FluidFlow can be used to model ducted ventilation systems. The below image of a more basic system captures the main nodes used in a ducted system.
When you place a "Steel Pipe or Duct" on the flowsheet, you can set the duct to be circular or rectangular from the Geometry row on the Input tab of the Data Palette.
The approach in the image below has been used to fix the flow rate at each outlet boundary with the fans being auto-sized based on a design pressure change across the system.
