District Heating Pipelines Research Articles

Finite-difference technique for optimizing the locations of district-heating pipelines

District heating is not a new technology. A major constraint in introducing district heating is the high investment cost for the distribution network. Thus, attempts should be made to obtain maximum financial returns if such an investment is to be made. So a mathematical model, using an adaptive and hierarchical finite-difference technique, has been developed for predicting the steady-state heat losses to the environment from the hotter (supply) pipe when vertically placed above the warmer (return) one in the trench. This computer model has been validated by comparing the predictions from it with pertinent experimental observations. The model has then been used to predict the dimensions of the optimal (i.e. maximum energy-thrift) configuration of the district-heating pipes in trenches filled with gravel.

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.

Steady State Heat Losses from Horizontal Pipes in an Air-Filled Rectangular Concrete Duct

Factors influencing the steady state heat loss behaviours of horizontal ‘supply’ and ‘return’ hot-water pipes, within an atmospheric pressure air-filled, relatively cold, horizontal rectangular trench, are considered. An experimental investigation concerning the effects of the displacement ratios for the two pipes revealed the optimal configuration, that is one which achieves a minimum steady state rate of heat loss from the supply pipe. For one set of temperatures for the pipes and trench walls, the optimal configuration of the supply and return pipes occurred at displacement ratios of +0.70 and −0.05, that is with the supply pipe in the upper region of the cavity and the return pipe vertically below it, the pipes being equidistant from the vertical walls of the trench. This configuration is of significance with respect to achieving maximum energy thrift for district heating pipelines, because it differs radically from the ‘side-by-side’ arrangement of pipes conventionally adopted in district heating practice.

Suggested design improvements concerning district-heating pipeline configurations

Factors influencing the heat loss behaviours of horizontal ‘supply’ and ‘return’ hot-water pipes, within a rectangular trench, are considered (see Fig. 1). The use of such atmospheric-pressure air channels for district-heating systems is prevalent in the UK in order to facilitate drainage and evaporation. By optimising the location of the two pipes within the rectangular-sectioned trench, it is possible to achieve considerable reductions in the steady-state rate of heat loss from the supply pipe compared with conventional practice. Recommendations for future designs are presented.

Optimal location of a single horizontal pipeline in a rectangular, horizontal air-filled enclosure to achieve maximum thermal insulation

An experimental investigation into the effect of the displacement ratio of a heated horizontal pipeline within an atmospheric pressure air-filled, relatively cold, horizontal rectangular cavity was made. The optimal position of the pipeline to achieve maximum energy thrift occurred at a displacement ratio approximately equal to 0·7, i.e. in the upper region of the cavity. This result is of significance with respect to the reduction of distribution heat losses from underground district-heating pipelines.