District Heating Diffusion Barrier Theory and Reality
Heat transport through insulation in a district heating pipe can be split into:
- Conduction in the cell gasses – 60%
- Conduction in the cell walls – 20%
- Radiation through the cells – 20%
The composition of the gas is changed and deteriorates due to diffusion and, consequently, the heat losses.
Theoretically, the heat losses increase substantially (~25%) during the lifetime of the pipe. It is therefore natural to look towards using a diffusion barrier to prevent the deterioration of the insulation capacity of the pipe.
The standard EN 253 §4.5.7 uses an accelerated test method to define the expected future insulation value after 30 years in operation.
- This test has been performed on Powerpipe’s product DN50/140 2013 by the Swedish National Testing Institute. A deterioration of only 1.3 mW/mK was measured. An increase in losses of only 5%.
- A comparative test of DN50/140 has been performed by the Danish Institute DTI where pipes with and without the barrier were tested. As shown in the table to the right, it is not possible to see any advantages for pipes with a barrier.
- Pipes (CO2-blown) from the start of Powerpipe were installed in a test facility in Gothenburg. Chalmers University of Technology documented the pipes at installation and has now, 28 years after installation, again measured the gas content and the thermal conductivity. The results showed a deterioration of 11% instead of the theoretical ~25% expected.
Powerpipe has found that the increased costs and technical risks cannot motivate using a diffusion barrier. So far, it has not been possible to practically document any of the theoretical benefits.