SFEE and steady flow engineering devices

in this ppt pdf derivation of SFEE can be understood easily and these how this equation applied to different engineering devices has been shown.

1. MECHANICAL ENGINEERING
SEMESTER –IV
MODULE –2
ENERGY BALANCE FOR STEADY -FLOW SYSTEMS AND STEADY FLOW
ENGINEERING DEVICES
BY
MANOJ KUMAR
DEPARTMENT OF MECHANICAL ENGINEERING
SCE SAHARSA
DATE 30-03-2020

2. STEADY FLOW PROCESS
Steadyflowmeans
fluidproperties
doesnotchanges
withtimeata
particularposition
inadefined
system/control
volume.
Uniformflowmeans
fluidproperties
doesnotchangein
entire
system/control
volumefora
particulartime
instant.
STEADY FLOW
UNIFORM FLOW
2

3. 3
Flowwork:Unlikeclosedsystems,controlvolumesinvolvemassflowacrosstheirboundaries,andsomework
isrequiredtopushthemassintooroutofthecontrolvolume.Thisworkisknownastheflowwork,orflow
energy,andisnecessaryformaintainingacontinuousflowthroughacontrolvolume.
Thetotalenergyconsistsofthreepartsforanon-flowingfluidand
fourpartsforaflowingfluid.
But the combination Pv+ u has been previously defined as the enthalpy h.
Hence,
And in the rate form,

4. 4
Under steady-flow conditions, the mass and energy contents of a control volume remain constant.
Similarly,
Notingthatenergycanbetransferredbyheat,work,andmassonly,theenergybalanceforageneralsteady-
flowsystemcanalsobewrittenmoreexplicitlyas

5. 5
Itiscommonpracticetoassumeheattobetransferredintothesystem(heatinput)atarateofQandworkproduced
bythesystem(workoutput)atarateofWandthensolvetheproblem.Thefirst-laworenergybalancerelationin
thatcaseforageneralsteady-flowsystembecomes:
A water heater in steady operation.
For single-stream devices, the steady-flow energy balance equation
becomes
Dividing above equation by gives the energy balance on a unit-mass basis as
The above equation is well known as steady flow energy equation (SFEE)

6. SOME STEADY-FLOW ENGINEERING DEVICES
6
1. NOZZLES AND DIFFUSORS
Nozzlesanddiffusersarecommonlyutilizedinjetengines,rockets,spacecraft,andevengardenhoses.A
nozzleisadevicethatincreasesthevelocityofafluidattheexpenseofpressure.Adiffuserisadevice
thatincreasesthepressureofafluidbyslowingitdown.Thatis,nozzlesanddiffusersperformopposite
tasks.Thecross-sectionalareaofanozzledecreasesintheflowdirectionforsubsonicflows(MACH
NUMBERLESSTHANUNITY)andincreasesforsupersonicflows(MACHNUMBERGREATER
THANUNITY).Thereverseistruefordiffusers.
ASSUMPTIONS:
1.Heattransferisnegligible(Therateofheattransferbetweenthefluidflowingthroughanozzleora
diffuserandthesurroundingsisusuallyverysmallsincethefluidhashighvelocities,andthusitdoesnot
spendenoughtimeinthedeviceforanysignificantheattransfertotakeplace).
2.Worktransferisnegligible.
3.Changeinkineticenergyandpotentialenergyarenegligible.
(For nozzles)
(For diffusors)

