Development, testing and design optimisation of a water and R134a based thermosyphon heat exchanger for air-water heat recovery systems

Abstract

Heat pipes find their increasing uses in heat recovery and energy saving in thermal systems. Our work involves the designing, building, and testing of a heat pipe heat exchanger made of 28 copper thermosyphons. We investigated the use of distilled water and R134a as the thermosyphons’ working fluid with optimisation. The thermosyphons were arranged into four rows by seven columns with a hot air flow at the evaporator and a cooling water flow at the condenser. The performance was tested in three working fluid cases: distilled water, R134a, and a hybrid. The hot air flowrate was varied and at temperatures of 80 °C, 110 °C, and 140 °C to investigate the performance. The results show that increasing the air flow temperature increased the effectiveness. Optimisation was examined using the ε-NTU analysis and revealed that R134a offered the highest effectiveness with a relatively small NTU. Tested using three values of Cr, highest effectiveness of 30%, 51% and 40% for distilled water, R134a, and the hybrid respectively, was achieved across all fluid cases at Cr = 0.15. A Reynolds analysis performed to compare the effect of relative flows over the evaporator and condenser showed that increasing Reh/Rec, increased the heat transfer at the condenser and the overall effectiveness. At Reh/Rec = 2500, heat transfer was 500 W, 2000 W and 800 W for distilled water, R134a, and the hybrid respectively. Our work could potentially be useful in the designing and optimising of a thermosyphon heat exchanger for saving energy in air-water heat handling applications.