller enjoysspeed reductiomust employ decrease in erepresents mooutput). In mechanical unwarmer climachillers will advantage.
Any speed
8 Maintaining hig
multiple chiller expected to opbuilding loads chillers needed3-chiller plant).
Figure 7Cond
FiguC
denser Wate
r this same opw chillers propeed and are nhe screw chil5 F entering
un at 50% speed reductionunderstand horessor just rood speed redustrates this nd screw perf
entering conspeed screw
rol reduces spnstant 85 F (2rature. Effic
than half loas the same reon is limited mechanical
efficiency is ore power conthe graphic nloaders are ates pure spe
provide a
reduction isand more
gh efficiency aboplants since only
perate below 40%below 40% x 1/Nd to meet building.
7 – Chiller Effidenser Entering
ure 7 – Chiller Condenser Ente
er Operatio
perating pointovide the samnot bound byller needs to
g condenser wpeed. This rn relative to ow the physiremember: duction and m
principle byformance at vndenser waterw chiller withpeed as the lo29.4 C) entericiency increaad.8 While theduction in lby the 2nd idemeans to unlobserved (hi
nsumed for a gbelow it is required anded capacity can enormou
s good speede is better.
ove 40% is highlyy the last chiller o
% load and this oN, where N is theg full load (for ex
ciency at Consg Water Tempe
Efficiency at Cering Water Te
on
t with a screwme amount ofy the ideal fan
provide 50%water, it willrepresents 34a centrifugalcs favors the
Any speedmore is better.y comparingvarying loadsr temperature.h pure speed
oad is reduceding condenserases until thehe centrifugalload, becauseeal fan law, itload, and theigher kW/tongiven coolingobvious that
d therefore incontrol screw
us efficiency
d reduction
y valued in on line is
only occurs at a e number of xample 13% in a
stant 85 F erature
Constant 85 F emperature
8
w f n
% l 4 l e d .
g s . d d r e l e t e n g t n w y
Cold
Is it opernot capplicontiguidbypaFiguovertempmay varyprofitons wate
A vacapaefficentemorcent
9 Un
equ
d Condense
possible to crating range wcome into plaication, with
tinuously, wode vanes, varass and flow rure 8 depicts r a wide varperatures. No
be fixed, they throughout file; significan
with signifier temperature
ariable speedacity controlciencies overering condenre than constrifugal chil
less there is conuator for example
Figure 8 – ChCondens
er Water Op
compare the twhere the mecay? It may sh chillers ruould provideriable geomerecirculation chillers operariety of enteote, that while outdoor amthe data ce
nt hours willicantly reduce.9
d screw chill performs arall, leveragnser water testant or variallers.
nstant entering coe.
hiller Full Loaser Entering W
peration
two technolochanical unloseem that a dunning at 10e a case whetry diffuserswould not beating at 100%ering condenle the data ce
mbient tempernter’s 24x7 l be spent at ed entering c
ller with purat higher opeging the reduemperature able speed
ondenser water,
ad Efficiency atWater Temperat
gies in an oaders will data center 00% load here inlet s, hot gas e required. % capacity nser water enter load rature will operating 100% of
condenser
re speed erating uction in even
on the
t Varying ture
Acitevrwaco2earfnbarudcc
BtwwbamccecorbsFftmhcr
As the condencooler in the cmproving ref
through the evaporator prvapor. Threfrigeration will not conalready changcompressor mof 120% (10020% due flaentering condand sub coolrefrigerant liqflashing acronow be moviboiling in thacross the orremains constunloading frodesign head compressor mchiller capacit
But even as tthe speed reqwider marginwater temperabecomes irreleat play, furmechanical comparing tcentrifugal cexclusively bcentrifugal opobserved. Threfrigerant floblade, from cspins, the morFigure 9 belofixed and the the 2nd ideal more energy head at the opcompressing required.
