Have you looked into AutoCAD MEP?
What is the best program for drawing 3d mechanical boiler plantroom, and to be able to draw them in 2d isometric
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AutoCAD MEP is currently the best one but Autodesk are beta testing a new release called AutoCAD 3D Plant that sounds like it would be more appropriate to you. There are several very useful YouTube videos done by Autodesk themselves about Plant 3D that you should look at for an overview.
If your budget (or time for training as there is a large learning curve) doesn't extend to that then try a predrawn library of 3D parts such as on the following websites: