I can tell you exactly how many BTU/hr it would take to boil any amount of liquid in any amount of time. That is a trivial matter. The great unknown is how many BTUs of gas you would have to burn to inject that amount of heat into a vessel. I clearly stated "pot" in my original post because you are currently using a "pot", and I suspected you were/are planning on using a 1.5BBL "pot". I was simply making you aware that the energy inputs for a simple 1.5BBL "pot" would not be cost effective, or timely. The most common premade jacketed (or shrouded) solutions available to homebrewers in a 1.5BBL size are used restaurant steam jacketed kettles. Many have their own steam generators, and they are not the size of your "Geo Otto" boiler- they are more like the size of a suitcase. Even a 1 million BTU/hr for a >10BBL system is only ~4'x4'x8', or smaller.
I fully acknowledged and knew there are shrouded direct fire kettles. That has already been covered. That was obviously not what you were talking about using for your 1.5BBL system. Feel free to use whatever you want. If you found a true shrouded direct fired "kettle", have at it. It should be efficient enough. The ventilation calcs need to be correct if you are planning on using that kettle in a basement.
Two stage gas burner
Re: Two stage gas burner
You would rather clean one of those, than this....
