Theory of Feed Heating

The purpose of feed heating is to increase plant efficiency. With no feed heating steam gives up three times more heat to the cooling water as it does in doing useful work in the turbine. Hence, if a proportion of the steam is bled off to a feed heater it can give energy ( its latent heat mostly) to the feed and so a higher proportion of the energy is reclaimed by the system as would have been gained by expanding it through the turbine to the condenser Feed heating can take two forms, one is contact heating which occurs in the Deaerator where the steam and feed water mix and and the water directly receives heat.

Theoretical cycle with all steam expanded through the turbine

Theoretical cycle where all of the steam used in feed heating with 100% efficiency

This could only be possible where the turbines where fitted with a water jacket through which the feed water flows.

Theoretical practical cycle where a portion of the steam is bled off in stages and used for feed heating

It should be noted that although thermal efficiency increases with the number of stages, it is governed by the law of diminishing returns and the improvement is reduces with successive stages. Hence the cost of the increased plant becomes a factor.
Bleeding off a portion of the steam gives the added advantage that the steam volume that has to be accommodated in the final stages of the turbine and condenser is reduced.