F
nser water temchiller becomfrigerant qualorifice from
ressure, somehis vapor w
cycle, includntribute to evged state. A
may be movin0% due to boishing acrossenser water telers in chillequid to the oss the orificeing a volume
he evaporatorrifice). Wtant at 100%, om 120% to
to a reducmass flow rety has been re
the speed neequired to gen. Therefore, ature is reducevant. Since rther insight unloaders
the constancurves. Sby mass flowperating withhe impeller wow throughoucenter to tipre energy is trow. Since tspeed is greafan law, the on the gas
perating poinand perfor
Figure 9 – Impe
mperature decmes more effeity. As refri
m condensinge refrigerant will move ding the comvaporator tonAt design cong a volume ling in the ev the orifice)emperatures ters provide corifice. The e. The come of 110% (r, and 10%
While the chithe compress110% of ma
ced head coequired to aceduced.
eded for massnerate head d
as the enterinced, the 2nd ithe 2nd ideal
as to thebecomes vi
nt and varSpeed is nw and the
h excess tip swill impart eneut the entire . The faster ransferred, asthe impeller
ater than requiimpeller bladflow than is
nt. The centrirming more
eller Blade
clines, the subective therebygerant moves
g pressure towill flash tothrough the
mpressor, butns as it has
onditions, theof refrigerant
vaporator, and). At lowerthe condensercolder denserresult is less
mpressor may100% due todue flashing
iller capacitysor is actuallyss flow fromndition. Thechieve 100%
s flow drops,declines by ang condenserideal fan lawfan law is not
e impact ofisible whenriable speedow dictatedimpact of aspeed can beergy onto thelength of thethe impeller
s illustrated ingeometry is
ired to satisfyde will impart
required forifugal is over
work than
9
b y s o o e t s e t d r r r s y o g y y
m e
%
, a r
w t f n d d a e e e r n s y t r r n
Screheadcomppoinspeeperfolevertempspeethe pthe mcentrexplsimpthe sefficflow
TheIdea
The ideabe.
The requrequcentrwhenrequunlolowespeecenteflowheadcompcomp
Comthat occuexacAbovcentrfor emplflowcompchillan afromwill
ew compressod at any speedpressing at th
nts. What maed screw wiorms at highraging the redperature evened centrifugal performance mechanical unrifugal compranation is bey
ple terms, thescrew compreciency of the
w, low head co
e Ideal Cental Operatin
e further the al speed curv
ideal fan uirement for fuirement for rifugal compn the speed r
uirements, meoad a centrifuer efficiency ed control aloer, we disco
w often exceed. In this cpress, but presses with l
mbining thesethe ideal sp
urs when thctly matches tve this sperifugal to op
capacity aloyed. Belo
w exceeds the pression. Inlers with pureadvantage. T
m the ideal spbe.
ors develop d, and so by dhe high capaciay be unexpecith pure spher operatingduction in enn more thanchillers. In tadvantage c
nloaders alonressors are oyond the scope resulting isessor is higher
centrifugal conditions.
trifugal Speg Condition
e operating pve, the great
laws defineflow and a sep
the generapressor. Earlrequired for hechanical meugal compres
than a scrone. Next, iovered that thds the speed case, both c
the screwless penalty.
e observationspeed for a ce speed reqthe speed requeed, the hierate at highand mechanow this speedhead requirem
n either case e speed capaThe further tpeed curve, t
the same adefinition areity, low head cted is that thpeed capacityg efficiencietering conden
n constant orthis operating
cannot be attrne. Both the sver compresspe of this papsentropic effir than the aercompressor at
ed and Lessns
point is fromter the impa
e a minimuparate minimation of helier, we obse
head exceededeans were ressor, which reew compresin the case ohe speed reqnecessary to
ompressor tyw compress
s, it becomescentrifugal coquired for muired to genegh head fo
her speeds thanical unloadd, the speed ments resultinvariable spe
acity control othe operatingthe greater th
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ler is taken ramp up to hore be runningnd other operat part load,water tempe chiller wenteed required tquired to genemechanical un
speed required for head .
commonly oversency seems illoghe head droppeds the condenser
ure 10 – Ideal
or example. size the chilat a price. A
ng at 85 to 90water would b
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e bigger the imved until the bn off line.
