Polymers for Solar Domestic Hot Water: Long-Term Performance of PB and Nylon 6,6 Tubing in Hot Water

Abstract

Polymers offer a lightweight, low cost option for solar hot water system components. Key to the success of polymer heat exchanger components will be the long term mechanical performance of the polymer. This is particularly true for heat exchangers in which one of the fluids is pressurized hot water. For domestic hot water systems, polymer components must not fail after many years at a constant pressure (stress levels selected to correspond to 0.55 MPa in a tube) when immersed in 82°C potable water. In this paper, the long term performance of two potential heat exchanger materials, polybutylene and nylon 6,6, is presented. Two failure mechanisms are considered: failure caused by material rupture (as indicated by the hydrostatic burst strength) and failure caused by excessive deformation (as indicated by the creep modulus). Hydrostatic burst strength and creep modulus data are presented for each material. Master curves for the creep compliance as a function of time are derived from experimental data. These master curves provide a mechanism for predicting creep modulus as a function of time. A case study is presented in which tubing geometry is selected given the hydrostatic burst strength and creep compliance data. This approach can be used to evaluate properties of candidate polymers and to design polymer components for solar hot water applications.