Most district heating pipes are insulated with polyurethane foam in order to minimise heat losses. A high-density polyethylene (HDPE) casing protects the insulated pipe, and its permeability properties for the polyurethane cell gases, including air, play an important role for the long-term insulating capacity. The permeability of the HDPE casing of a district heating pipe was studied. Two methods were used to determine the permeability: I) by measuring the mass transfer through a sample, and II) by measuring the sorption and desorption of the gas in a sample. The experimental procedures are described. For carbon dioxide a permeability coefficient of about 9·10-16 mole·m-1·s-1·Pa-1 was found by both methods. For oxygen and nitrogen only method I was used and the permeability coefficients found were 1.9 and 0.6·10-16 mole·m-1·s-1·Pa-1, respectively.

The mass transfer of carbon dioxide through the outer polyethylene casing of district heating pipes, at room temperature, was evaluated, using different test methods. The mass transfer either through polyethylene casings on polyurethane preinsulated district heating pipes or through polyethylene casings alone was mea sured. Permeability coefficients of different polyethylene casings were about 20 · 10-18 kg·m-1·s-1·Pa-1. Permeability coefficients for carbon dioxide in polyurethane foam is about 100 times lower, which means that the mass transfer resistance to car bon dioxide of the polyurethane foam in a district heating pipe is negligible in com parison with the polyethylene casing.

District heating pipes are usually polyurethane-(PUR)-insulated and the outside is protected by a polyethylene (HDPE) casing. The permeability of the cell gases (oxygen, nitrogen, carbon dioxide and cyclopentane) in the PUR foam and in the HDPE casing was determined for a temperature range of 20-60 C (PUR) and 5-40 C (HDPE).From the results, it was concluded that the casing acts as the main diffusion barrier for carbon dioxide, oxygen and nitrogen, whereas the main diffusion barrier for diffusion of cyclopentane is due to the PUR foam. This conclusion is valid for district heating pipes of small to medium dimensions.

This study focuses on the mass transfer properties of 1,1,1,3,3-pentafluorobutane (HFC-365mfc) in the insulating system used in most district heating pipes produced today, namely rigid polyurethane (PUR) foam with a protective layer of polyethylene (HDPE). The solubility, permeability and diffusion coefficients for HFC-365mfc in PUR foam and HDPE have been determined. The coefficients for HFC-365mfc in PUR foam are very similar to those of cyclopentane, currently the most common blowing agent in PUR foams used for district heating pipes in Europe. The polyethylene casing is a better diffusion barrier for HFC-365mfc than it is for cyclopentane. However, the main mass transfer resistance of HFC-365mfc in a district heating pipe is found in the PUR foam.

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