Enthalpy of Steam

What is the enthalpy of superheated steam (in kJ/kg)?
- Mike (age 17)
Edmonds, WA, USA

There's a bit of arbitrariness in defining enthalpies, because you have to pick which state you call the one with zero energy. Presumably here we don't want to count the rest energy of the water. So let's say you call the enthalpy zero for water at T=0K. Now you can get the enthalpy at some other T by just integrating C_pdt from t=0 up to the T you're interested in, where C_p(t) is the constant-pressure heat capacity at temperature t. For water at atmospheric pressure the biggest term in that comes from from the liquid to steam latent heat with other major terms from the ice to liquid latent heat and from heating the liquid water up to boiling. Those latent heats are 2260 kJ/kg and  334 kJ/kg, respectively. (You can think of the latent heat as a very big spike in C_p(t) at the phase transition temperature.) Heating the water from 0°C to 100°C takes roughly 410 kJ/kg. So that's around 3000 kJ/kg. There's also a few hundred more kJ from heating forms of ice up to 0°C, with some latent heats of conversion between ice forms as well. You can try to look those up. The key thing to remember is that C_p is not constant, it changes as T changes, so you can't just use a fixed value. Then there's some more because you say the steam is "superheated", but you don't say how hot. You get that term from the same integral.

Mike W.

(published on 06/19/2014)