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In summary, the f, g, and fg subscripts in the thermo steam tables refer to saturated liquid water, saturated steam, and dry steam respectively. The difference between saturated liquid water and saturated steam is that saturated liquid water is in equilibrium with the vapour, while saturated steam is not in equilibrium and has to be heated to get there. Finally, the superheat enthalpy is the gaseous specific heat of a gas times the temperature difference between the gas and its equilibrium temperature.
- #1
AriAstronomer
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Hey everyone,
I'm a bit confused with the meanings of f, g, and fg subscripts in the thermo steam tables (e.g. hf, hfg, hg). If there is saturated steam, part of the enthalpy is in liquid, hf, part is encapsulated in the vapour, hg, but what is hfg? Also h = hf + (x)hfg. h here is the total enthalpy of the entire system, and hf/hfg are the components?
Another question, is what is the difference between saturated liquid water, and saturated steam?
Some general help would be appreciated.
Ari
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- #2
Studiot
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Here is a quick rundown.
You should be aware that engineering steam is usually a 2 phase system ie both liquid and gaseous water are present. this is also called wet steam.
Starting with a pot of liquid water not in equilibrium with vapour (steam) we need to add energy to it to bring it to saturation (equilibrium with vapour)
This energy is the enthalpy hf kJ/kg at any temperature tf
Once we have the water in equilibrium we want to evaporate it so we have to add the latent heat or 'specific enthalpy of evaporation' hfg again inkJ/kg.
this energy is added at constant (saturation) temperature tf.
We now have our water as dry, saturated vapour. To get our water to this stage, we must have added the vapour specific enthapy hg - you guessed it in kJ/kg.
So hg = hf + hfg.
If we wish to heat this dry steam further we must add the superheat enthalpy.
The superheat enthalpy is the gaseous specific heat Cp times the temperature difference, as with any other gas.
hs = Cp(Ts - Tf) kJ/kg
Where T is in degrees Kelvin.
The total enthalpy of the gas is then
h = hs + hg
Hope this helps
go well
- #3
AriAstronomer
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thanks a lot for the speedy reply, it definitely cleared a lot of things up.
Related to What do the subscripts in thermodynamic steam tables mean?
What are thermodynamic steam tables?
Thermodynamic steam tables are reference tables that provide a comprehensive set of thermodynamic properties for water and steam at various temperatures and pressures. These properties include specific volume, internal energy, enthalpy, entropy, and quality.
Why are thermodynamic steam tables important?
Thermodynamic steam tables are important because they provide essential information for engineers and scientists working with steam systems. These tables allow for accurate calculations and analysis of steam properties, which are crucial for designing and operating steam-based systems such as power plants and steam engines.
How are thermodynamic steam tables created?
Thermodynamic steam tables are created by conducting experiments and collecting data on the thermodynamic properties of water and steam. This data is then organized and tabulated into tables for easy reference. Advanced mathematical equations and models are also used to calculate properties at values that are not explicitly included in the tables.
What is the difference between saturated and superheated steam in thermodynamic steam tables?
Saturated steam is the state where both liquid and vapor phases of water coexist at a given temperature and pressure. Superheated steam, on the other hand, is steam that has been heated above its saturation temperature while remaining at the same pressure. In thermodynamic steam tables, properties for both saturated and superheated steam are included.
How are thermodynamic steam tables used?
Thermodynamic steam tables are used by engineers and scientists to determine the properties of steam at different conditions. These tables are typically arranged in a grid format, with temperature and pressure as the input parameters. By locating the desired temperature and pressure values, the corresponding properties can be found, allowing for accurate calculations and analysis of steam systems.
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