This is very similar to the housing of a weather warning siren. I am in the process of attempting to re-create the housing to a Federal Signal Model 2 siren. I think it would be really cool to see you try and come up with that type of design tutorial.
Where do you find your information on details? Specifically when you say “it should be twice the diameter of the stack”. I’m genuinely interested in learning and hope to not sound rude at all. Do you use smacna? I love to know resources to learn from and own very many old sheet metal books
I’ve seen this in a few textbooks that have been around for many years. We also follow SMACNA guidelines and perhaps they do provide a chart? But we follow the chart from a Richard Budzik book, PRACTICAL SHEET METAL LAYOUT, Round Fittings used today, on page 213. Having said all that, you may find other resources that mention a slightly different slope angle and free Area.
