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Boil off evaporation rate

Gordon NJ

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Everything went perfect for my pale ale recipe (40L/10gallon) yesterday, up to the boil. My pre-boil gravity (1.041) came just under the estimate (1.045), which is fine. Beersmith asked me to top up kettle to 54L (pre boil) after 43L from mash out (didn't measure looked about right). However my post boil volume turned out much more (51L) than the estimate of 46L, and consequently the gravity also much lower (1.037 instead of 1.055). How could this be? Not vigourous enough boil? I kept a lid on to build up heat until the hotbreak, and then boiled with the lid off for 60 min with my hop additions. The bubbling was there but not massively so. Could the lid on pre-hotbreak and vigour have a massive influence? Did i get my calculations wrong somewhere? 
 

philm63

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If you started with 54L pre-boil, boiled for 60 minutes and ended with 51L, that's only about a 5% per hour reduction rate which is pretty low. While it varies widely depending on kettle dimensions and a few other variables, an average boil-off rate of around 14% is commonly accepted at the home-brew level, which also happens to be about what it appears BeerSmith was telling you.

Doesn't matter if you left the lid on at the beginning, many of us do this to conserve a little heat energy at the start. It's the intensity of the boil that matters. Sounds like your boil was lacking in intensity. Easy fix - boil harder next time, but be sure to watch out for boil-overs if your kettle is fairly full.

Something else I see - you're topping-up in the kettle. If you're doing this to hit pre-boil volume, that's fine if you're already above your estimated pre-boil gravity because you're diluting the wort down to what you expected by adding straight water.

If you came up short on volume AND gravity, seeing as the boil accomplishes a reduction in volume resulting in a concentration of sugars, you'd want to pull more sugars from the grain. Assuming a batch sparge, You can toss some more sparge water in the tun, stir, wait a few minutes, recirculate until clear, and run that off to the kettle until you hit your volume and pre-boil gravity targets. If you fly-sparge, just keep running until you've hit your targets (this assumes you have enough water in your HLT). Even if you end up with a little more liquid in the kettle than anticipated, just boil a little longer, no biggie.

As long as you're still pulling sugars from the grains, you can continue to sparge, just watch the pH and SG, don't go above pH 5.8 or below about SG 1.010. You may need to make more sparge water than BS tells you - I make around 5 gallons of sparge water every time (I brew 5 gallon batches) and even if I don't use all of it (I rarely do), it's good to have on hand for just such an emergency.

Your gravity tanked because you diluted 43L of an already short-on-gravity wort with 11L of straight water, and only boiled-off 3 of those liters. If that top-up water had contained sugars from the grain, you would've been further concentrating that liquid and likely would've come much closer to your gravity target. It takes practice getting used to how your system works - keep plugging away and take lots of notes - you'll get there.
 

Gordon NJ

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Thanks Phil! Yep, I think the boil vigour was a factor and should have just turned it up and kept on boiling. The boil pot was quite elongated so perhaps the smaller surface area also kept the rate down. I did a fly sparge, and will make sure to measure sg and not just trust the numbers. Thanks for help! Will enjoy the 3.5% ale and hope the flavour shows well. Thanks for the help! Metrically yours from South Africa!
 

jomebrew

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Is this your first run on this system?  You do need to establish the brewing parameters for your system.  So, now you know you will have a 5% boil off rate.  There are a few environmental factors that affect boil-off rate but mostly it is the dimensions of the kettle and the vigorousness of the boil.  Sounds like the boil was a fine and you just needed to learn what your rate will be. 

My converted keg was closer to 10% while my 15 gallon kettle was closer to 15%.
 

RiverBrewer

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Gordon NJ said:
Not vigourous enough boil? I kept a lid on to build up heat until the hotbreak, and then boiled with the lid off for 60 min with my hop additions. The bubbling was there but not massively so. Could the lid on pre-hotbreak and vigour have a massive influence? Did i get my calculations wrong somewhere?
jomebrew said:
You do need to establish the brewing parameters for your system.  So, not you know you will have a 5% boil off rate. 

I may be totally wrong but I get the feeling your trying to use your propane like Scrooge.
eg. cover on while heating & 5 % boil off.
Hot side, while sparging and transferring, I'd keep the cover ajar slightly. Cover off when you start the main burner & throw a lot of btu's at the kettle to get it up to a boil, then back it down to a nice roll.
 
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