3.3.4 Effect of final feed temperatures
In the previous section, the concept of regenerative feedheating was introduced as a means of improving cycle efficiency. The thermodynamic optimum final feed temperature for Rankine cycle plant is, by definition, that which gives the highest cycle efficiency. However, raising the final feed temperature would reduce the required size of the boiler economiser surface.
In order to avoid an increase in the stack temperature, the air heater would have to be increased in size, and the cost of this would exceed any saving on the economiser. Consequently the economic optimum feed temperature is somewhat lower than the thermodynamic optimum.
Raising the final feed temperature also has implications for the design of the HP turbine. Past practice has been to use bled-steam from the HP turbine exhaust. The saturation temperature of the bled-steam controls the maximum feed temperature obtainable. To provide steam at a higher pressure with a higher saturation temperature, requires steam to be bled from an intermediate point on the HP cylinder. This is an additional complication and cost to the turbine plant. Despite the increases in capital cost, the benefit from improved cycle efficiency is such that the additional heater is worth including.
The design of the Advanced Gas-cooled Reactor (AGR) has resulted in limitations on the final feed temperature. The feed train is curtailed in order to limit the final feed temperature to about 150°C. This limitation is imposed because of the allowable temperatures in the gas cooling circuit.