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CHEE 221Tutorial:SaturationandSteamTables

WhatdotheyreallymeanbySaturation?

Upon reachingand enthalpy of 2676 only saturatedvapour will remain. Further increasesin enthalpy will now causefurther increasesin temperatureabove 100C and the vapouris a superheated vapour or superheated steam, meaning thatthe vapour has beenheatedabove its saturationtemperature.

1. Saturationconditionsarethoseunderwhichtwoormorephasesof a puresubstancecanexisttogetherinequilibrium.However,notethatasecondphaseneednotactuallybepresent.A phaseis consideredsaturatedso long as it is at conditionswhereanotherphasecouldexistin equilibrium.In thecasewherea singlephaseis found,it is in aconditionwhereanysystemchanges(temperature,pressure,enthalpy)will causesomematerialtochangephase.

Important note: 419+2257=2676

Question: Why is this important?(this is thechange in enthalpybetweensaturatedliquid waterand saturatedvapour at 100C)

Here aresome examples.

Question: If, at I atm and 100C with no vapour present, 1000KJ of heat is addedto Ikg of water, what percentis convertedto vapour?(no vapour means that the enthalpy is that of liquid water at 419 kJlkg, then we add1000kJ which is entirely used to convert a quantity of liquid water into steam.

Since it takes a total of 2257 kJ to convert I kg of water,1000kJ will convert 1000/2257. 100%= 44.3%)

Question: What would you call I kg of water at I atm with the following enthalpies:

Anotherway of lookingat saturationconditionsis thata changeof phasecanoccurwithoutachangein pressureor temperature(whatoccursis a changein enthalpy). Yetanotherapproachis to considera saturationstateto betheconditionsatwhicha phasechangebegins,takesplace,orends. ..

2. Liquid water (a singlephase)can exist in equilibriumat varioustemperatures(betweenslightlyabove0 and100C)whileundera pressureQf I atm. However,liquidwatercannotexistattemperatureshigherthan100Cwhileat I atm.Watervapourcanexistatthispressureonlyattemperatureof 100Candhigher.Thereforeat I atm,threepossiblephasecompositionscanexist:(I) onlywater;(2) onlywatervapour;(3) waterandwatervapourinequilibrium.

Lets takea closer look at thesesituations.

a) 4 KJ (coldliquidwater)b) 419.1KJ (slightlyabovesaturatedliquidwater)c) 1500KJ (amixofliquidwaterandwatervapour)d) 2675KJ (almostsaturatedwatervapour)e) 2677KJ (justabovesaturation,superheatedwatervapour)I) 3000KJ (superheatedwatervapour)

ConsiderI kgof waterwithanenthalpyof 3000KJ at I atm.It is asuperheatedvapour.Question:Whatis itstemperature?(262C,by interpolationfromtableB.7)Supposewehaveasystematatemperaturebelow100Cwhileat I atm,withonly I kg

ofliquid water.Thereis nogasphase(wearenotconsideringthepresenceof air... yet).At this pointthewateris below its saturationtemperatureandis calleda subcooledliquid. Heatcanbeaddedandthetemperaturewill increase.At 100Cthefollowingenthalpiesare reported(relativeto waterat its triplepoint OC, wherethe relativeenthalpyis takentobe0).

As energyis removedandthetemperaturedecreasesitwill eventuallyreachitssaturationpoint(100Cwithanenthalpyof2676KJ). 11is thenasaturatedvapour.

WaterEvaporationSteam (Vapour)

419.1kJlkg2257kJlkg:2676kJlkg

As more energy is removed, the temperaturewill remain constantand saturatedvapourand saturatedliquid will exist in equilibrium until the enthalpydrops to 419.1 KJ.

OK this is really great and I'm happy that the water went from superheatedto saturatedand went throughvapour to liquid phase... but what about thosesteamtables?

Upon arrivingat 100Cwith an enthalpyof 419.1kJ,the liquidwateris said to besaturatedwater(thereis no watervapouratthispoint). Additionalheat(latentheatofvaporization)will causenofurtherincreaseintemperature,butwatervapourwill begintoform(saturatedvapour)andwill bein equilibriumwiththeliquidwaterso longastheenthalpyisbetween419.1and2676kJ.

All right, herewe go.(turn thepage)

SaturatedSteamtablesTwo types: Pressurebasedand Temperaturebased.

Conversely you could look up thepressureon B.6.