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mpact will be.building load
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e reality of acgant than IPnts where thover compres
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ween centrifumost commoton and IPLVlers are overss not accuwever, this reerences betwen in the subsiders four dis
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oaders or overhese four discropriate, but twn in the abo
% and 45% rating at 65 F
not supportperatures for
mates will experatures even
economizerrating hours ses that IPLesentation of ngle chiller oloy more thanner in which
HRI 550/590 reconducting a life cy
HRI 550/590 sectmprehensive too
Enterioad Water00% 5% 0% 5%
G FACTORMETRICS
ctual operatPLV, resultin
e use of mecssion are mo
oversizing chthe most com
s may not rugal and screonly referenceV (integrated psized, the sub
urately depiceality is not aeen compressbmittal data.screte operatin
nts assume thre is droppingperspective, quirements freducing ther compressio
crete operatinthis is rarely ove chart theof the time
F. The weatht 65 F ente
57% of thexperience con at high loars reduce th
at low loadLV “was dthe average p
only.”13 Mosn one chiller
h chillers are
ommends local wycle analysis. tion D.2 further e
ol should be used
ing Condensr Temperatur
85F 75F 65F 65F
R IN COMM
tion is muchng in operatichanical unore likely to
hillers providemmon metricreflect the dew compressoed metrics arepart load valubmittal full lct actual oacknowledgesion types w. The IPLng points:
he entering cg with load. this operatin
for flow ande need for mn. If chillers
ng points this the case (if e
e assumption e, the chillerher in warmerering condene ton hours.12
lder condensads while air he number ods. AHRIderived to ppart load efficst chilled wato meet the lsequenced in
weather to be use
explains that a md for life cycle an
ser re Weigh 1% 42%
45% 12%
MON
h less ing loaders occur.
es a good cs used to differences ors. The e full load ue). If the load point operation. d and the
will not be V metric
condenser From an
ng profile d head to mechanical s ran only would be
ever). As is that at
r will be r climates ser water 2 Cooler ser water and water of chiller I 550/590 provide a ciency for ater plants load. The n multiple
ed when
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chiller plants hours occurrinrarely depicts most chiller pthe local weatAHRI profilemuch less elepoints where over compresbecause the athe ideal speeAHRI IPLV chillers with possess induobserved in agreater.
Consider for operating prorepresented oyellow and bl
We observe t67% (two thimet by operatblue chiller chillers will lWhile the chtself is just 6
water temperaas listed as thFor this examagain when ththus requires 100% load ponot all 100% l
4 Subject to low
Figure 1
will typicallng in the 75%an actual chi
plants have mther likely doe. The realiegant than IPL
the use of sion are mor
actual operatind curve than load profile. pure speed
ustry leadingactual operatio
example a ofile of ch
on the chart iack lines.
hat when theirds load) theting just 2 ofis turned ofload up to 1
hillers are at 67% loaded, ature would nhe 100% loa
mple we will ahe building o
only one coints to the AHload points ar
delta T issues a
11 – Chiller Op
ly result in 2% load rangeiller’s load prmore than onoes not matchity of actual LV, resultingmechanical
re likely to ocng points arethe operating While thecapacity con
g IPLVs, on is expecte
three-chiller hillers 1, 2 in Figure 11
e building hase needed capf the 3 chillerff, the yellow100% to mee
100% load, so the enterin
not be expectead point on tassume 75 F.operates at 3
chiller. ComHRI submittare the same.
and other operatin
perating Profil
/3 of the tone. The IPLVrofile becausene chiller andh the standard
operation isg in operatingunloaders orccur. This is further from
g points in thee VFD screwntrol alreadythe benefits
ed to be even
plant. Theand 3 areby the blue,
s unloaded topacity can bers. After thew and blacket set point.14
the buildingng condensered to be 85 Fthe submittal. This occurs3% load and
mparing theseal reveals that
ng realities.
les
11
n V e d d s g r s
m e
w y s n
e e ,
o e e k 4 g r F . s d e t
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e screw chilletrifugal chilthis benefit mitted full lo
AHRI pointed curve for crating point d
w and head arr compressiciency as trifugal Speeler outperformrating points, er the submittuld be noted, tperating points observation h emphasis iwhen a ma
rs occur at ofaccept the
rsized, most coperation at denser water t
e BIN Metho
impacts of lugh that it isysis cannot b
dified such tect local weufacturers haprovide thesprofile, use o
ecognize that ric, and BIN
oad EnteringCondenWater Temper
00% 85 F
00% 75 F
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er further oller at operawill not be soad point or
t will fall vecentrifugal codoes not, as re now quite ion occurs,seen previod graph in Fms the centrbut this bene
ted full load that far more ts 3 and 5, tha
n raises the qis placed on ajority of theff peak tempeidea that chchillers will 100% load
temperature.
od’s Uninte
local weathers recommend
be performed,that temperaather and ch
ave simple mse values baof economizet IPLV or mometrics use a
g nser
rature
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~ 100
~ 100
~ 100
utperforms ting points 3seen in either the IPLV.
