Analytical modeling of simultaneous phase transitions in a low-temperature evaporator for a pilot-scale Organic Rankine Cycle using R134a: A comparative study with water coolant

This study focuses on comparing the heat and water recovery performance of two coolants, R134a refrigerant and water, in a low-temperature evaporator (LT-E) designed for a pilot-scale Organic Rankine Cycle (ORC). The primary objectives were to develop a one-dimensional analytical model capable of predicting simultaneous phase transitions—internal flow boiling of R134a and condensation of water vapor from flue gas on the outer tube wall—and to compare the heat and mass transfer performance of R134a with that of water. Baseline modeling conditions included a flue gas temperature of 57.3 °C, coolant inlet temperature of 16 °C, and a coolant mass velocity of 126.4 kg/sm², with inlet pressures of 630 kPa for R134a and 100 kPa for water. The model’s predictions showed average discrepancies of 10 % for water recovery efficiency and 3 % for flue gas exit temperature when compared to experimental data. Case studies revealed that R134a outperformed water in heat flux by 16 % to 67 %, and water recovery efficiency was 15 % to 68 % higher with R134a. Increased heat exchanger surface area improved recovery efficiency for both coolants, eventually reaching an asymptotic limit.