Eventually I figured it out that Revit won't allow us directly connecting 2 fittings to each other, however, is it a way to not showing broken gap between them ? Thanks.
I really hate Revit. The duct system in Revit is good for HVAC only. It's a nightmare using Revit for municipal odor control system. All ducting for odor control system will be slopped approx. 1-2% for draining condensation water.
Never mind. I figured it out.
Its customary that once you've solved a problem, you share that solution so others with the same problem might find an answer.
In other words, if you're going to ask for help, offer help in return.
Yes, that does fix the issue. But it happens because the two fittings 1) have coincident edges and (2) are not connected. This results in awkward rendering. The best solution is to force a connection, which is possible.
Make sure that you have Object Snaps turned on and make sure that the connectors you want to be connected are not coincident. Then select the fitting you want to connect (in this case, the elbow) and drag using the connector onto the connector of the second fitting (reducer with angled tap) until you see the connector snap appear (pink circle with an "X" superimposed over it) and release. It can take a couple tries but it is possible. You may need to cycle through nearby snaps with the Tab key. Be careful when editing though as Revit doesn't like these kinds of connections and they will break if you aren't careful. It may not be 100% accurate to leave an 1/8" stick of duct in between, but unless you are modeling for fabrication, it's well within most construction tolerances.
FWIW, you can slope duct in section, but not having a way to slope duct in plan sucks. Autodesk doesn't seem to think it's an issue, so consider throwing your support behind ideas such as those below:
https://forums.autodesk.com/t5/revit.../idi-p/9031678
https://forums.autodesk.com/t5/revit-ideas/draw-angled-or-sloped-ducts/idi-p/8862656
Thanks, Robert.