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Pressure losses and pipe sizing of 5GDHC (anergy) networks

The design of 5GDHC networks is more complex than that of normal heating networks. On this page you will find information on how 5GDHC networks can be designed hydraulically and which design factors have to be taken into account.

Why is the network design and planning more complex for 5GDHC networks?

Technically, the dimensioning of pipe diameters of 5GDHC networks is similar to the pipe dimensioning of conventional hot heating networks. However, there are some important differences: First, it is more difficult to calculate the maximum mass flows in the pipe sections. This is more complex for 5GDHC networks due to the balancing of heating and cooling demands. Furthermore, the operation of the 5GDHC network should be estimated more precisely already at the planning stage, which allows to consider not only the maximum mass flows for dimensioning, but also the distribution of the mass flows over the course of the year. Another important difference in dimensioning is that uninsulated plastic pipes are often used for 5GDHC networks. These are much less expensive to purchase and install than conventional steel pipes. This has an influence on the optimum pipe diameter, since this is the result of a techno-economic optimization. Lastly, the heat carrier medium can also have an influence on pipe sizing: In some cases, a water-glycol mixture is used instead of water. For networks with very low temperatures, this ensures that the fluid in the pipes does not freeze even at temperatures below 0 °C. Since a water-glycol mixture has different physical properties (density, specific heat capacity) than pure water, this also has an influence on the mass and volume flows and thus on the choice of the optimum pipe diameter.
In summary, the following influencing factors make the design of 5GDHC networks more difficult:

  • Demand balancing of heating and cooling demands leads to mass flow reduction
  • Influence of the later operation needs to be taken into account in the planning stage already.
  • Use of plastic pipes, which are less expensive to purchase and install
  • Use of water-glycol mixture as heat carrier (in networks with flow temperatures close to the freezing point)
The nPro tool helps to automatically determine and optimize pipe diameters for 5GDHC networks.

How are pipe diameters for 5GDHC networks determined?

To determine the pipe diameters, the mass flows to and from all buildings must first be determined. Due to the installed heat pumps in the buildings, this is more complicated for 5GDHC systems than for conventional heating networks, in which only one heat exchanger is installed in the building. The mass flows depend on the heat flow at the evaporator of the respective heat pump, which in turn depends on the coefficient of performance of the heat pump as well as the heat demand of the building. For a correct design, not only the hour with the highest heat demand should be considered, but ideally the entire course of the year. The mass flow in the building connection pipe can then be calculated using the well-known formula $$m = \frac{Q}{c_P \cdot (T_{\mathrm{supply}} - T_{\mathrm{return}})}$$ If the mass flow profiles are known for all buildings, they can be determined for each point of the network using mass flow conservation in the heat network - provided the network topology is a star network. In the next step, suitable diameters can be selected for each pipe section so that the flow velocities are within a suitable range. For this purpose, known design recommendations can be used, which are partly based on experience and partly determined by means of techno-economic optimization models. In order to calculate the pressure loss in a pipe section, the known relationships from fluid mechanics can be used: $$\Delta p_{V} = \lambda \frac{l}{d} \rho \frac{v^2}{2}$$ Here, \(l\) denotes the pipe length, \(d\) the pipe diameter, \(\rho\) the fluid density and \(v\) the flow velocity. The most interesting parameter is the friction coefficient \(\lambda\). This can be read from the Moody diagram for different flow conditions. The Moody diagram is shown below.

Moody diagram for the pipe dimensioning
Figure 1: Moody diagram for the pipe dimensioning, based on [1]
Table 1: Example flow condition for three different pipe diameters
Physical dimension DN 80 DN 100 DN 125
Mass flow 13,6 kg/s (49 t/h)
Volumetric flow rate 49,9 m³/h (13,9 l/s)
Flow velocity 2,76 m/s 1,76 m/s 1,13 m/s
Pressure gradient 993 Pa/m 312 Pa/m 99 Pa/m

Recommendations for the hydraulic design of heating networks

The determination of optimal pipe diameters depends on many influencing factors and cannot be calculated directly, as it is a techno-economic optimization. If very large pipe diameters are chosen, the initial investment increases, since both the pipes themselves and the installation (excavation, etc.) cause higher costs. On the other hand, large pipe diameters lead to low hydraulic losses (pressure losses) and therefore to savings in pumping work, i.e. electrical costs for the operation of the network pumps. The optimization problem to be solved therefore depends, for example, on factors such as pipe laying costs, electricity prices (for pumping operation), which are individual in each project. Therefore, there cannot be a design guideline valid for all heating networks. Particularly in the case of 5GDHC networks, the design recommendations used so far are only of limited value: if plastic pipes are used, the investment for the network is much lower than for conventional, insulated steel pipes. Larger pipe diameters can therefore be reasonable because this is only marginally reflected in greater costs. At the same time, the temperature differences between the cold and warm pipe (flow and return) in 5GDHC networks are smaller than in normal heating networks. This leads to high volume flows, which can be compensated by larger pipe diameters. In addition to economic influences, technical factors can also play a role. For example, if in a 5GDHC network the network temperatures must not drop or rise too much due to interactions with the surrounding soil, it might make sense to choose a smaller pipe diameter, as this leads to a higher flow velocity and thus reduces the temperature drop or rise per meter of network length. Figure 2 illustrates various design recommendations for coventional heat networks. Since no specific recommendations for 5GDHC networks, these can be used as a rough guide. These are also stored in the nPro tool and are used for the semi-automated network design calculation.

Recommendations for the pipe network design of heating networks
Figure 2: Recommendations for the pipe network design of conventional heating networks, figure based on [2]

Sources

  1. Original diagram: S Beck and R Collins, University of Sheffield (Donebythesecondlaw at English Wikipedia) Conversion to SVG: Marc.derumaux, CC BY-SA 4.0 , via Wikimedia Commons
  2. T: Nussbaumer, S. Thalmann: Influence of system design on heat distribution costs in district heating, Energy, 101, 496-505, 2016. DOI: 10.1016/j.energy.2016.02.062
  3. Österreichisches Kuratorium für Landtechnik und Landentwicklung: ÖKL Merkblatt-Nr. 67, 2nd edition, Vienna 2009 (withdrawn 2014).
  4. Frederiksen S, Werner S. District heating and cooling. Lund: Studentlitteratur AB; 2013, ISBN 978-91-44-08530-2.

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