ery close to mpressors. Tthe speed ndifferent. In, impactingously on tFigure 10. Trifugal chillerefit will not bpoint or the Iton hours are
an at operatinquestion as tothe full load e high load eratures. In adhillers are cexperience zwith design
ended Impac
r and staging ded that whe, the IPLV eqatures and whiller staging
modeling progased on yourers, etc. It is odified IPLVaverage value
n Design Head
% ~ 100%
% ~ 80%
% ~ 60%
the 3 and 5, er the
the ideal The actual needed for n this case g chiller the Ideal The screw r at these be seen in IPLV. It e expected ng point 1. o why so submittal operation
ddition, if commonly zero hours n entering
ct
are large en hourly quation be weightings g. Most
grams that r building important
V is a BIN es. Using
12
average values can be convenient but it can also oversimplify. Consider a spring day with a cool morning and a warm afternoon. In the morning the building requires some heat and in the afternoon some cooling. On average, the building requires neither heating nor cooling but you should not expect your utility bill to reflect this average. The same is true for BIN data.
Within each load BIN, chillers operate over a wide range of conditions. On average, the chiller may be operating at 75% load with 75 F entering condenser water temperature. In actual operation, the loads and condenser water temperatures vary above and below the average. Figure 12 compares chillers of similar full load kW/ton to evaluate the variance between the 75% point published and the surrounding operating points. We find that IPLV uses the average operating point, not the average efficiency for the range of operating points.
For the variable speed centrifugal chiller evaluated, the average of operating points A,B,C,D is 5.6% less efficient than the rating point. For the variable speed screw chiller evaluated, this variance is just 1.5%. These results are consistent with the compressor behavior presented earlier. For centrifugal chillers operating at warmer temperatures, the head limits speed reduction and mechanical unloaders must be used. At colder temperatures, the centrifugal compressor is over compressing. The average efficiency resulting from the use of mechanical unloaders and over compression is not equal to the IPLV rating point, but is actally 5.6% lower!
Earlier we presented AHRI certified data showing that oiled screw chillers have better IPLVs than centrifugal chillers. Here we see that in actual operation, the advantages are likely larger than the full load and IPLV metrics would indicate. As we
move from IPLV to actual operation, the impact of averaging alone accounts for a large variance in operation. This observation leads to the following question: What other common issues experienced in actual practice impact chiller performance, and do they impact screw chillers differently than centrifugal chillers?
The Counterintuitive Energy Retrofit Case
A facility manager once boasted that his load had been significantly reduced by installing a new roof, new lighting and other changes. Based on his new lower cooling demands, he intended to upgrade his chillers to new units that specifically featured great efficiencies at part load. The solution was counter intuitive. The chiller that had the higher kW/ton (worse) values listed at the 50% and 25% load points at AHRI operating conditions was actually better. Reviewing in more detail we find that while he lowered the cooling demand, he could not lower the outdoor wet bulb. The AHRI IPLV efficiencies listed at 50% and 25% load are based on 65 F entering condenser water. His new chillers would be operating at these new lower loads, but with the warmer condenser water temperatures normally associated with chillers operating at higher loads. In essence, he had shifted his operating profile, creating oversized chillers. Only after accounting for both load and condenser water temperature did the answer become clear. Centrifugal chillers operated at high speed due to head requirements and used mechanical unloaders to get to 50% and 25% load. Screw chillers could use speed control alone, even with high entering condenser water temperatures, yielding true energy savings.
A B A B62.5% 87.5% 62.5% 87.5%
80F 0.525 0.5 80F 0.448 0.483
70F 0.376 0.398 70F 0.346 0.381D Average of Four Corners = 0.45 kW/Ton C D Average of Four Corners = 0.415 kW/Ton C
Variance =1.056 Variance =1.015
Centrifugal Chiller Screw Chiller
0.426
Entering Cond Water Temp.
Load Load
Entering Cond Water Temp.
0.409
IPLV reflects the average operating point, but the numerical average of operating at points ±5ºF and ±12.5% capacity above and below the IPLV point can be 5.6% worse for centrifugal chillers.
