Thermal analysis of polymer heat exchangers for solar water heating: A case study

The feasibility of reducing the cost of solar water heating systems by using polymer heat exchangers is illustrated by comparing thermal performance and cost of heat exchangers made of nylon, cross linked polyethylene (PEX), or copper. Both tube-in-shell heat exchangers and immersed tube banks are considered. For the thermal analysis, the tube geometry and the arrangement of tubes are fixed and the heat transfer surface areas required to provide 3000 and 6000 W are determined. Thermal performance is estimated using published heat transfer correlations. The nylon heat exchanger outperforms the PEX design, primarily because nylon is a stronger material. Consequently, the ratio of diameter to wall thickness required to withstand the operating pressure is greater and the conduction resistance across the polymer wall is less. The cost of nonoptimized nylon heat exchangers is about 80 percent of the cost of heat exchangers made of copper. Significant additional work is required to optimize the tube arrangement and geometry and to validate our initial estimates of thermal and economic performance.