Question

In using departure functions we use a graph to get delta h at a reduced temperature and pressure and then we say that value is equal to the delta h at regular T and P but why are those delta h the same?

Answer 1

T_r = T/T_c and P_r = P/P_c

and all the enthalpy and entropy departure functions are defined and computed based on these reduced values of temperature and pressure. So there is a direct correlation between T_r & P_r and T & P.

So basically the equations demand that the property be reduced and each and every value of T & P has its own value of T_r and P_r. So all the values are equally reduced by the critical property and thus no overlap occurs in the values of departure function.