Originally Posted by larrybrewer
Somewhere around 1.5-2 gallons per hour will evaporate while boiling. It depends on the equipment.
I second what larrybrewer has posted.
Different people have a different opinion on what's called a rolling boil as I have seen agressive boiling with high evaporation rates vs a undersized burner on the edge below a rolling boil. On that one we used a welding blanket in a hurry wrapped it around the keggle for a big difference in the boil. The other thing is the diameter of the opening will change the boil off rate. My last set of keggles I cut a 10" opening vs 12". Those large diameter short pots expose a lot more surface area for a larger boil off amount. A 12" opening has 113 sq/inch area, 10" has 78.54 sq/inch area which is .694 of the 12". A 16" diameter pot has 201 sq/in surface area that is 2.56 times more than the 10" keggle. This is a reason why I went to 10" on the last keggles and will not own pots plus the kegs are free.