Thu Mar 25, 2010 6:46 pm
The engineer in me says that scientifically, evaporation rate is directly tied to the surface area of the liquid being boiled. In most cases, surface area for homebrewers is pretty much the same throughout the boil, whether you've got 10 gallons, 5 gallons, 3 gallons, or whatever. In fact, it's even almost consistent from brewer to brewer because many of our brew kettles are roughly the same diameter -- perfectly sized for putting over a big burner.
The only other variable, really, is how high you crank up your heat. If you have a standard sized brew kettle and you crank 'er up as high as she'll go without gurgling significantly over the side, chances are that your boil rate will be someplace between about 1.2 and 1.5 gallons per hour. Scientifically, it is not appropriate to use a percentage, although if you always make the same batch size, use the same kettle, and boil for the same amount of time, then you CAN utilize a percentage which will probably end up around 15 to 20% for the typical 5 gallon batch. Actually, I brew 3 gallon batches and I know my boil rate is about 22% for my standard batch size and my equipment. Since my brewing software uses percentage, I put 22% in and I always come out real close on all my volumes. When I brew bigger batches, I know from experience that I will need to lower this to the 15-20% range.
But "the right way" would be to figure out how typical your kettle size is, and then assume a certain volume per hour is boiled off, which would NOT change based on batch size. 3 gallons, 5 gallons, doesn't matter. The evaporation rate by volume per hour will stay the same, assuming the same boil vigor (is it just barely bubbling, or is it rolling big-time over the sides?).
Dave
"This is grain, which any fool can eat, but for which the Lord intended a more divine means of consumption. Let us give praise to our Maker, and glory to His bounty, by learning about... BEER!" - Friar Tuck (Robin Hood - Prince of Thieves)