Plastic helical coil heat exchanger as an alternative for a domestic water storage tank

A reduction in weight and cost of a domestic hot water storage vessels is an interesting case for the industry, that can be reached by an alternative material of the helical coil heat exchanger inside them. The goal of the present study was to design a fully polymer solar boiler demonstrator and to explore its thermal performance in the low pressure and low temperature conditions. The metal coil of the conventional solar boiler for domestic usage was replaced by a plastic tube and the heat transfer behavior of helically coiled smooth plastic tube heat exchanger was investigated experimentally. The heat exchanger is placed in the middle of the tank in two parallel coils that fill almost whole height of the vessel in order to achieve maximal surface area. The water inside the tank was heated by circulating in closed loop with heater to achieve constant initial temperature across the whole volume of the vessel. When reached, the heating was stopped and a cold water of the tap temperature started to flow inside the polymeric tube. All the required parameters like inlet and outlet temperatures of tube-side and stratified temperatures in fifteen different points, flow rate of fluids and pressure drop were measured using appropriate instruments. The test runs were performed for different initial temperatures inside the tank ranging from 30-60°C from which the overall heat transfer coefficient and thermal resistances were calculated. The validity of obtained results was compared with the numerical simulation and the experimental results on the initial metallic tube. INTRODUCTION Use of solar domestic hot water systems is a cost-effective and a sustainable way to provide hot water for the houses, which also can be used in any climate. The majority of solar water systems require a well-insulated storage tank. The water in a hot water tank can be heated either directly, by means of electric heaters or heated from another energy source as solar collectors. Helically coiled tubes are typically placed into the tank to discharge/charge the water inside the tank for several reasons: flexibility and therefore easy accommodating to required length, easy construction, low cost and enhanced inner convection heat transfer compared to straight pipes [1]. The heat transfer through the coil depends on the forced convection through the tube, natural convection around the tube and conduction through the tube wall. The use of polymer heat exchangers were introduced in 1988 by [2] and studied since then by several researchers as a replacement of a metallic heat exchanger due to their advantages in low cost, light weight and corrosive and fouling resistance [3, 4]. They can be used in wide range of applications such as for heat recovery application, evaporative, cooling and refrigeration, solar water heating, electrical fluid heating, electric device cooling, water desalination and distillation [5]. The thermal performance and the cost analysis of for tube-inshell heat exchangers and immersed tube banks, both made of polymers, for the purpose of solar water heating were studied by [6]. The high temperature nylon (HTN) and cross linked polyethylene (PEX) were chosen as materials and compared with copper. By determining the surface areas required to provide heat transfers of 3kW and 6kW with fixed geometry and arrangement of plastic tubes was found, that polymer heat exchangers can provide thermal output equivalent to conventional copper heat exchangers, with 80% of the cost of a copper tube-in-shell heat exchanger. The dimensions of the tubes were 9.53 mm of outer diameter (OD) and 1.78 mm of wall thickness for PEX; and 3.81 mm of OD and 0.2 mm of wall thickness for HTN. In another study, stability over the life cycle of the plastic heat exchanger at constant pressure was studied [7]: The solar collector system had a polymer tube bundle heat exchanger immersed in a tilted enclosure, filled with fluid, and heated by solar radiation. Via a study of natural convection involving a tube bundle, it was found that the nylon tubes were able to withstand a pressure of 0.55 MPa and temperature of 82°C for at least 10 years. In a study of polymer-based water storage systems, an immersed heat exchanger to discharge (and/or charge) the stored energy of unpressurized polymer water storage tanks [8]. They stated that the use of polymer heat exchangers for solar hot water storage systems posed thermal and material challenges but were also promising for lower cost systems. The possibility of using polymeric helical coil heat exchanger as an alternative to metallic helical coil was investigated by [9], where they provided a model to calculate plastic coil dimensions with a thermal output equivalent to that of a conventional solar boiler. More experimental results on an ad-hoc demonstrator with a 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics