Solution:

If you want to have separated parts STL geometry, you can model your geometries with different bodies in freeCAD.

#1. Select the body you want to export on the tree diagram, click file > export > give the part name. Make sure the extension is STL

#2. Repeat step #1 for each part you want to export

#3. Import model to tensorHVAC-Pro with “Import STL” button

#4. Check your geometry on the viewport, make sure the size is correct by inspecting the ruller.

#5. If it is too huge, sometimes it is 1000x larger due to the meter to mm conversion. You can scale the model by click the part on tensorHVAC-Pro tree diagram > transform > input 0.001x on scale, and apply to each part