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About the Air Duct Network Calculation

Explains the concept of air duct network calculation.

Detection of Network Geometry and Components

Before calculation, the duct network is detected from the model in all detail. This is done automatically after clicking Calculate. Only those components are detected whose system types are entered in the Settings dialog.

In order to analyze the geometry of the duct network, the section parts are detected first. Section parts in the duct network will be delimited by T-pieces or by components of dimension changes (e.g. transitions).

The program then searches for the initial component of the duct network. This is a fan or a partial network start. If no fan is available, the program searches for a partial network start. The program creates one system for each connection of an initial component. Accordingly, one fan generates two plants, a partial network start generates one plant. The fan and partial network start must not be installed in the same flow path. The system type is determined on the basis of the system of the first duct after the initial component.

Starting from the initial component, the flow paths in the duct network are determined. The flow paths always lead from the initial component to an end component such as air diffuser, protective grating or a partial network end. The number of flow paths in a duct network thus always is equal to the number of installed end components. Several outdoor and exhaust air systems may be led onto one protective grating. Extract air and supply air systems, however, always require exactly one fan per system. The flow paths of different extract or supply air systems must not be connected.

The results of the sound calculation are displayed as they occur: The protective grating emits sound to the outside, the results of the sound calculation for the flow path with the protective grating refer to the environment. Diffusers emit sound in the rooms in which they are installed, so their results are output in relation to the corresponding room.

Calculation steps

Prior to calculating the duct network, you can set some parameters in the Settings dialog, limit values for velocities and the pipe friction pressure gradient.

When starting the duct network calculation, the smallest dimension is selected for all section parts that have not been fixed. The smallest dimension always is the first active dimension in the Ducts and pipes dialog. If you do not wish to go beyond a certain dimension, deactivate smaller dimensions in the Ducts and pipes dialog.

Based on the air volume flows of the air diffusers and partial network ends of supply and extract air systems, the program first determines the required air volume flows in the duct network. The accumulated air flows are transferred to the outdoor or exhaust air system. If several protective gating or partial network ends are connected to the fan, the volume flows at each branch are divided proportionally between the protective gating or partial network ends behind the respective branch. The volume flows are automatically divided in the outdoor and exhaust air systems on the basis of the pressure drops to ensure that the same pressure drop is assigned to each flow path that ends at an outside grille or partial network end. This simulates a natural distribution of the volume flow rates in the design. This initial allocation of air volume flows can be adjusted by the user after deactivating the calculated checkbox in the component data dialog of the protective grating or the partial network start.

If an air diffuser or a partial network end of a supply or extract air system only contains information about the capacity, the air volume flow is calculated using the set supply and extract air temperatures.

This air volume flow rate is the basis for the calculation of the flow resistances in the duct network. As soon as the limits for the R-value or the speed is surpassed, the program applies the next larger dimension to the affected section part (unless the dimension is specifically fixed). Here as well, the program proceeds according to your specifications in the dialog Ducts and pipes.

The last step for the calculation is the hydraulic balancing. This is done by comparing the pressure losses in the flow paths and the throttling of all flow paths to the same pressure loss level, as it occurs in the most unfavorable flow path behind a ventilator. The network is hydraulically balanced using available adjustment options for outlets and other regulating devices.

The least favorable flow path is detected automatically by determining the pressure losses of all flow paths first. This most unfavorable pipe run is marked in color in the table for Flow paths of the calculation dialog. When pressing U on your keyboard, the most unfavorable flow flow path is selected in the table.