Pipe Dimensioning in District Heating Networks

Pipe diameter in district heating networks is determined from the required mass flow, the allowable flow velocity (0.5—2.5 m/s) and the maximum specific pressure loss (typically 100—200 Pa/m). Since the pipe network accounts for 40 to 60 % of total investment costs, correct dimensioning is decisive for both economic viability and energy efficiency of the entire system.

Why Pipe Dimensioning Is So Important

The pipe network typically accounts for 40 to 60 % of the total investment costs of a district heating network. The diameters determine not only material costs but also influence civil engineering costs (larger trenches), pump energy demand and the level of heat losses over the entire operating life. Careful dimensioning is therefore key to an economically and energetically optimal network.

The Central Equation: Mass Flow and Capacity

The starting point for any sizing is the heat flow to be transported. From the required thermal output $\dot{Q}$ the necessary mass flow follows as:

Here, $c_p$ is the specific heat capacity of the medium (approximately 4.18 kJ/(kg$\cdot$K) for water) and $\Delta T$ is the temperature spread between supply and return. The volume flow $\dot{V}$ follows from the mass flow $\dot{m}$ and the density $\rho$:

Together with the chosen flow velocity $v$, the volume flow determines the required inner pipe diameter $d_i$:

This equation already shows that the temperature spread $\Delta T$ has a direct influence on the pipe diameter. Doubling the spread halves the volume flow and reduces the required diameter by approximately 30 %.

Pipe Dimensioning Methods

In practice, two methods have become established and are often applied in combination.

Method 1: Velocity-Based Dimensioning

With this method, a maximum flow velocity $v_{max}$ is specified. The pipe diameter is chosen such that the velocity at full load does not exceed this value. Typical limits:

Velocities above 2.5 m/s should be avoided, as they can lead to noise problems, increased wear and erosion corrosion. Below 0.3 m/s there is a risk of deposits forming in steel pipes.

Method 2: Pressure-Gradient-Based Dimensioning

The second method limits the specific pressure loss per metre of pipe, the so-called pressure gradient $R$ (in Pa/m). The pipe diameter is chosen such that $R$ does not exceed a specified maximum value:

where $\lambda$ is the Darcy-Weisbach friction factor. Typical guide values for the maximum pressure gradient:

Combined Application

In practice, both criteria are checked simultaneously. The chosen diameter must comply with both the velocity limits and the maximum pressure gradient. In transmission lines, velocity is often the governing criterion, while in distribution networks it is the pressure gradient.

Influence of Network Temperatures

The operating temperatures (network temperatures) of the network significantly influence dimensioning. A conventional district heating network with a spread of 50 K (e.g. 90/40 °C) requires significantly lower volume flows at the same output than a low-temperature network with only a 20 K spread (e.g. 60/40 °C). Specifically:

• At $\Delta T = 50$ K and $\dot{Q} = 1$ MW: $\dot{m} \approx 4{.}8$ kg/s
• At $\Delta T = 20$ K and $\dot{Q} = 1$ MW: $\dot{m} \approx 12{.}0$ kg/s

The 2.5 times higher mass flow in the low-temperature network requires approximately 60 % larger pipe diameters. This is one of the reasons why lowering network temperatures must be carefully weighed against the resulting additional costs.

Discrete Nominal Sizes and Practical Implementation

In reality, not every diameter is available. Pipes are manufactured in standardised nominal sizes (DN). The common nominal sizes in district heating networks range from DN 20 for small house connections up to DN 500 or more for large transmission lines. The calculated ideal diameter almost never corresponds exactly to a nominal size, so a decision must be made between the next smaller and the next larger step.

An economic evaluation is worthwhile here: the smaller nominal size saves investment costs but causes higher pressure losses and therefore higher operating costs over the entire service life. A typical service life of 30 to 50 years makes clear that operating costs play a substantial role.

Stepwise Dimensioning Across the Network

A district heating network typically consists of a main trunk with distribution and connection lines branching off from it. The main trunk must transport the entire volume flow, while side branches carry only partial flows. The pipe diameter therefore decreases in steps from the heat source towards the consumers. For meshed networks (network topology), dimensioning becomes more complex because the volume flows are distributed over several paths.

Simultaneity and Load Profiles

In practice, not all consumers ever demand their maximum output simultaneously. The simultaneity factor $g$ (typically 0.5 to 0.8 for residential buildings, see heat load demand) reduces the design-relevant volume flow in the main trunk:

The more consumers connected to a branch, the lower the simultaneity factor generally is. This must be taken into account during dimensioning to avoid oversizing.

Software-Supported Dimensioning

For networks with more than a few branches, manual dimensioning quickly becomes unmanageable. Modern planning software such as VICUS Districts enables automatic pipe dimensioning based on hydraulic calculation. All branches are considered simultaneously, simultaneity factors are taken into account and the optimal nominal sizes are determined under the defined boundary conditions (maximum velocity, maximum pressure gradient).

Conclusion

Pipe dimensioning is far more than simply inserting values into a formula. It requires balancing investment and operating costs, considering network temperatures and simultaneity factors, and knowledge of standardised nominal sizes. The velocity- and pressure-gradient-based methods form the foundation upon which an economically and technically optimal network design is built. For more complex networks, using appropriate simulation software such as VICUS Districts is recommended to reliably capture all interactions.

Further reading: Pressure Loss Calculation in District Heating Networks — the hydraulic constraints that govern pipe sizing, Linear Heat Density — economic sizing considerations based on heat sales per metre, Network Temperatures in District Heating Networks — how temperature levels influence required pipe diameters, Pump Sizing in District Heating Networks — how pump selection interacts with pipe dimensioning.

References and Standards

• AGFW FW 401 — Installation and Structural Analysis of Pre-insulated Bonded Pipes for District Heating Networks
• DIN EN 13941 — District heating pipes — Design and installation of pre-insulated bonded pipe systems
• AGFW FW 524 — Hydraulic Calculation of Hot Water District Heating Networks