A Theoretical and Experimental Investigation of a Shell-and-Coil Heat Exchanger for a Solar Domestic Hot Water System

Abstract

Solar energy is an important form of renewable energy that can be used as an alternative to fossil fuels. It can be used to produce electricity or to provide heat. One particular application is using solar energy for a domestic hot water system.

The purpose of this research is to improve the thermal performance of a solar domestic hot water (SDHW) system. Experimental research was conducted to study the thermal performance of a shell-and-3coil heat exchanger and a shell-and-4coil heat exchanger using either water or glycol as working fluids on the tube side.

An experimental set-up simulating a SDHW system was designed and constructed. The set-up contained a 270 L storage tank, a shell-and-three coil heat exchanger or a shell-and-four coil heat exchanger, and electrical heaters to simulate the solar collector. At the inlets and outlets of the storage tank and the heat exchanger the temperatures, pressures, and flow rates were measured to determine the thermal performance. The results from the experiment tests were analyzed in terms of the overall heat transfer coefficient product (UA) and the pressure drop (?P) between the inlet and outlet of the heat exchanger.

The UA value of the shell-and-4coil heat exchanger was higher than the UA value of the shell-and-3coil heat exchanger. For example, at a heat transfer rate of 2000 W for water, the UA values were 240 W/K and 270 W/K for the shell-and-3coil heat exchanger and the shell-and-4coil heat exchanger, respectively. With respect to glycol, at a heat transfer rate of 2000 W the UA values were 197 W/K and 215 W/K for shell-and-3coil, and shell-and-4coil heat exchanger, respectively. The degradation of the thermal performance of the shell-and-3coil was offset by benefits, such as reduction in mass, volume, labor cost and the final cost. A reasonable agreement between theoretical and experimental results in terms of the UA value was observed.

The thermal performance of each coil in both heat exchangers was below that predicted by the relevant heat transfer correlations. A performance factor was calculated for each coil. For both glycol and water, and both heat exchangers, the performance factors for the inner most and outer most coils were 0.70 and 0.53, respectively. However, there is a slight difference in the performance factors of coils between the inner most and the outer most coils for the 3-coil and 4-coil heat exchangers. For these coils the performance factors varied from 0.55 to 0.67.