A double-pinch criterion for regenerative Rankine cycles

A double-pinch criterion for closed feedwater heaters (FWH) of regenerative Rankine cycles is presented. The FWHs are modeled as counter-current heat exchangers. Thus, two potential pinch positions in the FWH exist: (i) at the exit of the bleed (drain) and (ii) at the onset of condensation. For a given heat duty in the FWH, feed inlet temperature and flowrate, the extraction flowrate and pressure should be chosen to achieve the same minimal approach temperature at the two potential pinch points. An analytical proof is given for a fixed pinch value for the case that the drain enters the condenser, based on weak assumptions. Additionally, the criterion is numerically demonstrated for fixed pinch value and for fixed heat exchanger area using the most common configurations: drain to condenser, drain to deaerator, and drain cascaded to next FWH. A similar criterion is developed for the case that the drain is pumped (upwards or downwards) and mixed with the feedwater. The double pinch criterion simplifies the optimization procedure and results in significant efficiency increase for fixed heat exchanger area. For numerical reasons it is advisable to use the pressure as the optimization variable and calculate the heat duty and mass flowrate.