Heat transfer measurement in a three-phase spray column direct contact heat exchanger for utilisation in energy recovery from low-grade sources

Abstract

Energy recovery from low-grade energy resources requires an efficient thermal conversion system to be economically viable. The use of a liquid-liquid-vapour direct contact heat exchanger in such processes could be suitable due to their high thermal efficiency and low cost in comparison to a surface type heat exchanger.In this paper, the local volumetric heat transfer coefficient (Uv) and the active height (Hv) of a spray column three-phase direct contact heat exchanger (evaporator) have been investigated experimentally. The heat exchanger comprised a cylindrical Perspex tube of 100cm height and 10cm diameter. Liquid pentane at its saturation temperature and warm water at 45°C were used as the dispersed phase and the continuous phase respectively. Three different dispersed phase flow rates (10, 15 and 20 L/h) and four different continuous phase flow rates (10, 20, 30 and 40 L/h) were used throughout the experiments. In addition, three different sparger configurations (7, 19 and 36 nozzles) with two different nozzle diameters (1 and 1.25mm) were tested. The results showed that the local volumetric heat transfer coefficient (Uv) along the column decreases with height. An increase in both the continuous and dispersed phase flow rates had a positive effect on Uv, while an increase in the number of nozzles in the sparger caused Uv to decrease. The active height was significantly affected by the dispersed and continuous phase flow rates, the sparger configuration and the temperature driving force in terms of the Jacob number.