An investigation of shell-helical coiled tube heat exchanger used for solar water heating system

Abstract

An experimental and theoretical study has been performed of thermal and hydrodynamic characteristics of a new type shell-helical coiled tube heat exchanger used as a storage tank in a closed loop solar water heater system for no water withdrawn from storage tank. The parameters studied are collectors heat gain, heat rejected from coils to shell side of the storage tank, average temperature of storage tank, friction factor, pressure drop, heat exchanger effectiveness, and collector efficiency for circulating mass flow rates of (1.8, 3, 6, 9 l/min), with no water consumption during a clear days in Baghdad. A FORTRAN 95 computer program was developed to evaluate theoretically the pressure drop across the helical coil and to process the experimental data. The results show that heat transfer inside helical coiled tubes is increased with increasing the circulation flow rates and coil diameter ratio. A transition from laminar to turbulent flow has been noticed at (6 and 9 l/min) circulation flow rates. For circulation flow rate of (6 l/min) the transition from laminar to turbulent flow is faster indicated for the outer coil than that for central coil, while the flow is laminar for inner coil. For (9 l/min) circulation flow rate the transition from laminar to turbulent has been noticed for all coils. Collector efficiency is increased with increasing circulation flow rates. The pressure drop decreases with the increase of solar radiation. Friction factor decreases with increasing circulation flow rate. The shell-triple concentric helical coil heat exchanger is found more effective than shell-single coil heat exchanger when the experimental results are compared with previous related work.

Keywords: Solar hot water, Storage tank, Heat exchanger, Helical coil tube, Pressure drop, Effectiveness