Figure 12 – Comparison of Actual Operating Loads to Average Loads
13
Warm Climates
In warmer climates, the speed of a centrifugal chiller is more likely to be dictated by the head required. This increases the use of mechanical unloaders and increases the expected relative operating savings of using variable speed screw compressors with pure speed capacity control.
CHILLER PLANT OPERATION:
IDEAL vs ACTUAL OPERATION
A typical life cycle analysis often assumes that a chilled water plant will not only achieve ideal operating conditions, but sustain them over a 25-year period. Ignored are the less than stellar maintenance practices, wear and tear, imperfect controls, low delta T syndrome, the fact that the equipment is oversized by 10 to 15% and, of course, unusual override requirements. What impact will these factors have on the chiller operating condition? The operating point is moved further from the ideal speed curve of a centrifugal chiller. As we move away from this point, the difference in compression matters more.
An interesting exercise to demonstrate this impact would be to perform a sensitivity analysis on a life-cycle evaluation. Change the simulation assumptions to reflect an actual load of 85% of the nominal chiller nameplate capacity. To simulate poor maintenance, use a tower approach of 10 F instead of 7 F. The results will show the effects of actual operating conditions on chiller performance.
Performance Degradation over Time
The concept of performance degradation over time occasionally sparks some debate. Some have suggested oiled chillers may worsen over many years of operation due to an assumed increase in oil concentration. There is no evidence to support this occurrence on modern positive pressure chillers. With respect to oil concentrations in the field, an ASHRAE subcommittee noted that, “the data is extremely sparse and statistically insignificant.”15 Any nominal impact of oil is already included in the AHRI submittal data and reflected in the approach temperature both on the submittal and in test results.
15 RTAR from Subcommittee 8.2 “Oil Concentration of Field-
Installed Chillers with Flooded Type Evaporators.”
Despite the seemingly large numbers often referenced in claims of oil impact on heat transfer, the actual impact of oil is a fraction of a fraction of a fraction, totaling just 0.1 % to 1% of the total losses in a chiller, as shown earlier in Figure 3. Oil-related losses are included in the AHRI certified performance. Comparisons of AHRI submittals reveal oiled chillers may have lower approach temperatures than oil-free chillers. Much is made of the impact of oil, but it is machine design and heat exchanger surface area that drive performance.
The approach temperatures of chillers and the instances requiring corrective action are commonly logged. Common causes of increasing approach temperatures are tube fouling, low refrigerant charge or issues with the metering device between the condenser and evaporator. Low refrigerant levels may contribute to oil accumulation in the evaporator and therefore, oil is often blamed; however, the actual cause of the change in performance is the low refrigerant charge. If the refrigerant charge level is adjusted to the appropriate level, the approach temperature will decrease. If all of the oil is removed from the evaporator and the refrigerant level is not corrected, the approach may actually get worse because the refrigerant level is still not covering the tubes and without the wetting benefit of oil, further degradation may occur.
Rather than focus on any one hypothetical cause of performance degradation for marketing gain, we should instead recognize that the maintenance of high performance variable speed chillers is more valuable than ever. Poor maintenance practices of many kinds have a common symptom. The head required of the compressor will rise. Poor tower maintenance, poor water treatment, skipping tube brushing, low refrigerant charge and many other issues all result in higher approach temperatures which increase the head on the compressor. In the case of centrifugal compressors, raising the head will raise the speed, and now the poor maintenance practice has elevated the power by the cube. A variable speed screw chiller speed is largely unaffected as lift increases. So while power will also increase, it is less sensitive to such issues.
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SUMMARY
Screw comprfan laws andcan be preciswithout the ugas bypass oa significantadvantage, imaccounts forchiller.
Not all compoefficiency equrefrigerant filHowever, evatotal losses accounting fevaporator loosses (water
not change wDespite the swith respect transfer coeff0.1 and 1% debenefit of oil)
In contrast, vaefficiency bywhere over hCentrifugal coto operate arequiring inletbypass or flrefrigerant tha
COMPARISO
Y
ressors are nd are not subsely matcheuse of mech
or flow recirct compressiomproving th
r over 50% o
onents and suually. Oil-frelm resistanceaporator losse
in a chillefor just rousses. The vafilm, fouling
whether the cseemingly larto the impac
ficient, the reepending on .
