Design sensitivity analysis of a direct evaporator for low-temperature waste heat recovery ORCs using various flow boiling heat transfer correlations.

The study evaluates 14 flow boiling heat transfer correlations in an annular-finned horizontal round tube direct evaporator design problem for a low-temperature waste heat recovery Organic Rankine cycle (ORC) case. The direct evaporation is realized through an industrial flue gas having a volumetric flow of 10000 Nm³/h, with inlet and outlet temperatures of 232°C, which is representative for typical waste heat recovery operations for annealing furnaces, drying, baking, exhaust gas internal combustion engine. An iterative evaporator design is performed, where each of the considered flow boiling correlations are separately used for finding the in-tube convective heat transfer coefficients. The error margins of the tube length, overall heat transfer coefficient, transferred heat, cost, fan power and pressure drops are assessed. The considered working fluid is R245fa and the deduced optimal evaporation temperature (OET) is 120°C. The corresponding total transferred heat in the evaporator is approximately 300 kW in all calculations.