Customer Substations
Customer Substations
The customer substations constitute the points of delivery. In Sweden, customer heating systems are separated from the district heating systems by the use of heat exchangers. This choice of indirect connection of customers has been made for both historical and practical reasons:
- In many Swedish towns, significant differences in ground level occur. For example, the difference in altitude between the highest and the lowest customer connected is 75 m at Borås and 127m in Göteborg. This gives differences of static pressure is 7,5 bar and 12,7 bar in the distribution networks. An integration of the customer systems and the district heating system would in these cases in crease the risk of pressure problems in the customer radiator circuit systems.
- In 1954, an accident in a direct connected (without the use of heat exchangers) area in Göteborg occurred. Nitrogen gas escaped from the pressure maintenance system and found its way to 2800 radiators. During a weekend, all these radiators had to be deaerated before heat supply could start up again. In the childhood of Swedish district heating , this accident guided Swedish district heating engineers towards the method of indirect connection.
- An indirect connection splits the responsibility between the customer system and the district heating system , and the heat exchanger will be an efficient and distinct divider of the responsibility.
Ordinary Swedish district heating systems are built as closed systems, which means that the water in the distribution pipes is not directly used for hot water supply. Hot tap water is prepared by the use of heat exchangers. However, some new small district heating systems are built as open systems, where hot tap water is used as heat carrier like in some Russian systems. The choice of open systems has given lower temperature levels and the possibility of using plastic service pipes in the distribution instead of steel pipes.
Customer substations are normally built as two and three stage heat exchanger systems. An ordinary substation has three heat exchangers, one for the radiator circuit system and two for the hot water supply (one preheater and one secondary heater). In the three stage arrangement, launched in 1957, the district heating water first reaches the secondary heater, then the radiator exchanger, and finally the preheater. In the two stage arrangement, the secondary heater and the radiator heater are connected in parallel. By these arrangements, the counter flow principle is employed in order to maximize the cooling of the district heating water.
In the childhood of Swedish district heating, voluminous shell-and-tube heat exchangers were used. Within a few years , new compact heat exchangers had been developed. A customer substation, which in 1952 occupied a floor area of 5 m2, occupied only 0,4 m2 in 1966.
Each customer substation has its own control units (often microprocessor-based) in order to maintain both the indoor temperature and the hot tap water temperature at constant levels without fluctuations.