- If thetemperatureis higher the steamis superheated.- If it is thesame, you havesaturatedsteam.- If it is lower somebody lied to you and you don't have steam atall but a sub cooled liquid.Table B.5: TemDeraturebasedSaturatedSteam:

For the temperatureslisted, we are at saturation conditions (along the Vapour-LiquidEquilibrium Curve). The corresponding saturation pressure is also listed here. Anychange in T, P, or H will cause material to changephases.

The temperaturemust exactly match the given pressure in order to havesaturatedsteam.

Information on this table:

Saturationpressureat the listed temperatureSpecific Volume (inverse of density)Internal EnergyEnthalpy: Saturatedwater

Evaporation/condensationSaturatedSteam

LookatthePT phasediagramonpage327toconvinceyourselfthatthisistrue.

You can also look on B.7. If theT and P you are interestedin, intersectin

the boxed region, you have a liquid not a vapour and someone lied to youagain. If you are right on the line then it's a saturatedvapour, and if youareoutside thebox, you've got superheatedsteam.

TableB6:PressurebasedSaturatedSteam:The only differenceis thatthe pressureis the indexby which you are lookingupconditions.Forgivenpressures,youcanlookupsaturationtemperatures.(therangeofTableB.6is moreextensive,whichcanbeusefuldependingontheinformationsought).

4. You aretoldthatyouhavewateratacertaintemperatureandnopressureis given.Enthalpyis a veryweakfunctionof pressureso valuesfromB5 canbeused. Evenif thewateris "subcooled"thevaluesfrom B5 arecloseenough.

Informationonthetable:SaturationtemperatureatthelistedpressureSpecifieVolume(inverseof density)InternalEnergyEnthalpy: Saturatedwater

Evaporation/condensationSaturatedSteam

4. You aretold thatyou have a subcooled liquid.You cannot use the values from B.5 and B.6 becausewe are interestedin

the enthalpy difference between the subcooled and the saturation state(this won't happenvery often in CHEE22I ). You may just be betteroffusing the Cp integrals. However, you could use the Cp integrals to findtheheatneededto get you to saturationconditions and thenuse thetables.

Uses:I. For a situation where you have saturatedsteamata certain T.

All propertiescan be found on B.S.

5. You havesuperheatedsteamandthe pressureand temperaturevalues arenot in TableB.7.

Seethenext Section on how B.5 can help in this situation.

SuperheatedSteamTables2. ForasituationwhereyouhavesaturatedsteamatacertainP.

All properties.canbefoundonB.6.

3. May havesteam~ta certaintemperatureand pressureandneedto know if it issaturated.

Look up the temperatureon B.5 and compare your pressure with the onefrom the table.

- If your pressureis lower, you have superheatedsteam.- If it is thesame,you have saturatedsteam.- If it is higher, somebody lied and you havesub cooled liquid.

TableB.7To be superheated,you are at a temperaturein excessof the saturationtemperature. Thistable is a little morecomplicated, so I'll go into more detail on eachcolumn.

ColumnIThis is whereyoulocatethesystempressure.Thetemperatureatwhichthesteamwillbecomesaturatedis listedin brackets.This temperatureis alsothedew point for thesystem.

Columns 2 and 3

The saturatedpropertiesare listed. This is the same information you would find on B.5and B6.. It is here for convenience. You will frequently run into questions where

superheated steamis being cooled past saturation. This information savesyou having tolook on otherchartsand table.

The restof the table

The system temperature is located across the top row. Intersect it with the systempressure and thereyou find the propertiesat the systemT & P. If the T and P you areatintersectwihtin the boxed region, it's a liquid not a vapour. If you are exactly on the linethen it's saturated vapour (check column 2 and 3 for the properties) and if you areoutside the box, it's superheated steam! And then you can determine the degrees ofsuperheat (which is the difference between the temperatureof your vapour and its dewpoint... The dew point is the temperatureat which this vapour becomessaturated).

OhNO! thetemperatureandPressureI wantarenotinTableB.7! Whateverwill I do!

Don't jump just yet, there is still hope. Table BS can save you. If you areat less than 10bar or if the pressure is not given then use the saturatedenthalpy values from Table B5.If the pressureis greaterthan 10bar thenuse theformula H = U+ PV and the values fromB5 to getthe enthalpy.

Terms to Remember,and be able to differentiatebetween:

Subcooled waterSaturatedwaterSaturatedstearn

SuperheatedsteamDegreesof superheatSuperheatedwater

See definitions and furtherdetails in your textbook.