Author Topic: boil off rate  (Read 13142 times)

Offline arctic78

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boil off rate
« on: January 30, 2016, 10:02:37 PM »
 I just done the math to work out my boil off rate per hour for my boil kettle and it seems wrong to me because in all the brews i have done so far it has been less than what i got from the equation. I have been keeping track off the amount of water that i start the boil with and what goes into the fermentor at the end of the boil once it has reached pitching temp. on average so far it is around 2.8 litres. ( .739 Gallons ) i loose from the start of the boil until it enters the fermentor.
When i work it out using this method ( which i found on here)

pi*(diameter of kettle in Cm/2)*(diameter of kettle in Cm/2)*0.00428

i get 4.1 litre/hour boil off ( my kettle diameter is 35cm ) 

SO........ 3.1415*(35/2)*(35/2)*0.00428 = 4.11 Litre/hour

I am guessing that there must be other factors which are not accounted for in this equation so i am wondering if i am wrong somewhere or i am doing something wrong in my brew process or if this is something others have found when they do the math.??? 

any advise would be greatly appreciated

Offline brewfun

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Re: boil off rate
« Reply #1 on: January 31, 2016, 07:57:30 AM »
Nope. The difference between the predicted maximum boiloff and your actual has to do with how much heat is applied to the wort. I'd expect that to be a lot less than the maximum simply because of the surface tension created by sugar (boil over).

To not boil over, most brewers reduce their flame significantly once the hot break begins to foam up.

The reduced evaporation is due to the flame delivering fewer BTUs. It takes about 2200 BTU to create 1 kg of steam. This is in excess of what is needed to keep the wort at the boil point (about 400 btu/kg).

What matters is the percentage of evaporation, not the volume. If your evaporation is more than 7%, then you've gotten rid of enough DMS to be undetectable to most of the population. For commercial brewers, this can take 60 to 90 minutes with steam, but homebrewers routinely have 15% in 60 minutes.
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Offline Oginme

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Re: boil off rate
« Reply #2 on: January 31, 2016, 08:36:36 AM »
This is a post I made on another forum which helped out users, it covers calculating the factors needed to set up your process (equipment profile) in BeerSmith:

Here's what I do/did when dialing in a system or process change:

Measure any dead space using cold water. I then convert to hot water using a 4% expansion factor [ V(ds) = vol retained in system / 1.04; where V(ds) is the dead space volume at sparge/mash temperatures]

On your next brew do the following:

Measure your water carefully into the pot: V(m) as mash water and V(s) as sparge water.

Dough in after weighing the grains carefully: W(g) as weight of grains.

Measure the volume and gravity (cooled to room temp) your wort from your mash [V(pbh) = pre-boil volume and SG(pb) = gravity of your wort, preboil]

After your boil, measure your ending volume and gravity [ V(abh) = volume after boil and SG(ab) is your gravity after boil (taken at room temperature.

Convert your hot volume readings to room temperature assuming a 4% expansion factor. Basically: V(pb) = V(pbh) / 1.04 and V(ab) = V(abh) / 1.04

You can now calculate your water absorption by the grain as V(m) + V(s) - V(ab) - V(ds). Divide this number by the weight of your grain and you will get the amount of water retained in the grains after mashing and sparging. You will need to convert this to oz of water per oz of grain to enter it into BeerSmith.

Your boil off rate can be calculated by [V(pb) - V(ab)] / time of boil. Convert this to volume per hour to enter into BeerSmith.

You can enter the numbers into BeerSmith to get the Mash Efficiency (which is what I use to dial in my process. None of this brewhouse crap as I vary that to accommodate the amount of hops and other additives).

I use the gravity readings to verify the accuracy of my volume measurements. Basically I convert to gravity points and if I am accurate V(pb) * SG(pb) should equal V(ab) * SG(ab) within the margin of my least accurate measurement (for me it is volume, measured to the nearest half liter when hot).

This should get you close for your next brew. I have found that averaging 3 to 4 brews gets my processes very close and repeatable.
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Offline arctic78

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Re: boil off rate
« Reply #3 on: January 31, 2016, 02:08:48 PM »
 @ Brewfun - Ok that make sense i thought that the amount of heat applied must have something to do with it but i was not aware of the % of evaporation that was required to get rid of the DMS but it sounds like that is not to big an issue for home brewing. I have one question though , If 60min is enough time to get around 15% evaporation is there any need to do longer boil times???

@ Oginme - thanks for that for that explanation for calculating everything to set up my equipment profile i will go through all this on my next brew to start getting things dialed in. I really appreciate the help.

Offline brewfun

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Re: boil off rate
« Reply #4 on: January 31, 2016, 03:39:01 PM »
If 60min is enough time to get around 15% evaporation is there any need to do longer boil times???

Not unless you're trying to increase your OG from more evaporation. And, there is one homebrew experiment that used triangle taste tests to see if even 60 minutes was needed.  It wasn't, and I think a lot of that was due to a fast chill. That's an advantage that probably won't translate to commercial scale, which is what homebrewers tend to copy.

On the other end of the DMS spectrum is the source, which is low color malt (like Pilsner). In kilning, the DMS precurser is burnt off as color is gained. Maris otter is relatively low SMM, but still requires boiling to eliminate. SMM level loosely follows diastatic potential, so you can make some assumptions from there.

What this means is that in shorter boils, SMM continues to convert to DMS as long as the wort is hot. Conversely, a 90 minute boil takes the SMM level low enough that what DMS is created is below the perception threshold.
« Last Edit: January 31, 2016, 03:42:00 PM by brewfun »
Beer Appreciation is the space between pints.

Offline arctic78

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Re: boil off rate
« Reply #5 on: February 01, 2016, 03:19:54 AM »
Ok  so if you have the means to chill your wort quickly and you are not looking to increase your OG  you should be fine with 60 min boils then?  But i guess if you want to be on the safe side a longer boil pays off , Especially if using lighter color grains.
Thanks for lesson , There is so much in brewing beer and the more i get into it the more interested i become i guess that is one of the things that i enjoy most besides the beer .

 

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