7. 7
2. TURBINES AND COMPRESSORS
Insteam,gas,orhydroelectricpowerplants,thedevicethatdrivestheelectricgeneratoristheturbine.Asthe
fluidpassesthroughtheturbine,workisdoneagainsttheblades,whichareattachedtotheshaft.Asaresult,
theshaftrotates,andtheturbineproduceswork.
Compressors,aswellaspumpsandfans,aredevicesusedtoincreasethepressureofafluid.Workissupplied
tothesedevicesfromanexternalsourcethrougharotatingshaft.Therefore,compressorsinvolveworkinputs.
Eventhoughthesethreedevicesfunctionsimilarly,theydodifferinthetaskstheyperform.Afanincreasesthe
pressureofagasslightlyandismainlyusedtomobilizeagas.Acompressoriscapableofcompressingthegas
toveryhighpressures.Pumpsworkverymuchlikecompressorsexceptthattheyhandleliquidsinsteadof
gases.
ASSUMPTIONS:
1.Heattransferisnegligible(Heattransferfromturbines/compressorsisusuallynegligiblesincetheyare
typicallywellinsulated).
2.Changeinkineticenergyandpotentialenergyarenegligible.
(For turbine)
(Since turbine is producing work that is sign of work is positive hence inlet enthalpy is greater than outlet enthalpy)
(For compressor)
(Since compressor is consuming work that is sign of work is negative hence inlet enthalpy is less than
outlet enthalpy)

8. 8
3. THROTTLING VALVES
Throttlingvalvesareanykindofflow-restrictingdevicesthatcauseasignificantpressuredropinthefluid.
Somefamiliarexamplesareordinaryadjustablevalves,capillarytubes,andporousplugs.Unliketurbines,
theyproduceapressuredropwithoutinvolvinganywork.Thepressuredropinthefluidisoften
accompaniedbyalargedropintemperature,andforthatreasonthrottlingdevicesarecommonlyusedin
refrigerationandair-conditioningapplications.
Throttlingvalvesare
devicesthatcause
largepressuredrops
inthefluid.
ASSUMPTIONS:
1.Heattransferisnegligible(Throttlingvalvesareusuallysmalldevices,
andtheflowthroughthemmaybeassumedtobeadiabatic(q=0)since
thereisneithersufficienttimenorlargeenoughareaforanyeffective
heattransfertotakeplace).
2.Worktransferisnegligible.
3.Changeinkineticenergyandpotentialenergyarenegligible.
Henceinletenthalpyisequaltooutletenthalpyhenceisenthalpicprocessoccurin
throttlingdevices.
Thetemperatureofanidealgasdoesnot
changeduringathrottling(h=constant)
processsinceh=h(T).Thatisinlet
temperatureequaltooutlettemperature.

9. 9
4. BOILERS AND CONDENSERS
ASSUMPTIONS:
1.Worktransferisnegligible.
2.Changeinkineticenergyandpotentialenergyarenegligible.
(For boiler)
(For condenser)
(Since in boiler heat is added tin water to produce steam, sign of heat is positive hence inlet enthalpy is less than
outlet enthalpy)
(Since in condenser steam get condense hence losses its heat thus heat is rejected, sign of heat is negative hence
inlet enthalpy is greater than outlet enthalpy)

10. 10
5. HEAT EXCHANGERS
Asthenameimplies,heatexchangersaredeviceswheretwomovingfluidstreamsexchangeheatwithout
mixing.Heatexchangersarewidelyusedinvariousindustries,andtheycomeinvariousdesigns.
Thesimplestformofaheatexchangerisadouble-tube(alsocalledtubeandshell)heatexchanger,shown
belowinfigure.Itiscomposedoftwoconcentricpipesofdifferentdiameters.Onefluidflowsintheinner
pipe,andtheotherintheannularspacebetweenthetwopipes.Heatistransferredfromthehotfluidtothe
coldonethroughthewallseparatingthem.Sometimestheinnertubemakesacoupleofturnsinsidetheshell
toincreasetheheattransferarea,andthustherateofheattransfer.
ASSUMPTIONS:
1.Understeadyoperation,themassflowrateofeachfluid
streamflowingthroughaheatexchangerremainsconstant.
2.Heattransferisnegligible(Whentheentireheatexchangeris
selectedasthecontrolvolume,heattransferbecomeszero,
sincetheboundaryforthiscaseliesjustbeneaththeinsulation
andlittleornoheatcrossestheboundary)
3.Worktransferisnegligible.
4.Changeinkineticenergyandpotentialenergyare
negligible.