How does pre-heating feedwater in a steam power plant improve overall efficiency?
I get that warmer water going into the boiler means less heat is needed to make it boil. But that pre-heating comes at a cost in your turbine, right? Here's what I don't understand:
I'm looking at this diagram. You're extracting hot steam from various stages in your turbines and sending it to feed water heaters to pre-heat your feed water. Heat is transferred from this extracted steam into the feed water. But aren't you making your turbine less efficient in the process? Wouldn't the efficiency gained in the boiler by pre-heating the feed water be cancelled out by efficiency lost in the turbines?
I'm guessing the answer is that the efficiency gained by pre-heating vs efficiency lost by turbine steam extraction is not equal, but can someone point me to the phenomenon that explains why?
Thank you!