Evaporation percentage rates don't work. Boiloff rate for my system is usually 6L per hour.
Screwy
Of course they do - they just aren't for what people think they are for.
The boil off rate in % is to let you know a bit about your level of boil vigor and to let you know whether you have atually evaporated enough to carry away all the volatiles etc that need to be gotten rid of.
Say I do a 25L batch and I boil off 2.7L per hour. Thats 11% of my starting volume and thats right in the middle of the rates that are accepted as being acceptable in order to ensure the quality of your beer.
Great - so now I want to make a batch starting with 45L - my kettle is a 50L keg - so it will fit. And I boil off 2.7L per hour. So I can do all my calculations and know I will get 42.3 at the end of the boil.
sweet - right?? So who the hell needs the % figures for boil off. They are obviously of no use to anyone and should be ignored
Except that now - if I
do look at the % figure - I am only boiling off 6% of my starting volume - and there is a goodly chance that I will not have evaporated all the bad out of the beer. Its also possible that there wasn't enough heat in the system to give me a good hot break, or utilise my hops fully etc etc.
Now if I turned my burner up a bit higher - got the wort jumping around a bit more (which effectively increases the surface area) and managed to up that figure by a litre or so (up to 8%) - then I am back into the comfort zone of the generally accepted figures. Anyone who says you cant do it is full of the proverbial. A strong, very active boil will evaporate more than a gentler boil. Pot surface area is the
most important factor, but its not the only one.
So - different figures for different purposes. L/hr lets you do your numbers. Its about starting and finishing volumes.
% of starting volume per hour is about whether you L/hr figure is high enough in the first place (or maybe too high) - and whether you should consider changing something about your system to get your boil to a point where you know its doing everything it should be doing.