Science Applied to Wort Chilling

It's been a while since I've worked a problem like this, a long while, however...

In terms of chilling my wort, I've been using a series of water baths, the first one or two using room temp water to pull off the worst of the heat, and then for the final cooldown, adding ice to the bath water. On my last cooldown, I topped up with nearly freezing water (34*) chilled down in my fermentation chiller.

Reading a recent thread about chilling wort directly with ice made me ask myself, "How much ice would it take?"

Well, I turned here:

http://www.hk-phy.org/contextual/heat/cha/la_he07_e.html

This equation makes a few assumptions not true in the brewing environment.

#1: The ice is at 0* Celsius. Most of our freezers will chill the ice below this temperature.

#2: No heat is lost to the atmosphere/environment (Not true - though this one cuts both ways. Our wort will cool by releasing heat into the surrounding air, but our ice will also absorb some heat from the surrounding air too).

#3: This does not account for the residual heat contained in the kettle, which isn't insubstantial.

Granted, some of these assumptions offset each other to a certain extent, but to how much, I have no clue. Imperfect as it is (and that may be an understatement), I ran the equation anyway. If nothing else, it might give me a ballpark of how much ice I want to have on hand if I want to chill my wort directly with ice.

I fired up Excel (my tool of choice, since I use it every day in the office) and ran the equation, and converted to gallons.

Assuming 2 gallons of boiling wort, with all the imperfect assumptions noted above, how many gallons of 0* ice do I need to get to a pitching temp of 20* Celcuis (68* Farenheit)?

Answer = 1.6 gallons.

My plan:

Freeze about 2 gallons of topoff water in plastic containers ahead of time, and when I'm ready to cool my wort, spray the outside of the containers with StarSan and drop them in. Stir the wort around the containers to distribute heat, and as the ice melts, perhaps occasionally lift the containers out of the wort and pour the water off into the kettle.

Edit: Based on advice I've gotten on another thread, I am not going to drop plastic bottles in near boiling wort. What I will do is perhaps simply freeze an amount of ice in my bucket the night before, and simply pour atop that. I figure with all of the headspace and the pliability of plastic, I shouldn't have a problem as the ice expands a bit when freezing - I hope!

Anyone use this method or something like it? I'd imagine so. How has it worked? How long has it taken to chill wort this way?

Anyway, if nothing else, I got to play with science equations that I haven't played with in... well... a very, very long time.

Cheers!

I haven't done anything like you're discussing. But I did discover that with my HERMS, chilling 5 gallons of boiling wort to 70 degrees took 42 pounds of ice. That is with stirring/mixing/agitation. I only did it once, because hauling 42 pounds of ice to put into my HLT sucks.

seems to me you are making this more difficult than it really is

I agree that with all your assumptions, i.e. equalling heat removed (as 2 gallons drops form boiling to 68°F) = heat absorbed by ice melting, about 1.6 gallons is right. Don't know about factoring the other assumptions in but by then you have 3.6 gallons of beer, another 2 gallons of tap temperature water should get you pretty much at the temperature of that top up water.

I actually did this on my brew yesterday.

Instead of 1.6 gallons of ice, I had one gallon, separated into about a half dozen small plastic containers. I poured my 2.5 gallons of wort right over the ice, stirred until the ice melted.

While brewing, I also moved my topoff water into the fermentation chiller. Topped off with 40* water (or so), and low and behold, I was at pitching temp!

The one advantage of this method I see is that I'm only removing the heat from the wort, not also removing the heat from the kettle (as in water/ice baths).

Of course this method might not work as well for those needing to chill larger volumes of wort than my partial boils.