District Heat Supply System

a system of pipes for transporting and distributing a heat-transport medium, such as hot water or steam, in a district heat supply system. A distinction is made between mains and distribution systems; customers are connected to distribution systems through lateral mains. Heat supply systems may be either underground or aboveground. In cities and towns it is most common to lay pipes underground in trenches and sewers together with other utility lines or to lay them directly in the ground without trenches. Overhead systems may use trestles or special supports and are usually installed on the grounds of industrial enterprises and outside city limits. Steel pipes with diameters between 50 mm (for delivery to individual buildings) and 1,400 mm (for supply mains) are ordinarily used to construct the system.

The temperature of the heat-transport medium in the system varies over a wide range. Devices are installed to compensate for elongation of pipes caused by temperature changes. Flexible, U-shaped compensators are used for small-diameter pipes (up to 300 mm), and axial (gasket-type and lenticular) compensators are used for large-diameter pipes. Heat losses in the system’s pipes are reduced by thermal insulation. Mineral wool is usually used as insulation in trenches and overhead ducting. Where pipes are laid directly in the ground, insulating materials, such as foam concrete and asphalt perlite, are applied to the pipes at the factory; loose materials, such as insulation asphalt, may also be put in the trench during installation of the system. Insulation is also used to protect the outside surfaces of the pipes against corrosion; a layer of waterproof material is applied to the thermal insulation coating for this purpose. Special coatings of Izol (insulating mastic), vitreous enamels, epoxides, and similar substances may also be applied directly to the surface of pipes. The water supplied is treated to protect the inner surfaces of pipes against corrosion and to prevent the formation of deposits.

The mains of a district heat supply system may be arranged in a radial layout with dead ends or as a circulation loop. Provisions are made to interconnect individual mains systems and to install connecting lines between lateral mains in order to prevent interruptions in heat supply. When the main nets are very long, pumping substations are installed in the systems. Underground chambers are installed along the supply system route and at places where lateral mains connect; the chambers house shut-off and regulating devices, gasket compensators, and similar equipment.