What factors determine the optimal extraction fractions for feedwater heaters in a regenerative Rankine cycle?

The optimal extraction fractions for feedwater heaters in a regenerative Rankine cycle are determined by a complex interplay of factors aimed at maximizing cycle efficiency. Several factors influence this optimization, including the number of feedwater heaters, the steam conditions (pressure and temperature) at each extraction point, the cost of the heaters, and the overall plant operating conditions. Ideally, the temperature rise of the feedwater across each heater should be approximately equal. This distributes the heat addition more evenly, improving cycle efficiency. The extraction pressures are chosen to match the saturation temperatures required for effective heat transfer in the feedwater heaters. Higher-pressure extraction improves thermal efficiency. However, excessively high extraction pressures reduce steam flow through the lower stages of the turbine, decreasing power output. The cost of each feedwater heater influences the number of heaters used. More heaters increase cycle efficiency but also increase capital costs. An economic optimization balances the efficiency gains with the capital costs. Finally, plant operating conditions, such as load and ambient temperature, can affect optimal extraction fractions. Plant performance is often slightly adjusted based on load and conditions.