The polyethylene casing as diffusion barrier for polyurethane insulated district heating pipes

Most district heating pipes are insulated with polyurethane foam in order t o minimise heat losses. A high-density polyethylene (HDPE) casing protects the insulated pipe, and its permeability properties for the polyurethane ce ll gases, including air, play an important role for the long-term insulatin g 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 m ass transfer through a sample, and II) by measuring the sorption and desorp tion of the gas in a sample. The experimental procedures are described. For carbon dioxide a permeability coefficient of about 9.10(-6) mole.m(-1).s(- 1).Pa-1 was found by both methods, For oxygen and nitrogen only method I wa s used and the permeability coefficients found were 1.9 and 0.6.10.16 mole. m(-1).s(-1).Pa-1, respectively.