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Old 06-06-2008, 01:10 AM   #1
aa8jzdial
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Default big kettle-physics problem

One of the brew pals built a big kettle for our group of 4-6 garage brewers.
20" dialmeter x 36". stainless steel. Approximately 46 gallons to the top.
We noticed a problem during the first boil.of about 40 gallons.
It was impossible to maintain a steady boil.
With a nice healthy flame, the boil would rise up very energetically and cycle back down to nothing. After a time of 45 seconds, the boil would return and shortly thereafter subside. On and on.
It has us confused as to what is taking place and also searching for a solution. Towards the end of the boil, the immersion chiller was lowered in place and it seemed to minimize the cycling after the wort returned to boiling temp.


Thinking of dropping a large stainless funnel upside down with a ss pipe venting partially to the top of the wort.
Thinking the tall kettle has high pressure on the bottom, raising the boiling point and as does cycle up to a boil, the pressure drops and boil becomes near violent.
Any one have insight as what is going on?
tnx
rick


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Old 06-06-2008, 01:40 AM   #2
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I have thought of this but never on the scale of a 45 gallon brew pot ! Damn !

As a chemist I used to use "boiling" stones with volatile solvents, basically small pumice stones that prevented boil overs in the flask.

In home brewing I often thought of using gas grill pumice ( no flavored ) ones of course ) to provide extra surface area and to even out the boil.

Never got a chance to do it as I got some huge boil pots and a keggle.

Edit: Immersion chiller ? What do you use 100 ft of 3/4 coil?


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Old 06-06-2008, 02:11 AM   #3
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The idea also bantered about was tossing in some marlbles to help the boil nucleate. Not sure what that really is other then a neat word.
The first brew used 30' of 3/8 tubing. Not enough.
Now we have 50' of 1/2 " with an real nice motor driven agitator to keep the wort swirling.
Hope to check that out this weekend for a warm weather brew.
The kettle is made of 3/8 " ss pipe x 20" dia. with a 3/16 bottom welded on.
One of the brews brothers is a pipe fitter/welder.
Nice addition to the group.
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Old 06-06-2008, 02:26 AM   #4
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I agree with Vorlauf, is the temp of the wort above boiling? Have you tried a lower flame to even out the boil. The bottom will get much hotter first and take much longer for the whole amount to boil without a violent eruption. The marbles would be a good idea. I will do a little geeking and dive into my chemistry books to try to give you a better explanation for this and maybe a better solution.
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Old 06-06-2008, 02:39 AM   #5
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I would guess you aren't putting enough heat into the kettle to keep it boiling. If it's swelling up you are definitely heating the bottom and getting convection, but in a nice rolling boil the bubbles make it all the way to the surface before they either pop or condense from the cooler wort stealing their heat. This isn't happening, it sounds like.

You'll lose heat out the sides to the air and out the top from evaporation. You could try insulating the sides of the kettle or building a windbreak (I assume you don't fire this monster inside) to minimize heat loss out the walls, or put a lid on to reduce the speed of evaporation. Don't seal it tight so you can still blow off DMS and other non-condensables.

A third option would be to get a bigger burner (or two).

Our brew rig does the same thing, although I have a flat-top stove and the world's thinnest-bottom turkey fryer (called Angry Pot because it warps and shakes constantly). Normally if I adjust the heat a little bit or wait a few minutes, this behavior goes away...although as long as it keeps stirring itself vigorously, I don't worry.
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Old 06-06-2008, 02:25 PM   #6
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I have no help but I'm putting together my 55 gal kettle so I want to watch this and see what you all come up with.

What burner are you using?
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Old 06-06-2008, 02:52 PM   #7
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No answer but I'd love to see a picture of your huge kettle in action!

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Old 06-06-2008, 03:01 PM   #8
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What is happening is a layer of slightly super-heated wort forms at the bottom. Eventually, a bubble of steam cuts loose. This stirs the area near it and more bubbles release. The effect cascades until all of the super-heated wort has mixed in. Then another layer forms. This is nucleate boiling.

When the heat is high enough to make bubble formation constant, you are into bulk boiling. (AKA boil-over h3ll)

Adding the chiller increases nucleation sites, which are small spots where vapor bubbles can form.

I saw this to a lesser extent in my kettle. Simple solution: put the chiller in at the beginning.
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Old 06-07-2008, 02:59 AM   #9
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Is the inside bottom of the kettle polished, if so maybe a quick scuff with some 220 grit sand paper would help make some nucleation sites.

I have a homemade ss kettle 25 gallons and I can boil it over easy. 150 degree to boil in probably 25 minutes or less without pushing my propane burner to the point were flames are rolling out the sides, I think the burner is an old hot water heater burner, it was free.
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Old 06-07-2008, 07:43 AM   #10
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It's a thermal problem. The tank is to narrow for a boiler. The hotter liquid percolates in the long column. Boilers are usually no higher than they are wide and if you look at comercial tanks they are wider than they are tall. Sorry about bringing this up but it needed to be explained.


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