ariable speed y focusing ohalf of the lompressors (oat higher spt guide vaneslow recirculaan needed) t
ON CHART
not bound bybject to surg
ed to the opeanical unloaculation. Thon efficiencyhe componenof the total lo
ubsystems inflee chillers se
e associated es represent jur, with refr
ughly a quast majority og and metal rchiller is oilerge numbers ct to refrigerasulting benefload (ignorin
screw chilleron the composses in a coiled or oil-frpeeds to prs (flow restrication (compro reduce cap
by the ideal ge, so speed rating pointaders, hot his providesy nt that osses in a
fluence chillereek to reducewith boiling.ust 5% of therigerant filmarter of allof evaporatorresistance) dod or oil-free.often quotedant side heatfit is betweenng the wetting
rs garner highpressor itself,chiller reside.ee) may need
revent surge,ction), hot gasressing morepacity for the
14
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s
r e . e
m l r o .
d t n g
h f, .
d , s e e
operby thspeepoingas signiimpr50%
Whilat colikelreduneedflowthe melegaand in mthe IstagiredulocatWarmcondwithCoolThe mechwithpoinconseveryenergchanless becochillwith
We counundemechin towate
16 IP
avbececoth
rating point. he ideal fan laed can be prnt without the
bypass or fificant comroving the c
% of the total l
le screw chilommon indusly much higheuces head andd for mechan
w recirculationmetric. The ant. Chillerscapacity con
multiple chilleIPLV load ping (on/off)
uced. Furthertions will nmer weathe
ditions that reh mechanical uler weather maverage of
hanical unlohout either.16
nts, the moresider the imyday occurrgy upgrades
nges in occupthan ideal
omes a great lers with purh such consiste
place great vnt on, not juser pressure. hanical equip
otality and in er plants.
PLV uses averagverage of operatelow the 75% IPentrifugal chillersonsistently, with he IPLV point.
Screw compaws and are nrecisely matce use of mecflow recirculmpression ecomponent thlosses in a chi
llers often prostry metrics lier in reality. d capacity innical unloaden at the four reality of acs operate at anditions. Moser plants that profile (and wof chillers armore, the we
not match ther results iequire centrifuunloaders and
may contributover compre
oaders is no The further
e significant mpact of acturences such such as lig
pancy or use,maintenanc
comfort thatre speed capency.
value on peost on their be
The sapment. Chillerecognition o
e operating pointing ±5ºF and ±12LV load point ca
s. In contrast, scthe numerical av
pressors are nnot subject toched to the chanical unloaation. This
efficiency ahat accounts iller.
ovide better ke IPLV, the The IPLV lo
n tandem, limers, hot gas br points that ctual operatioa wide varietst chillers areare unlikely
weighting) ds the buildineather profile
he IPLV assin the high
fugal chillers td their resultite to over comession and thot equal to r off of the IP
the impact. ual operationover sizing
ghting and i, varying weace and opet variable spepacity contro
ople and toolest day, but
ame can be ers should be of our imperfe
ts, but the nume2.5% capacity abn be 5.6% worsecrew chillers opeverage falling wit
not bound o surge, so
operating aders, hot
provides advantage,
for over
efficiency benefit is
oad profile miting the bypass or comprise
on is less ty of head e installed to follow
due to the ng load is es in most umptions. her head to operate ing losses. mpression. he use of operating PLV load
As we n due to
g chillers, insulation, ather, and ration, it eed screw ol operate
ls we can when put
said of evaluated
ect chilled
erical bove and e for erate thin 1.5% of
15
REFERENCES
1. AHRI Standard 550/590 or Performance Rating of
Water-Chilling and Heat Pump Water-Heating Packages Using the Vapor Compression Cycle (2011).
1. ASHRAE Research Project 751 (RP-751),
“Experimental Determination of the Effect of Oil on Heat Transfer with Refrigerants HCFC and HFC-123 and HFC-134a,” July 1999.
2. ASHRAE Research Project 601 (601-TRP), “Chemical
Analysis and Recycling of Used Refrigerants From Field Systems,” April 1990.
3. Brasz, J. J., "Aerodynamics of Rotatable Inlet Guide
Vanes for Centrifugal Compressors" (1996), International Compressor Engineering Conference. Paper 1196.
16
This paper is provided for informational and marketing purposes only and shall not be deemed to create any implied or express warranties or covenants with respect to the products of Carrier Corporation or those of any third party.
© Carrier Corporation 2015 www.carrier.com/commercial 04-581080-01 Printed in U.S.A. 5-15 Replaces: New