Calculating amount of superheat produced

[ThunderChemE]

Currently reading through my textbook and came across a problem asking to find the amount of superheat when saturated steam is expanded across a valve from 500 psig to 15 psig. Correct me if I'm wrong, but when saturated steam is expanded such as this, isn't the enthalpy constant? I used the steam tables and found that decreasing the pressure of the saturated steam from 500 psig (giving a temperature of 470 F and an enthalpy of 1204.9 Btu/lb) to 15 psig would just produce superheated steam at 15 psig and about 331.7 degrees Fahrenheit. I guess the question is confusing me a bit because until now I believed that the "amount of superheat" had to do with a change in enthalpy when going from saturated to superheated conditions. Any help would be greatly appreciated.

 

[jari001]

Yes, the expansion is treated as isenthalpic. The two states to compare are the saturated 15 psig steam to the superheated 15 psig steam, not the superheated 15 psig steam to the saturated 500 psig steam.