There are lots of design tools out there that can be used to generate MEP or HVAC fabrication drawings, including spooling and integration with Vulcan or CAMduct. Some are FABmep, CADmep, ESTmep, and Sysque.

There is an interesting discussion going over on LinkedIn regarding Revit and duct fabrication. The overall message seems to be this: if you want to use Revit as an integral piece in your MEP fabrication process, you should probably be looking seriously at SysQue:

CADmep Object Enabler is a free downloadable collaboration utility that
enables ObjectDBX host applications (i.e. AutoCAD, AutoCAD LT, Volo-View
and Navis) to view the enhanced objects from CADmep.

Read more:

Autodesk Knowledge Network

The Autodesk Fabrication CADmep Object Enabler applies to both 32-bit and 64-bit Windows.

Autodesk Fabrication CADmep 2014 OE — Build 3.02.600 SP2b (exe – 270334Kb)

Autodesk Fabrication CADmep 2013 Object Enabler
– 246856Kb)