The exhaust gas from the engine is passed through, inlet duct, bundle of tubes, and outlet duct, in a smoke tube boiler. How can I calculate the reduction in noise level? Thanks.
Save yourself the trouble and possible blame. Put in the engine manufacturer recommended stock muffler. They can be mounted outdoors if there is no room indoor.
For gas engines with exhaust gas hear recovery we used to do precisely that. Ignore the noise reduction (if any) of the heat exchanger. For a start, getting a heat exchanger manufacturer to give the information, let alone a guarantee can be difficult.
Also, if the heat exchanger is removed, there needs to be no additional silencer.
At MW size of engine, the cost saving is marginal.