Long-Term Thermal Performance of Polyurethane-Insulated District Heating Pipes
Doctoral thesis, 2001
The distribution of hot water in district heating pipes gives rise to energy losses. In order to minimise heat losses and maintain a high water temperature most district heating pipes produced today are insulated with rigid polyurethane foam.
The polyurethane foam has a very low thermal conductivity, which is due to the porous structure and the low conducting gases trapped in the cells of the foam. Over a considerable part of a pipe's lifetime the gas content of the foam alters. This is due to diffusion. The insulating gases diffuse out of the foam simultaneously as the air components, which are more conductive, diffuse into the cells of the foam, having consequences for thermal conductivity. As the cells of the foam eventually become air-filled, the thermal conductivity of the district heating pipe increases by about 30%. The level of thermal conductivity and its rate of degradation can be interpreted as a cost or gain, economically and environmentally, when examining alternative pipe solutions.
The overall concern of this work has been to enable the prediction of long-term thermal performance of both new and existing pipes. Methods for determining current thermal conductivity of a pipe already exist, whereas conditions for predicting the rate of change are lacking. This thesis focuses on determining of the gas transfer process in the insulating and casing materials, a factor which is considered decisive for the issue.
A methodology, which is applicable to both new and existing pipes of various dimensions, is proposed. It includes experimental determination of gas permeability in polyurethane foam, and gas permeability in the high-density polyethylene casing, which surrounds the foam. Since it takes years for the diffusion processes to take place, one challenge has been to obtain results of reasonable accuracy within a reasonable amount of time.
Polyurethane foam for insulating purposes formerly contained CFC-11 or similar chlorofluorocarbons. Due to their ozone depleting potential, CFCs have now been abandoned by a majority of the polyurethane producing countries and less ozone depleting alternatives are in use. In district heating pipes produced in Europe, cyclopentane has become the dominating alternative. Experimental results presented in this thesis concentrate on the relevant cell gases for cyclopentane-blown foam: nitrogen, oxygen, cyclopentane and carbon dioxide (the latter a co-blowing agent during foam production). The results of the thesis emphasise the importance of understanding the gas transport processes of each of the cell gases in order to accurately determine long-term thermal performance.