For the typical(ultra)supercritical units with double-reheat cycle

on the basis of unchanged new steam parameters and the first reheat parameters

a double-reheat cycle is added to establish the double-reheat unit model.The power coefficient method and the isentropic expansion process line of the steam in turbine were applied to perform theoretical analysis.Taking five typical(ultra)supercritical units as the mother units

the average heat absorption temperature and thermal efficiency of the ideal cycle and doublereheat cycle were calculated.The results show that

for different units

the best ratio of the secondary reheat pressure to the first reheat pressure ranges from 19%to 23%

and that for the same unit is between15% and 30%.Within this pressure ratio selection range

the thermal efficiency of the ideal double reheat cycle is about 1% higher than that of the conventional reheat cycle.The research has certain reference significance for engineering application.