In the childhood of district heating in Sweden, distribution pipes were insulated with mineral wool and laid in a common concrete square box duct. This first generation of distribution pipes was expensive to build, but was reliable, if the ducts were well ventilated and well drained. In order to reduce the cost of distribution , the second generation of distribution pipes was introduced during the 60's: Pipes in two separate asbestos cement casings. This type of piping was not so expensive, but more unreliable! In order to lower the distribution cost further and to increase the reliability, the third generation was introduced during the 70's: Prefabricated pipes with polyethylene casing and insulated with polyurethane foam, (Andersson 1980). However, there have been some problems with the reliability of the joints bringing together the prefabricated pipes, (Ljungqvist 1988). Nowadays, more than 90 % of all pipes buried in the ground during a year are pipes of this third generation, while this type of distribution pipes accounts for 65 % of all pipes being in operation in the Swedish district heating systems.
The Swedish District Heating Association has gathered statistics about pipe failures in Swedish district heating distribution networks since 1968, (Thunström 1983).
The specific heat supply is defined as the quotient between the total heat generation and the total route length. The parameter is a measure of the overall heat density of all customers connected to district heating. Figure 7 reveals that the specific heat supply was more than 10 MWh/m in the beginning of the 70's. Then it has fallen to a level of 5 to 6 MWh/m in the 80's. This implies that district heating first was built up in areas with high heat densities, and later, areas with lower heat densities were connected to district heating. In order to maintain a competitive price of district heat, the overall specific heat supply must be kept on this level, since lower specific heat supply increases both the specific distribution cost and the specific distribution heat losses (Werner 1982 and Werner 1985).
In Swedish district heating networks, the specific distribution heat losses vary between 6 and 15 % due to prevailing conditions like degree of insulation, sizes of distribution pipes, temperature level in the pipes, and the actual specific heat supply.