The polytropic exponent for saturated steam

An isentropic expansion for superheated steam is well characterized by taking the exponent as y = 1.3, but the exponent changes markedly when the steam becomes saturated. The value of y appropriate in the saturated region will depend on the dryness fraction, a o, at the start of the expansion, where xq will depend on the local thermodynamic properties at stage inlet, ho. So, Vo, etc.  

Zeuner s equation for the saturated region gives the isentropic exponent as  

If the steam is superheated at the start of the expansion, then the value y = 1.3 is used, the reason being that condensation will not occur immediately the saturation temperature is reached. Instead, the steam stays wholly in dry vapour form, when it is known as supersaturated . This metastable state cannot be maintained indefinitely, however, and condensation will occur suddenly at some point after the steam has passed through the nozzle. 

---