Can I model ductwork?

Yes. When you have added a "Steel Pipe or Duct" to the flowsheet, select the Input tab of the Data Palette and set the "Use Database Size" to "No". You can then define the "Inside Diameter" for the duct and the associated "Diameter Unit" (see image below).

If you wish to model a Rectangular/Square duct, you can set the "Geometry to "Rectangular/Square" and define the "Rectangular Width" and "Rectangular Height" as well as the "Width/Height Unit" (see image below).