Evaporation rate

Question

Right now, when brewing, I have one step that is never the same : the boil. Actually, the boil phase is going alright, my problem is with the evaporation rate of my wort. I have a few questions...

• What is the "standard" boil rate? Beersmith gives me 12% per hour by default. This is based on a 23 liters batch I think. That would mean an evaporation of 2.73 liters per hour, but normally I lose at least twice that amount.
• What if I make a 40 liters batch? will evaporation be 12%, or the same liters/hour? (In this case it would mean an evaporation rate of 6.8%)
• What are the advantages of a higher evaporation rate ?
• What are the advantages of a lower evaporation rate ?
• Is my evaporation rate way too high at 6 liters / hour?

And one big question here:

• How do you standardize your evaporation rate? I never evaporate the same amount, but I would like that part becoming a constant, not a variable.

I am using a propane outside burner. Should I get something to regularize the psi of the propane?

Thanks!

Answer 1

First, expressing evaporation rate as a % is completely the worng way to do it. Boil is a constant gal./hr. and does not change due to batch size. You don't boil off twice as much for a 10 gal. batch as a 5 gal. batch. 6 liters/hr. is about what my boil off is using a converted keg kettle and a propane burner. I try to set the flame to the same level every time and I get a pretty consistent rate. There are theoretical downsides to too high or low a rate, but the reality is that consistency is much more important than amount.

Answer 2

I agree with @Denny that stating evaporation as a fixed percent in your assumptions is somewhat meaningless for homebrewing because boiloff is constant based primarily on your kettle geometry and BTUs of heat. In other words, the percent being evaporated constantly goes up as the remaining volume shrinks (with 100% evaporation when the last drop is evaporating).

I also agree that "there are theoretical downsides to too high or low a rate, but the reality is that consistency is much more important than amount."

That being said, the biggest downside to too high of a boiloff rate is waste of energy. Brewing water is not necessarily free either because you may have bought RO water or spent time dechlorinating, collecting, or adjusting your water. Commercial brewers shoot for an evaporation rate of 6-8% in the first hour, and that is sufficient to release any DMS and SMM. Because a boil does not get hotter the more vigorous it is, and hop alpha acids will isomerize at any boil, we should shoot for the gentlest boil that we can. So while using percent evaporation may not make sense for homebrewers, we can learn something from commercial brewers about not having an excessive boil.

You should calibrate your kettle with 1/2 gallon or quarter gallon graduations (either etched on kettle or marked on an unvarnished/unpainted wooden dowel you can immerse in the wort). Then calculate your boiloff rate with those graduations.

You can standardize your evaporation rate by doing the same thing every time. Same amount of flame, same kettle, same burner, and same vigor of boil. Then check it at intervals to see if you are spot-on. Boiloff is constant (linear). Even if you do this, you may get different rates -- my rate in Winter is higher than in Summer due to the extremely low humidity here in Winter, and high humidity in Summer.