Calculating fermenter addition to hit temp

[Mr B]

Hey brains trust

I'm looking at helping a mate to do an extract brew.

Propose to do a boil with the extract and add hops etc.

What I'm trying to work out is the calculation to hit an 18 degree ferment temp, where X litres of boiled extract etc at say 100 degrees would be mixed with X litres of cold water at say 4 degrees to hit the fermnent temp. Cant find an online calculator or equation to work out the right proportions.

Any ideas?

 

[wide eyed and legless]

Well you would have to get it to yeast pitch temperature first, I wouldn't have a clue what the mathematical equation is but there should be something on google.

 

[MHB]

It's called the standard mixing equation Aa+Bb=Cc
Where A+B=C
A,B and C are a quantity
a,b and c are a condition
All units must be the same, in this case (L) litres and (oC) temperature.

Say you want to make 23L of wort at 18oC
You are starting with 3L of 100oC concentrated boiling wort
obviously what you need to add is 20L of colder water (question being how much colder)
Put what you have into the equation
(3L*100oC)+(20L*XoC)=23L*18oC
Solve what you can
3*100=300, 23*18=414 and 20*X
300+20*X=414
Rearrange to solve for X (temperature)
(414-300)/20=5.7oC
Answer being 20L of 5.7oC water

There will be some error, that 100oC will fall fairly fast, there is going to be a difference between the heat capacity of the 3L of concentrate and water.
Be a good idea to make the water a bit cooler and have a little extra on hand, stop when you get close to 23L (say 20-21L) and check the temperature, if its at 18, add as much 18oC water as you need...

Hope that helps - very handy equation, basis for strike water, infusion, decoction... calculations.
Mark

 

[Mr B]

Thanks Mark much appreciated.

I did see that equation but didn’t get how it worked for this situation - you made it very simple (it probably is )

 

[MHB]

Good glad it helps. Now for the warning!
If you went much over 4L of boiling wort, the temperature of cold water required goes below freezing.
If you play with the numbers (best to put it in a spread sheet and play with goal seek), if you don't have enough cold water the only thing that works easily is to let the "Boiling Wort" have some time to cool, let it cool to say 80oC.
Re-do the numbers and add the required cold water, than add the balance with ambient.
Should get you fairly close.
M

 

[Mr B]

Thanks again, will do.

Will do a boil for hop additions, and assume that the volume boiled wouldn’t matter for the purpose of hop flavour and bitterness?

I.e work out a volume for the boil that would suit the temperature calculation (so probably proportionally small to the batch size), and can rely on normal bitterness predictions from beer smith?

 

[MHB]

Err - maybe, I'm not sure what beer smith has to say, but as gravity goes up your utilisation goes down.
At very high gravities, you might be getting pretty crappy isomerisation. personally I would boil a small wort at or around the gravity the wort is going to end up at (i.e. 1.050), probably more important than boiling all the extract.
Naturally as we are brewing, it cant be a simple yes, a higher hop concentration also lowers the utilisation, as does lower pH's (but taste better than hops boiled in alkaline wort)...
Bit of an element of suck it and see, keep notes and revisit your method until you get the beer you want.
Best option is to go all grain and full volume, but that's not my call.
Mark

 

[Brewnicorn]

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