0
$\begingroup$

By Googling I can find basically two types of steam tables for water. They are either saturated steam tables, which I understand are for steam in saturated state. Here pressure are temperature are fixed together as saturated steam states form a curve in water phase diagrams. Then there are some tables for superheated steam, where temperature exceeds critical temperature for water.

But what if I'm interested in values for water vapour/steam that is neither superheated nor saturated, that exists under the saturation curve in water phase diagrams?

Why are we specifically interested in saturated steam?

$\endgroup$
1
  • 1
    $\begingroup$ That would be liquid water. $\endgroup$
    – TimWescott
    Oct 14 '19 at 19:50
1
$\begingroup$

Not sure this is universal but in my field of interest (steam turbines), we are interested in saturated steam because that is what we get in the last stages of the low pressure turbines. In our case, moisture is detrimental to the performance of the turbine, so it is important to be able to predict the moisture content aka liquid fraction aka (1-steam quality).

Saturated steam tables at thermodynamic equilibrium are one way to get this information.

Now, we could be (and are) interested in steam in a subcooled state, i.e. steam that remains dry at temperatures lower than the saturation temperature. This state is in fact often encountered in steam turbines, because the flow is expanded so fast that steam "doesn't have time" to condense at equilibrium. It remains dry, until the temperature/pressure conditions reach a threshold called the Wilson line. Then, condensation happens and thermodynamic equilibrium is restored.

wilson line in enthalpy/entropy diagram

The properties of subcooled steam are usually extrapolated from the properties of superheated steam.

Whether you are interested in saturated steam at equilibrium, or subcooled steam, the reference data for the industry is the one from the International Association for the Properties of Water and Steam (http://www.iapws.org/relguide/IF97-Rev.html)

$\endgroup$
0
$\begingroup$

Because for any mix of water vapor we re-calculate using (1-x), x being the dryness fraction.

$\endgroup$
1
  • $\begingroup$ Dryness fraction aka steam quality. You use sat steam data for the vapor fraction and sat liquid data for the liquid fraction. $\endgroup$
    – Phil Sweet
    Oct 13 '19 at 19:52
0
$\begingroup$

Water vapor below saturation is the vapor pressure of liquid water. By definition, saturated steam is steam for which the vapor pressure equals boiling point pressure/temperature. Vapor pressure below the boiling point is easily determined by the appropriate vapor pressure tables/graphs/calculators.

Saturated steam is important because it is the state before steam is heated above boiling. So: boiler exit conditions for saturated steam boilers, superheating boilers before superheating, condenser conditions, and some turbine conditions. Enthalpy of vaporization, hfg, is the heat required to fully vaporize water at the boiling point. It is what most heat is used for in making steam.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.