Regenerative Power Plants

El Moriana

Hi, this question is posed merely out of curiosity:

Background
My thermodynamics textbook tells me that in regenerative power plants (i.e. ones making use of feedwater heaters), the steam extracted during expansion in order to feed the feedwater heater(s) results in less work being developed in the subsequent stages of the turbine. It then tells me that in practice, the systems are such that the advantage of the water heater ends up being greater than the lost turbine work.

Question
This leads me to assume that there is a range in size/pressure/temperature in which regen power plants are viable. What is this range? (this question could also be answered by telling me situations in which regen plants are not viable)