Recommended location of a hot horizontal pipe in a relatively cool rectangular trench filled with thermally-insulating spheres

The steady-state rate of heat loss from a horizontal hot pipe, enclosed in a relatively cold rectangular trench which is filled with a single size of air-filled plastic spheres, was measured. The procedure was repeated successively with each of two other sizes of spheres. This system was chosen in order to simulate the heat-transfer behaviour of a thermally-insulated district-heating pipeline in, for instance, a gravel-filled trench. The optimal configuration (for the system employing spheres having a diameter of 10 mm) had the pipe (i.e. from which the least rate of thermal losses occurred) located at a displacement ratio of 0·01 (i.e. with the pipe in the upper half of the trench). By adopting such a configuration, an improvement of approximately 4% in the overall thermal resistance of the system was achieved, compared with that having the horizontal pipe symmetrically placed in the trench with these same spheres present. Such an improvement can be gained without incurring extra constructional costs, yet the financial savings over the lifetime of the district-heating system should be appreciable.