How to calculate Tsat when given a pressure 1

[waveout]

I need the formula to calculate the saturation Temp. given the steam pressure. (Tsat)

Thanks,
Dave

 

[Cwiz]

i havent really seen any equations, what i would do is punch in Tsat vs. Psat data from a thermo/HT book into excel and fit a line to it. get the equation from the line

 

[timbones]

There is no formula (unless you curve fit one).

Tsat is given in the steam tables for a whole range of pressures in the saturated water/steam section of the tables.

If you haven't got a steam table, go to this site:

http://www.chemicalogic.com/download/phase_diagram.html

Look for the data under "saturation line" under the "data" tab of the spreadsheet.

 

[katmar]

Dozens of these formulas have been developed. One that I use from time to time was published by V Ganapathy, Hydrocarbon Processing, Nov 1988, pgs 105-108. Ganapathy references his source as V Gonzales-Pozo, Chem Eng, May 12, 1986, pg 123.

It has the form

T = Ax + B/x + C(x^0.5) + D(ln(x)) + E(x^2) + F(x^3) + G

where
T is temperature in Fahrenheit
x is pressure in psia
A = -0.17724
B = 3.83986
C = 11.48345
D = 31.1311
E = 8.762969E-5
F = -2.78794E-8
G = 86.594

For quick estimates in the field you can use
T (deg C) = 100 * (bar(abs)^0.25)

This is easy to do with a hand held calculator because you enter the pressure in bar(abs), hit the square root button twice, and mutiply by 100. The answer is within 1 or 2 degrees for "normal" plant pressures. For example, this short cut gives the temperature for 15 bar(abs) steam as 196.8 C, but a more accurate formula will give 198.3 C

regards
Harvey



Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Montemayor]

Harvey is absolutely correct in logic and technique. I do the same, except I use a technique that has proven much more accurate and flexible than V Ganapathy's 1986 version. I use present-day, free, authoritative and readily accessible thermodynamic properties for water available at:

http://webbook.nist.gov/chemistry/fluid/

I take the tabular results and regress the data using my favorite regression program, DataFit. I can choose the resulting equation(s) I wish, depending on the accuracy and range I opt for. The result is a mathematical equation for saturated steam that I use in Visual Basic programs I write sometimes. The result is as "accurate" as you're going to get in the USA - mainly because it's authoritative and backed up by NIST.

With the advent of readily available thermodynamic databases such as NIST, timbones' comment about there not being an equation has become history. There is an equation available. I know Harvey and I have had accurate ones for years now. All you have to do is ask - and that is one of the functions this Forum has as its goal: furnish an "asking" platform. I wish I had been born 30 years later; life would have been much easier in engineering.

 

[athomas236]

You could always try this which I think is the correct equations behind the steam tables

Public Function tSatW(pressure)
'
' saturation temperature of water
' tSatW in K
' pressure in bar
'
' tSatW = -1: pressure outside range
'
'
If pressure < 0.00611213 Or pressure > 220.64 Then
tSatW = -1#
Else

' Initialize coefficients for region 4
'
nreg4(1) = 1167.0521452767
nreg4(2) = -724213.16703206
nreg4(3) = -17.073846940092
nreg4(4) = 12020.82470247
nreg4(5) = -3232555.0322333
nreg4(6) = 14.91510861353
nreg4(7) = -4823.2657361591
nreg4(8) = 405113.40542057
nreg4(9) = -0.23855557567849
nreg4(10) = 650.17534844798

bet = (0.1 * pressure) ^ 0.25
eco = bet ^ 2 + nreg4(3) * bet + nreg4(6)
fco = nreg4(1) * bet ^ 2 + nreg4(4) * bet + nreg4(7)
gco = nreg4(2) * bet ^ 2 + nreg4(5) * bet + nreg4(8)
dco = 2 * gco / (-fco - (fco ^ 2 - 4 * eco * gco) ^ 0.5)
tSatW = 0.5 * (nreg4(10) + dco - ((nreg4(10) + dco) ^ 2 - 4 * (nreg4(9) + nreg4(10) * dco)) ^ 0.5)
End If
'
End Function

 

[zerok]

Hi waveout,

The formula you're looking for is available at the iapws (International association for the properties of water and steam) web site :

http://www.iapws.org/relguide/IF97.pdf

(pp 33-35)

a free software (WSProps) which is based on the iapws releases is available at

http://www.sourceforge.net/projects/innohex

Regards

B-)