Pipe dimensioning for heat networks in nPro

Pipe sizing and dimensioning in nPro

In the nPro tool, the pipe diameters of heating networks can be estimated and dimensioned in the early planning phase. The procedure for this is as follows: In the pipe dimensioning of nPro (in the module "Load profiles and heat network"), the network pipe, that shall be dimensioned, is selected on the left-hand side (building selection): In the nPro tool it is assumed that the heat network has a star topology (a star topology has no meshes). In the building list, all buildings are selected that are supplied via the pipe that is dimensioned. This is illustrated in Figure 1: In the depicted district, 3 groups of buildings are supplied via a heat network by an energy hub. In order to dimension pipe I of the heating network, all buildings in the district are selected, since pipe I supplies all buildings in the district. Consequently, this pipe has the largest pipe diameter in the heating network. Only building A is selected for sizing pipe II, since pipe II only supplies this single building. Accordingly, for the dimensioning of pipe III, buildings B and C would be selected.

Figure 1: Exemplary district with heating network in star topology.

How is the optimum diameter calculated?

Design recommendations from pipe manufacturers are stored in the nPro tool. The manufacturer guidelines are used to determine the optimal pipe diameter for a specific mass flow. In principle, there is no clear physically justifiable design decision. The calculation approach underlying these design recommendations is generally based on cost assumptions for the installation of the heating network, the material price for the pipes and other techno-economic cost influences. nPro suggests a nominal diameter for pre-dimensioning, which is determined on the basis of the manufacturer recommendations. Nevertheless, other nominal diameters should be considered. For this purpose, a detailed analysis of several pipe diameters can be created in nPro. Here, key figures for pumping work and maximum pressure gradients are calculated and graphically displayed. For the calculation of the pressure gradient in the laminar range up to a Reynolds number of 2300, the pressure loss coefficient is determined with the formula 64/Reynolds number. For the turbulent range with a Reynolds number greater than 2300, the iterative approach of Colebrook is used. The transition range between Re=2300 and Re=5000 is deliberately not attempted to be calculated. Instead, a "jump" in the pressure loss coefficient at Re=2300 is assumed, which has the advantage of additional safety in the calculation of the pressure loss.

Figure 2: Analysis of pressure gradients in the district heating network pipe with nPro.

Validation of the pressure loss and diameter calculation

The calculation methods in nPro were compared and validated with different hydraulic calculation tools. As an example, Tables 1 to 3 show a quantitative comparison of the pressure loss calculation in nPro and data from the pipe manufacturer ENERPIPE. The ENERPIPE data are based on a company brochure with data for a CaldoPEX pipe at 80 °C supply temperature and 60 °C return temperature (brochure: "ENERPIPE - Nahwärmetechnik die ankommt.", 01/2021, pp. 34-35). Tables 4 to 7 present a quantitative comparison between the pressure loss calculations of nPro and the results of the online tool druckverlust.de. Tables 4 and 5 specifically validate the results for laminar flows and Tables 6 and 7 for turbulent flows.

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