Properties Formula

[dastagg]

I have two formulas which I need assistance on as they are close but not precise.
First is for saturated steam properties above atmospheric pressure.
Formula: 117.31*(Steam in PSIG+14.696)^.2226 = Saturated Steam Temperature
This is used to read a gauge pressure to convert to the temperature in steam properties pertaining to water.

Second formula is converting vacuum pressure (inches on gauge) to saturated steam temperature.
Formula is: 117.3*((Inches/-29.921*14.696)+14.696)^.2226 = Temperature

Any idea what number is not correct. Like I said, it is close, but not precise.

 

[Flareman]

Dastagg
These formulae are essentially the same formula.
The first gives the pressure in psig because that's what you read on the gauge (above atmospheric) and the second gives the pressure in inches of mercury for the same reason (below atmospheric) I would retstate second formula with extra brackets for clarity as
117.3*(((Inches Hg/29.921)*14.696)+14.696).
{this uses 29.921 inches of mercury = 1 atmoshere and then converts to psi}
The 117.31 or 117.3 is just a matter of numeric accuracy which in this case is a moot issue because, clearly, the formulae are not quite accurate anyway as they give the boiling pint of water as 213 degF. They are essentially someone's wet finger (forgive the pun) estimates. We all do this ... saves looking up tables which is a drag when you're writing a spreadsheet.


for the but it seems as though your converting inches to psig using a divisor of

 

[dastagg]

Thanks,
This was my point, it showed 213 as boiling water at atmospheric. Not accurate but probably close enough for me. Thought maybe I could make small change and get correct answer. Either way, I can live with this.

 

[dastagg]

Flareman
By the way, the last part of your message was lost.

 

[poetix99]

Read the Appendix of "Steam Tables" by Keenan, Keyes, et.al. (or any other similar book) - O.K. don't REALLY read it (you would fall fast asleep), but just scan through it sometime. The steam properties are a masterpiece of curve-fitting. Very complex overall interrelationship of the various properties.

After reviewing these, you might appreciate that your convenient equation is indeed an approximation that will not necessarily be "made correct" by merely adjusting a coefficient.

There is easily a tendency to overapply your kind of simplified equations, or to forget their known range of acceptable accuracy, or to have never established such a range in the first place. Use your temperature calculation with caution.