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Formula to calculate Enthalpy from steam Pressure and temperatureHelpful Member! 

jswpower (Mechanical)
15 Dec 12 13:00
I want to show online turbine heat rate in DCS screen of the power plant.Please any one tell me formula(or Algorithm) to calculate enthalpy by using steam pressure and temperature with out using steam table.
SNORGY (Mechanical)
15 Dec 12 15:44
Google IAPWS-IF97 and you will find all the formulae you need.
There are a few of them.
MortenA (Petroleum)
17 Dec 12 3:51
The table will always be most accurate (especially for steam). HYSYS e.g. uses tables for steam. If you have access to the table making an intrapolation e.g. in excel is not difficult.

Best regards

Morten
Helpful Member!  PaoloPemi (Mechanical)
17 Dec 12 7:09
water is a very non-ideal fluid and the cp values are not constant even in small ranges of t, p,
you should define a maximum acceptable error for yor model,
simplified correlations (based on one or two parameters) can produce relatively large errors (i.e. 5-20% or more when applied outside a relatively limited range of t,p), if you need to follow that way (for example because the limits in DCS programming do not allow to include complex models) then you could break down the whole range and include several correlations for limited t,p ranges, this usually works, Wagner and Pruss and others have described many different simplified and complex models which you may adopt.
A well tested solution is IAPWS formulation for industrial use (IAPWS 1997) as suggested by Snorgy,
you can easily find the source code on the net,
finally if you need the best accuracy you may adopt the IAPWS formulation for general and scientific use (IAPWS 1995),
it is a quite complex model to program,
a solution could be to link an external dll (if your DCS allows that) which provides all the calc's,
this can be useful if you need to do some simulation as for example turbine, flash operations etc.
I use Prode Properties (which includes IAPWS 1995), other well known brands could be Chemicalogic and others.
ElSid1 (Mechanical)
7 Jan 13 13:07

Quote:

IAPWS-IF97 and you will find all the formulae you need
IF you use MathCAD, it has already been done. Valery Ochov (Professor from Russia), has a file that does the properties of Steam per the IAPWS-IF97 criteria. He is also a programmer for SteamPro software. I have used the MathCAD sheet and found it to be accurate.

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