Really high boiloff percentage?

[mpruett]

I brewed a CAP this weekend. Coming out of the mash, I had what appeared to be close to the right amount of wort, and it was on the money for my pre-boil gravity (1.050).

After my 90 minute boil, I came out about a gallon lower and about 0.010 higher in post-boil gravity (1.070) than I (or BeerSmith) expected. I did some back-fiddling and figured out that an evaporation rate of about 20% would explain my situation.

Is that insanely high, or is that actually in the realm of reasonable?

For what it's worth, I have a 52 quart aluminum tamalera that I'm using as a boiler. It's about 18" in diameter, which is pretty wide for a brewpot. I was also boiling at a good rolling boil.

 

[Benthic]

A large diameter pot with a lot of surface area, like the one you have, would produce a higher boil off rate. So the numbers you posted above don't seem to farfetched to me. Since your gravity was right on at the end of your mash, you can always add top up water to the fermenter to bring your OG back into line.

Brian

 

[MalFet]

I'd love more scientific feedback on this, but I've always noticed evaporation to be more a function of fixed rate (Xgal/hour) than of an proportional rate (X%/hour). 1 extra gallon in 90 minutes seems like a normal difference. Once you get a feel for your boil off rate, it's easy enough to adjust your pre-boil volume. In the meantime, you can always periodically adjust with a cup or two of hot water.

edit: misread before

 

[ajwillys]

Yeah, I think it is a fixed rate based on equipment and environmental conditions. Pot size, boil vigor, humidity, wind, air temperature, etc... can all play a factor.

 

[mpruett]

Thanks. That makes me feel a little better. I boiled off quite a bit, I think- a couple of gallons at least.

I really need to inscribe markings inside my pot for measuring volume by sight.

 

[Benthic]

For comparison purposes--I boil my 10 gallon batches in a 15 gallon kettle. The kettle is roughy 15.75" in diameter and during a typical 1 hour boil I usually boil off about a gallon. It's worth noting that I keep the kettle covered while heating the wort and then remove the lid just before it reaches the boiling point. I'm sure that affects my boil off rate.

Oh, and as for marking your boil kettle for volume measurements--you might consider marking your stirring device instead. I did that for a long time before I got a kettle with a sight glass. When I needed to know volume I just touched the stir paddle to the bottom of the kettle and looked at the marks on the handle.

Brian

 

[Jawbox0]

Seems just right to me, unless I misheard the numbers. You said it looked like a 20% boiloff rate for 90 minute, so assuming a final volume of ~5.5G and a starting volume of 7G you'd be boiling off one gallon per hour... I know that's pretty much what I boil off with a light rolling boil in my 32quart pot.

7G * .2 = 1.4G boiled off ~= 5.6 Gallons final volume - hop loss ~= 5.5 Gallons

I've hung a piece of aluminum stock off the side of my pot marked with half gallon markings to make it easy to see if I'm on track for my final volume.

 

[RM-MN]

It requires a certain amount of BTU's to boil off a gallon of water regardless of the shape of the vessel. My burner has a control on it, I turn mine down.

Being that your vessel has a larger surface, it will take more heat to keep it boiling but to boil off that much seems like excessive heat to me.