12th International Symposium on District Heating and Cooling

The <strong>12th</strong> <strong>International</strong> <strong>Symposium</strong> on District Heating and Cooling,September 5 th to September 7 th , 2010, Tallinn, Estoniawould result in the ―red‖ heat loss values duringlifetime. Moreover, a new generation of blowing agentsis under development. These new agents aim at lowerconductivity values for the gas and larger molecules.This may result in lower conductivity values for theproduct as well as a slower ageing process.6. HYDRAULIC CALCULATIONSThe pipe per pipe comparison between Steel-PUR-PEand PB-PE-PE as demonstrated in Fig. 8, gives anindication of field results, but is not conclusive. Internaldiameters differ, as do friction coefficients. Therefore,for the comparison between distribution systems fittedwith either pipe, hydraulic calculations are needed. Tothis end, a reference network is introduced in Korsmanet al. 2008 [2]. The same network is used here. It isinstalled in a housing estate near Arnhem, theNetherlands, and has been designed using Pipelab,developed by Prof. Dr. Pàll Valdimarsson in 1995 [6].See www.pipelab.nl. Standard design criteria wereused. A total of 247 houses are connected by 3.02 kmof DH network (6.05 km of pipe), 12.2 m per house.The graph in Fig. 10 represents the pressure in thesupply network (in m water column), as a function ofthe distance from the source. For standard symmetricalnetworks, the return network is similar, but mirroredover a horizontal axis.Using the flexible and smooth PB pipes allows forsmaller diameters, mainly because PB is less prone tothe transmission of hydraulic noises. This is due to thelow modulus of elasticity of PB when compared tosteel. In contrast, a steel pipe filled with water is quite agood conductor of sound. To prevent noise caused byhigh flow velocities, these are limited in the design forsteel networks to 1 m/s.A network, specifically designed for PB, is shown inFig. 11. Smaller diameters in the periphery of thenetwork as a result of a higher permitted fluid velocitycauses higher pressure drops. This has to becompensated by bigger pipes closer to the source toreach the same overall pressure drop.Fig. 9, Aerial photograph of reference housing estateFig. 11, Design pressure drop PB networkFig. 10 shows an output graph of Pipelab.Fig. 10, Design pressure drop steel networkIn the design of district heating networks, the maximumdesign point is chosen considerably below the sum ofthe installed power in the connected buildings. It is notuncommon to have a design point of 50% of the totalinstalled power for larger numbers of connections,depending on the experience and the courage of thedesigner. A design point of 50% of the total installedpower was used in both designs in this paper. Inpractice, no problems have arisen with this designpoint, partly because not all installed power is used atthe same time. However, this statistical effect does notapply to individual connections. Therefore, a designtrick is used in the periphery of the network, to preventproblems in the service pipes connecting the buildings.The flow in these pipes is raised artificially above thedesign point, up to 100% load. The result of thiscalculation is shown in Fig. 12, which can be comparedto Fig. 10.314