Influence of oxidation on radiative heat transfer in polyurethane insulation used for district heating pipes

Thermal conductivity of cellular rigid polyurethane foam (PUR) increases by time which leads to higher heat energy losses in district heating pipe networks. The main reason for increased thermal conductivity is diffusion of low conductive gases out of the PUR and diffusion of surrounding air into the PUR. However, oxidation of the PUR occurs during the service-life of the PUR and is accelerated by the higher temperatures close the fluid pipe. The effect that oxidation has on the thermal conductivity is not yet fully understood and existing models for prediction of long-term thermal performance of PUR insulation in district heating applications does not take oxidation processes into account. It is possible that the radiative heat transfer is affected by the oxidation and changes over the service-life of the PUR. In order to investigate the influence of oxidation on thermal conductivity, the extinction coefficient was therefore calculated for samples subjected to different levels of ageing. The input data for the calculations were measured by FTIR. The extinction coefficients were then used to calculate the overall thermal conductivity of the PUR with typical gas compositions. Results indicated that the extinction coefficient was 22 % higher in the samples exposed to lower temperatures. However, the effect on the overall thermal conductivity of the same samples was an increase of about 1.8 %. Since the comparison was made between two samples subjected to different levels of ageing, the increase in total thermal conductivity should be interpreted as a minimum if considering the total service lifetime of the PUR insulation.