Thermal Insulation for a Novel Flexible District Heating Pipe System

Abstract

The utilization of industrial waste heat and cogeneration heat requires efficient systems for the distribution of hot water or other heating media. In Europe the “Fernheizkabel” (FHK) has been applied for this purpose since 1967. It is basically a flexible preinsulated pipe which consists, from inside out, of a welded and corrugated copper or stainless steel tube containing the heat-carrying medium, a thermal insulation, a welded and corrugated outer steel tube, and finally an extruded corrosion protection. The FHK line comes with an inner tube diameter from 25 to 150 mm (1 to 6 in.) and is shipped in lengths of 230 to 550 m (750 to 1800 ft) on cable reels. The efficiency of a district heating system depends mainly on the thermal losses of the distribution lines which are determined by the thickness of the insulation and its thermal conductivity. The thermal insulation between the inner and outer tube is the subject of this paper. For this application the insulation material is required (1) to keep the heat transfer between the inner and outer tube as low as possible, (2) to support the inner tube mechanically, (3) to absorb movements and pressures resulting from temperature changes and temperature differences, and (4) to permit bending of the pipe assembly. The insulation must provide a good mechanical bond between the inner and the outer tube. Two different materials were found to be suitable: polyurethane (PUR) and polyisocyanurante (PIR). For these materials in the foamed state, one additional condition arises: the foaming process must be compatible and coordinated with the application of the outer welded and corrugated steel envelope. The two foam materials were tested thoroughly. Performance at different temperatures makes possible extrapolation of expected service life. As a result of these investigations the PUR is used as standard insulation in the FHK line, for nominal operating temperatures of up to 130°C (266°F). The more expensive PIR insulation is recommended for applications with nominal operating temperatures of up to 180°C (356°F). FHK installations with PUR insulation have been in operation in Europe for several years. It is safe to expect a service life of about 30 years for the PUR-insulated FHK pipes. A test installation with a PIR foam-insulated pipe was set up in the United States and yielded good results.