Heat Supply

Heat Supply

 

(teplosnabzhenie), the provision of heat to residential, public, and industrial buildings and structures to meet customers’ residential (heating, ventilation, and hot-water supply) and industrial needs. A distinction is made between building and district heat supply. Building heat supply systems serve one or several buildings; district systems serve a residential or industrial area. In the USSR, district heat supply has proved most important; therefore, the Russian term teplosnabzhenie is ordinarily used in reference to district heat supply systems. The principal advantages of district heat supply over building heat supply are significant reductions in fuel expenditures and operating costs (for example, by automation and increased efficiency of boiler units), the possibility of using low-grade fuel, reduction of air pollution, and improvement of health conditions in populated areas.

A district heat supply system includes a source of heat, the heat supply system, and the heat-consuming installations, which are connected to the system through heat distribution points. With district heat supply, the sources of heat may be district heat and power plants, which combine the production of electricity and thermal energy; large boiler units, which produce only thermal energy; devices to recycle the thermal waste of industry; or installations to harness the heat of geothermal sources. In building heat supply, the sources of heat may be furnaces, hot-water boilers, or water heaters, including solar heaters.

In district heat supply, the heat-transport medium is usually water at temperatures up to 150°C or steam under pressures of 0.7–1.6 meganewtons per m2 (7–16 technical atmospheres). Water is usually used for municipal and domestic purposes, while steam is used for industrial purposes. The temperature and pressure in heat supply systems are determined by customer requirements and economic considerations. The cost of the higher temperatures and pressures of the transport medium becomes more justifiable the longer the distance of heat transport. The distances over which heat is transported in modern district heat supply systems reach several dozen kilometers. The consumption of standard fuel per unit of heat delivered is determined principally by the efficiency of the heat supply source. Current development of heat supply systems concentrates on increasing the capacity of the heat source and the unit capacities of installed equipment. The thermal capacity of modern district heat and power plants reaches 2–4 teracalories per hr, and that of regional boiler installations is 300–500 gigacalories per hr. In some heat supply systems, several sources of heat are used together to feed a common heat supply net; this technique increases reliability, flexibility, and economy.

Heat supply systems are classified according to how the heating units are connected as dependent or independent systems. In dependent systems, the heat-transport medium passes directly from the heat supply system into the customer’s heating units. In independent systems, it passes into an intermediate heat exchanger installed at the heat distribution point and heats a secondary heat-transport medium, which circulates in the customer’s heating unit; the customers’ units are thus hydraulically isolated from the heat supply system. Independent systems are used primarily in large cities to increase the reliability of heat supply; they are also used in cases where the pressure in the heat supply system exceeds the pressure ratings of customers’ heating units or the static pressure created by the customers’ units is unacceptable to the heat supply system (for example, in heating systems for highrise buildings).

A distinction is made between closed and open heat supply systems, depending on how hot-water supply units are connected. In closed systems, water for hot-water supply comes from a water line. This water has been heated to the required temperature (usually 60°C) by water from the heat supply system in heat exchangers installed at heat distribution points. In open systems, the water is fed directly from the heat supply system. Water losses caused by leaks in the system and normal supply are replenished by the delivery of additional water into the heat supply system. Water fed into the heat supply system is treated and deaerated to prevent corrosion and the formation of deposits on the inner surfaces of pipes. In open systems, the water must also meet requirements for drinking water. The choice of system is determined chiefly by the presence of sufficient water of drinking quality and the water’s corrosion and deposit-forming properties. Both types of system are common in the USSR.

Depending on the number of pipes used to carry the heattransport medium, heat supply systems are classified as single-pipe, double-pipe, or multipipe systems. Single-pipe systems are used in cases where the heat-transport medium is entirely used by the customer and does not return (for example, in steam systems without condensate return and in open water systems where all water from the source is distributed for hot-water supply to customers). In double-pipe systems, the heat-transport medium is fully or partially returned to the heat source, where it is replenished and reheated. Multipipe systems are installed where it is necessary to separate distinct types of heating loads, for example, in hot-water supply systems. This simplifies regulation of heat delivery, operating conditions, and methods of connecting customers to the heat supply systems. Double-pipe heat supply systems are the most common type in the USSR.

The delivery of heat in heat supply systems is regulated daily and seasonally both at the heat source and at the heat-consuming installations. Water heat supply systems usually have central temperature regulation of heat supply according to the primary type of heating load: heating only or a combination of heating and hot-water supply. Regulation consists in changing the temperature of the heat-transport medium delivered from the heat source to the heat supply system according to an established relationship between the required water temperature in the supply system and the temperature of the outside air. District temperature regulation is supplemented by local quantitative regulation at the heat distribution point; this is most common for hot-water supply and is usually done automatically. In steam heat supply systems, local quantitative regulation is the type usually used. The pressure of the steam at the heat supply source is held constant, and the consumption of steam is regulated by the customers.

REFERENCES

Gromov, N. K. Gorodskie teplofikatsionnye sistemy. Moscow, 1974.
Safonov, A. P. Avtomatizatsiia sistem tsentralizovannogo teplosnabzhennia. Moscow, 1974.
Sokolov, E. Ia. Teplofikatsiia i teplovye seti, 4th ed. Moscow, 1975.
Zinger, N. M. Gidravlicheskie i teplovye rezhimy teplofikatsionnykh sislem. Moscow, 1976.

N. M. ZINGER

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