MODERN DECISIONS
The best option for installation of new networks is to use pre-insulated steel pipes. It guarantees the smallest losses in the network. So called "fixed underground installation" technology is the most economically viable and expedient from the modern point of view. Concrete channels, expansion bends and support points are not used in this technology. Тhis technology is used in Finland for more than 20 years. Average lifetime of these networks without major repairs comes up to 30 years, but if these pipes are installed correctly in accordance with technology their lifetime can be even 50 years.
Usually temperature difference in the heating network is 115/70°C in winter and 70/40°C in summer. Network consist of two parallel pre-insulated pipes, so called supply and return pipes. Temperature amplitude is between 70-115°C, depending on outdoor temperature. Consumer use heat from the network through individual heating points for heat and DHW. Temperature of return water to the boiler house varies from 40 to 70°C.
Pre-insulated heating pipe is a steel pipe in the box of foamed polyurethane and covered with plastic. One or two pipes could be placed in that kind of box.
Pipe tension, caused by heat carrier temperature difference is compensated by steel pipe, and friction between pipe plastic cover and soil.
The following parameters are normal for this type of heating network: max. working pressure 1,6 MPa (16 bar) and max. temperature 120 °C (permanent) and 140 °C (momentary).
In normal conditions technical lifetime of pipe elements and pipes is min. 50 years
Assembling of pipe elements is done by so called "fixed underground installation".
This method has two main installation technologies:
1. With preliminary heating of the pipeline;
2. Without preliminary heating of the pipeline.
Both installation technologies have the following general characteristics:
· No special compensation system for thermal expansion of the pipeline;
· Minor axial fluctuation is compensated in corner elements of the pipeline;
· Most part of the pipeline is static;
· Minor differences in depth of installed pipelines don't affect system efficiency;
· Anchoring points are needed only in exception case.
Long-term experience of western countries has shown that annual costs for maintenance and running of heating networks of this type are about 1.5% from investment volume for installation of whole network. If lifetime of the network is about 30 years, so annual investment to network renovation is only 1/30 of all investments to the network or 3.3%, if use flat rate depreciation method. If costs for maintenance and running are getting higher, it is necessary to check condition of the pipes and may be to replace depreciated parts of the pipeline.
Cross-cut of the central heating system is shown below. Minimal depth of pipe installation is 600 mm.
Dimensions of pre-insulated elements of the network are shown on the following layout.
Pictures of network installation are shown below.
Statistic data on heat losses in the heating networks in Finland are shown below. Real heat losses are about 8-9%. It is very close to theoretical calculations. Very low figures in 90-th years are because of imperfection of heat metering system at that time.
NETWORK CALCULATION
Dimensions of network pipes are always depending on the peak loads needed for space heating and DHW production. Internal network dimensions and networks to small private houses should be calculated in accordance with amount of water needed for space heating. Dimensions of the network depend on the pressure losses in the network, max. losses could be 1 bar/km in the main pipeline and 2,5 bar/km in building internal networks. Min. difference in different consumer facilities should be about 0,5 bar. For example, pumps are very often chosen to achieve this pressure change in the remote points of the network.
Pressure difference transmitter usually controls pumps, located in the remote areas of consumption. Supplied heat is calculated by multiplying of total supply at the turn of the houses by the isochronal index. This index could be used when peak loads are different at different consumers. For example, if there are different types of consumers buildings in the current area. Time of peak loads in residential, industrial and commercial buildings varies very much. Isochronal index can be 0,85 and it can fluctuate from 0,65 to 0,9. Correct pipe dimension is selected taking into account water flow, its speed and pressure losses. Losses in the network can be 5 - 12% depending on connected loads.
In Finland central heating networks are nowadays calculated with the help of computer software, for example «Heat Nexus» and «Flowra».