What do the subscripts in thermodynamic steam tables mean?

[AriAstronomer]

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

 

[Studiot]

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 h_f kJ/kg at any temperature t_f

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' h_fg again inkJ/kg.
this energy is added at constant (saturation) temperature t_f.

We now have our water as dry, saturated vapour. To get our water to this stage, we must have added the vapour specific enthapy h_g - you guessed it in kJ/kg.

So h_g = h_f + h_fg.

If we wish to heat this dry steam further we must add the superheat enthalpy.
The superheat enthalpy is the gaseous specific heat C_p times the temperature difference, as with any other gas.

h_s = C_p(T_s - T_f) kJ/kg


Where T is in degrees Kelvin.

The total enthalpy of the gas is then

h = h_s + h_g

Hope this helps

go well

 

[AriAstronomer]

thanks a lot for the speedy reply, it definitely cleared a lot of things up.