Surging and choking

Surging is associated with sudden drop in delivery pressure & with violent aerodynamic pulsation which is transmitted throughout the machine
the position is reached where no further increase in mass flow can be obtained no matter how wide open the control valve is ‐ CHOKING

1. BRANCH :-MECH(5
th
)
SUBJECT:-FLUID POWER ENGINEERING
GUIDED BY :-V.N.KALOLA
NAME ENROLLMENT NO.ROLL NO.
CHAUDHARI BRIJAL170433119004 2097
CHAVDA MAYUR 170433119006 2098
DOSHI PARAS 170433119008 2099
HINGU RAVI 170433119009 2100
SHANTILAL SHAH ENGG.
COLLEGE

2. •Letsanalyzewhatwill
occurwhenavalve
placedinthedelivery
lineofacompressor
runningatconstant
speedisslowly
opened
•Thevariationin
pressureratiois
shownabove
Centrifugal Compressor
Performance

3. •PointAoccurswhenthevalveisshut&massflowiszero.
Itcorrespondstocentrifugalpressureheadproducedby
actionofimpellerontheairtrappedbetweenthevanes.
•AtpointB,efficiencyandpressureratioapproach
maximumvalue.Furtherincreaseinmassflowwillresult
infallofpressureratio.
•Formassflowsgreatlyinexcessofdesignmassflow,air
angleswillbewidelydifferentfromvaneanglesleadingto
breakawayofair&fallinefficiency.
•Thepressureratiodropstounityat'C',whenthevalveis
fullyopenandallthepowerisabsorbedinovercoming
internalfrictionalresistances
Centrifugal Compressor
Performance

4. Surging
•Theoperatingpoint'A'couldbeobtainedbuta
partofthecurvebetween'A'and'B'couldnot
beobtainedduetoSurging.
•Surgingisassociatedwithsuddendropin
deliverypressure&withviolentaerodynamic
pulsationwhichistransmittedthroughoutthe
machine
•ForanyoperatingpointDonthepartof
characteristicscurvehavingapositiveslope,a
decreaseinmassflowwillbeaccompaniedbya
fallindeliverypressure.

5. •Ifthepressureoftheairdownstreamofthe
compressordoesnotfallquicklyenough,the
airwilltendtoreverseitsdirectionandwill
flowbackinthedirectionoftheresulting
pressuregradient.
•Whenthisoccurs,thepressureratiodrops
rapidlycausingafurtherdropinmassflow
untilthepoint'A'isreached,wherethe
massflowiszero.
Surging

6. •Surgingstartstooccurinthediffuser
passageswhereflowisretardedbyfrictional
forcesnearthevanes
•Tendencytosurgeincreaseswithnumberof
diffuservanes
•Severaldiffuserchannelstoeveryimpeller
channel–tendencyforairtoflowupone
channel&downanother
Surging

7. •Onlyinonepairofchannelsthedelivery
pressurewillfall&increaselikelihoodof
surging
•Thusnumberofdiffuservanesislessthan
no.
ofimpellervanes
•Surgingisthennotlikelytooccur
Surging

8. Choking
•Atsomepointsay'E',thepositionisreached
wherenofurtherincreaseinmassflowcan
beobtainednomatterhowwideopenthe
control valve is ‐ CHOKING
•Thispointrepresentsthemaximumdelivery
obtainableattheparticularrotationalspeed
forwhichthecurveisdrawn.

9. LossesinCompressor
1.FrictionalLosses
2.IncidenceLosses
3.Clearanceandleakagelosses

10. FrictionalLosses
•Majorportionofthelossesisduetofluid
frictioninstationaryandrotatingblade
passages
•Flowinimpelleranddiffuserisdecelerating
in
nature
•Frictionallossesareduetobothskinfriction
andboundarylayerseparation
•Dependonthefrictionfactor,lengthofthe
flowpassageandsquareofthefluidvelocity

11. IncidenceLosses
•Duringthe off‐design conditions, the direction
ofrelativevelocityoffluidatinletdoesnot
matchwiththeinletbladeangle
•Hence,fluidcannotenterthebladepassage
smoothlybyglidingalongthebladesurface
•Thelossinenergythattakesplacebecauseof
thisisknownasincidenceloss

12. •Thisissometimesreferredtoasshocklosses.
•However,thewordshockinthiscontext
shouldnotbeconfusedwiththe
aerodynamicsenseofshock
IncidenceLosses

13. Clearanceandleakagelosses
•Certainminimumclearancesarenecessarybetween
theimpellershaftandthecasingandbetweenthe
outletperipheryoftheimpellereyeandthecasing
•Theleakageofgasthroughtheshaftclearanceis
minimizedbyemployingglands.
•Theclearancelossesdependupontheimpeller
diameterandthestaticpressureattheimpellertip.
•Alargerdiameterofimpellerisnecessaryforahigher
peripheralspeedanditisverydifficultinthesituation
toprovidesealingbetweenthecasingandthe
impellereyetip.

14. LossesinCompressor

15. •Theleakagelossescompriseasmallfraction
of
thetotalloss.
•Theincidencelossesattaintheminimum
valueatthedesignedmassflowrate.
•Theshocklossesarezeroatthedesignedflow
rate.
LossesinCompressor

16. •However,Incidencelossescomprisebothshock
lossesandimpellerentrylossduetoachangein
thedirectionoffluidflowfromaxialtoradial
directioninthevanelessspacebeforeentering
theimpellerblades.
•Theimpellerentrylossisverysmallcompared
to
otherlosses.
•Thisiswhytheincidencelossesshowanon
zerominimumvalueatthedesignedflowrate.
LossesinCompressor

17. 