Hi! Is it possible to analyze or design a boiler/steam generator with integral economizer/preheater by a CFD tool like FLUENT/CFX etc ? Is there any example ?

In my experience, Fluent or CFX is not the correct tool for analyzing overall boiler performance. In order to look at factors like economizer or air heater performance, you are better off to use a 1-D analysis.

Of course, if you are looking at something specific like fuel or air delivery, fluid flow and heat transfer to a tube or bank of tubes, local steam or water flows, etc., CFD can be a good tool.

You surely will be able to find a sampling of what has been done by doing a quick search on the internet.