Making my own Boil Kettle: Formula for pre-boil vs post boil

[brewjedi]

I'm one of those lucky guys with a small machine shop. I shopped around for equipment and figured out it would be cheaper and more fun to fabricate my own equipment. What I can't seem to find is an actual formula to figure a pre-boil volume for a given post boil volume. I know that the diameter of the BK factors in as well as the boil time. As far as I can tell so far, air temp, humidity, and surface area are the biggest factors. Anyone have info or thoughts on this? I'm not going to buy software to make beer and from what I've read Beersmith doesn't account for these factors accurately..... The reason this maters is I plan to make a matched set. HLT, MLT,BK, and conical fermenter. I'm thinking I want about 25 gallons post boil minimum, but also considering using 55 gallon plastic barrel fermenters with which I think I would want about 40 gallons post boil. So this problem factors in to the size of every other piece of equipment and what size sheets of metal I will be ordering. It makes a big difference at $200 - $300 a sheet for 18 gauge 304 stainless.

[Stinkfist]

easy put 28 gallons of water in there and boil for an hour...and measure again...that is your boil off rate for 1 hour....no real math involved it won't be quite exact...as wort will boil a slight amount different than water and you will want to take your measurements where either both are hot or both are cold...to account for volume differences...

[Ozwald]

easy put 28 gallons of water in there and boil for an hour...and measure again...that is your boil off rate for 1 hour....no real math involved it won't be quite exact...as wort will boil a slight amount different than water and you will want to take your measurements where either both are hot or both are cold...to account for volume differences...

Rather difficult when the kettle hasn't been built yet. I know ProMash has a boil off calculator that's pretty dern accurate, I would assume BeerSmith would as well. I'd get a trial copy & plug in some of your recipes & see what it tells you... plus you might enjoy the software, but since it's a trial copy, no loss if you don't. Many folks don't use every feature of their software & you might find that it helps you in some odd way or another, perhaps not formulating the recipe itself, but being able to archive them or print off multiple copies when you soak one is rather handy. If you just refuse to have any part with the software, sorry, can't help ya. The software must be working off some formula, but I think it takes a bit more than volume & time into consideration.

I'm assuming since you're building a system of that size, this isn't your first rodeo. If you've been doing small batches on the stove, use the brewing software to scale the recipe up first. How vigArously you boil plays a big part as well. Even though we're all aiming for a similar effect, it's still going to vary slightly from brewer to brewer - especially of those of us who live at altitude.

Best of luck & I sure hope we get some pictures of the final product... if not a couple along the way. Cheers!

Edits: Just woke up & the ability to form a proper sentence must be waiting for the coffee to finish.

[spiderwrangler]

Keep in mind that while most sources will refer to this as a % loss per hour, that only works assuming that you keep things standard from batch to batch. In other words, a guy brewing a 5 gallon batch on his 10 gallon system is going to have a much higher % loss on the small batch than on the large.

[anday6]

The goal is somewhere between 10-15% boil-off, but just add 25-30% onto your post boil to account for boil overs and piece of mind. That'll get you close if you need to guess before hand and it's better to have more than you need.

[brewjedi]

I appreciate the insight guys. What I'm looking for is a formula. I'm sure I'm not going to always brew 25 or 40 gals at a time. I'll test batch new recipes in 5 or 1 gallon carboys. I got a friend that loves banana heffwiezens . Not going to even use a giant BK for them. Hard to believe how much attention and detail is paid to water chemistry and yet no standard formula for boil-off. I believe I will attempt to create and publish this formula for all to use.

Definitely going to post photo of this build as I go.

[Ozwald]

The goal is somewhere between 10-15% boil-off, but just add 25-30% onto your post boil to account for boil overs and piece of mind. That'll get you close if you need to guess before hand and it's better to have more than you need.

That's some solid advice. Every BK I've had I've always wished there were a couple extra inches... but then again, I'd probably just increase the batch size. Wait... not probably, I know I would. The current one's perfect for 10 gallons. All my recipes are written for 13.8

[tlael]

If you stay in a 1.5:1 height to depth ratio area it'd probably be in the 10-20% range per hour. (I'd probably use 25-30% if I were building a kettle just to allow extra room. Yes that is excessive, but if your cost doesn't skyrocket because of it, why not?)
Even if the height to width varied a bit, you may not be too far off of that.

Per hour is the key since the result will not be linear.

Here's the formula I use to calculate loss over time with any length of boil and boil off rate:
((d+l)+c)/((1.00-(r/100))^(t/60))

where:
c = cooling loss (desired to primary+equipment loss)*(cooling loss percentage/100)
d = desired volume to primary
l = equipment losses (dead space)
r = boil off rate (percent)
t = boil time (minutes)

I use 4 as my cooling loss percentage. I think that is a pretty accepted standard.