LOGSTOR ROR A/S, DENMARK / AREA SALES MANAGER as the shipbuilding and chemical trade. District Heating Distribution Systems The distribution of heating in piping networks is known as district heating. it is widely used in Northern European countries. in Germany, Denmark, Sweden and Finland more !han 25% of ali space heating is district heating. in facı, in many towns more !han 70% of the total heat demand is covered by district heating. in the beginning of the 20th century, the basic idea of common use of energy resources appeared and !his concept has been refined conlinuously until today. Over the years !he energy production has been optimized. Conventional condensing power stations generally utilize less !han 40% of the fuel !hey combust to generale electricity. More !han 60% is lost as flue gases, in cooling towers or cooling water. Today, combined heat and power plants use this waste energy by recycling hol water or steam to buildings, for space heating or industrial processes. Waste heat can also be used for driving chillers for cooling. These processes provide an overall energy efficiency of more !han 90%. This proves !hat heating and cooling combined with electricity production resul! in very efficient utilisation of energy [1 ]. Development of Distribution Systems The method of energy distribution has developed continuously. it has always been evident !hat the distribution pipes have to be insulated in order to save energy during distribution. lnitially, !he distribution pipes were laid in concrete ducts and the pipes were insulated with mineral wool or cellular concrete. in the 1960'ies the first pre-insulated pipes with polyurethane foam and PVC jacket pipes were developed. Later on the PVC was substituted by polyethylene. Up to approximately 1987 it was common to use CFC 11 as P :pe C om ponents Ca�rPjpe Steel Pex Copper Polyurethane Foam Po:VOl Cycbpentane bxyanate JacketP :Pe H zjh Den�iy Pol/<!ıhylıne // / Logstor Ror Figure 2: Components of Pre-insulated Pipe Systems physical blowing agent because of !he relatively low costs and favourable heat conductivity. As il became evident !hat CFC 11 is extremely harmful to the earth ozone layer !he industry stopped using it and replaced it with less harmful compounds such as CO2, which is stili widely used. However, after some other intermediate solutions the state of the art is now to use cyclopentane, CP in abbreviation, as !he physical blowing agent !he insulating foam. Cyclopentane consists of carbon and hydrogen and has an ozone depletion potential (ODP) of O. in other words, it does not affect the ozone layer. The other components of the foam, named polyurethane (PUR) foam, is polyol and isocyanate [2]. Figure 2 shows pipe components of modern pre-insulated pipe systems. Comparisons Between the Systems - Heat Loss When comparing the insulation properties of the different systems, !he main parameter is the thermal conductivity, I-value, of the insulation. Table 1 indicates !he I-value of the insulation material of the different systems. Thermal Conductivity ıı,-Value Polvuretane foam w. cvclopentane "' 0.028 W/mK Polvuretane foam w. CO2 "' 0.032 W/mK Mineral Wool "' 0.043 W/mK Cellular Concrete "' 0.1- 0.5 W/mK Table 1: Thermal Conductivity - Comparison The traditional district heating pipes, which consists of steel pipes with post-insulation of cellular concrete or mineral wool, are obviously much poorer than the insulation property of preinsulated pipes. When computing the heat loss in a pair of pipes, allowance must be made for the laying depth, the distance between the pipes, the flow and return temperatures, as well as the insulation properties of foam, carrier pipe, jacket pipe and soil. The heat loss per metre for a pair of pipes can be approximated by means of the following formula: Where, ? = heat loss per metre for a pair of pipes, W/m U = heat transmission coefficient for a single pipe, W/mK t f = flow temperature, °C 1 r = return temperature, °C 1 s = soil temperature, °C The heat transmission coefficient U is calculated using the following formula: 1 u = ECOGENERATION WORLO � 3 s
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