Evaluation of the entropy integral for steam

Specific enthalpy and specific entropy are measured relative to the properties of saturated water at its triple point both specific enthalpy and specific entropy are taken by convention to be zero for saturated, liquid water at a temperature of 0.0l°C. 

Although the steam tables list steam properties against pressure and temperature, the fact that specific enthalpy and specific entropy are themselves thermodynamic variables means that it is possible to construe the steam tables as providing specific enthalpy as a tabular function of pressure and specific entropy. Alternatively, we may regard the tables as providing specific entropy as a tabular function of pressure and specific enthalpy. These two statements may be summarized mathematically by the two equations  

We may apply this interpretation to find the specific enthalpy following an isentropic expansion, given the initial specific enthalpy, h, the initial pressure, pa, and the final pressure, pb- The specific entropy in state A is found by first moving to the column where the pressure is pa, finding the row at which the specific enthalpy is Ha, and then reading off the specific entropy, sa, on the same row  

To find the specific enthalpy in state B , we select the column for the final pressure, ps, find the row where the specific entropy is the same as at the beginning of the expansion, sa, and read off the specific enthalpy along this row. This will be the desired value following 

We treat the real case where the expansion is fric-tionally resisted with efficiency, by first carrying out the tabular exercise for an isentropic expansion as above and then proceeding as follows. Since the real enthalpy drop will be the isentropic enthalpy drop multiplied by rj, the actual specific enthalpy at the end of the expansion will be  

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