Heat Exchange of the Routes
Assign heat-exchange definitions to the routes: model types, parameters and their effect in steady-state calculation and dynamic simulation
Overview
Each route of the network can carry a pipe heat-exchange definition. It describes the thermal boundary condition between the pipe and its surroundings: either a dynamic ground model, a constant ambient temperature, or a user-defined temperature time series. Routes without a definition are adiabatic, i.e. they exchange no heat with the surroundings. The definitions take effect both in the steady-state calculation (there reduced to a single temperature) and in the dynamic simulation (there evaluated time-dependently).
Access
Tab Heat exchange via pipes of the network properties window.
Table of the definitions in use
The table lists all heat-exchange definitions used in the active network with a color column as well as the columns Name and Model type. Adiabatic routes appear as a separate entry Adiabatic. For the definition highlighted in the table, the following actions are available:
| Button | Function |
|---|---|
 Edit… | Opens the database dialog to edit the highlighted definition. |
 Exchange… | Replaces the highlighted definition on all routes that use it with another definition from the database. |
 Select | Selects all routes with the highlighted definition in the scene. |
Assigning to selected routes
Below the table, the Selected heat transfer area shows the definition of the routes currently selected in the scene (or Nothing selected, Adiabatic or Multiple heat exchanges selected). Three buttons are used for assignment:
| Button | Function |
|---|---|
 Assign from DB… | Opens the database dialog and assigns the definition chosen there to all selected routes. |
 Assign from table | Assigns the definition highlighted in the table to the selected routes. |
| Set adiabatic | Removes the definition from the selected routes; the routes then exchange no heat with the surroundings. |
Model types
The database dialog sets the Type: for each definition. Four model types are available:
| Type | Parameters | Description |
|---|---|---|
| Ground model | Soil type, laying depth, pipe spacing, moisture content | Dynamic coupling of buried pipes to a soil model, see ground model. |
| Constant temperature | Temperature: in °C | Temporally constant ambient temperature as boundary condition. |
| User-defined time series from values | Value table time/temperature | Temperature time series stored directly as a table in the project; a chart shows the annual profile. |
| User-defined time series from file | TSV file | Temperature time series from an external TSV file; the file is read at simulation time, so external changes take effect without re-importing. For the format see TSV files. |
Outer heat transfer coefficient
For the temperature model types (not the ground model), the heat transfer coefficient: in W/m²K can additionally be activated. It describes the convective transfer resistance between the outer pipe or insulation surface and the surrounding medium:
- Disabled (default): A practically infinite coefficient is assumed; the outer transfer resistance is neglected, and heat exchange is limited only by the pipe wall and insulation.
- Enabled: The entered value is used; the value 0 completely suppresses heat exchange across this boundary.
Typical values: free convection in air 5–25 W/m²K, forced convection in air (wind) 10–100 W/m²K, convection in water or moist ground 100–10000 W/m²K.
Effect in steady-state calculation and dynamic simulation
In the steady-state calculation, pipe heat losses are only taken into account if the option Consider pipe heat losses is enabled; otherwise all routes are treated as adiabatic. The boundary condition is thereby reduced to a single temperature per model type:
| Model type | Temperature in the steady-state calculation |
|---|---|
| Ground model | The dynamic soil model is not simulated; instead, the mean annual outdoor air temperature from the climate data serves as an approximation for the undisturbed ground temperature. |
| Constant temperature | The configured constant temperature is used directly. |
| User-defined time series (values/file) | The time series is evaluated at the peak hour of the current operating point (hour of the maximum total load of the network); in the operating-point mode Connection load (nominal), the hour of the lowest outdoor air temperature of the year is used instead. |
In the dynamic simulation, the constant temperature is applied as a fixed boundary condition, while time series are evaluated over time. Routes with a ground model are coupled to a linked soil model (co-simulation), see ground model.
Notes
- Heat-exchange definitions are database elements and are thus available across projects, see database concept.
- As long as no heat exchange is assigned, the simulation computes the routes adiabatically; heat losses of the network are then not represented.
Important in practice:
Routes without an assigned heat-exchange definition compute adiabatically – the network then appears loss-free, and your heat-loss balance turns out unrealistically good. Therefore always assign a definition to buried pipes (ground model, constant temperature or time series). Also note: in the steady-state calculation, pipe heat losses only take effect if Consider pipe heat losses is enabled.