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

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?

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.
r

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.

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.

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?

No answer but I'd love to see a picture of your huge kettle in action!

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.

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.

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.

Thanks for all the insight folks.
I was somewhat concerned about the kettle dimensions being so far from "square". But the possiblility of having a relatively cheap big kettle
caused us to throw caution to the wind. The price of a commercial kettle of proper shape is way to high for us po' Michigan boys.
The kettle bottom surface is presently rough ground so nothing to gain there.
I believe next brew we will try running with chiller in during entire batch.
We have a real nice agiatator system and will more then likely just wind up running with chiller and agiataion.
I will try and come up with a photo or 2 of the kettle and chiller.
Also have a real neat homemade flash heater that we us for continuos hot liqour up to about 1 gallon per minute at 170 degs. will try to jpg that also.
thanks again.
rick

I have read that a few pennies tossed into the pot will even the boil??? For that monster I'd toss in half a buck or so??

aa8jzdial said:

Thanks for all the insight folks.
I was somewhat concerned about the kettle dimensions being so far from "square". But the possiblility of having a relatively cheap big kettle
caused us to throw caution to the wind. The price of a commercial kettle of proper shape is way to high for us po' Michigan boys.
The kettle bottom surface is presently rough ground so nothing to gain there.
I believe next brew we will try running with chiller in during entire batch.
We have a real nice agiatator system and will more then likely just wind up running with chiller and agiataion.
I will try and come up with a photo or 2 of the kettle and chiller.
Also have a real neat homemade flash heater that we us for continuos hot liqour up to about 1 gallon per minute at 170 degs. will try to jpg that also.
thanks again.
rick

Click to expand...

Where in MI are you?

Hey, just thought I'd chime in real quick. I use a 30 gallon aluminum pot. I ended up using 2 burners to bring to a boil. Once boiling I'm able to maintain a nice rolling boil with one. I also partially cover the pot with a lid, and watch it like a hawk during the initial boil. I tried marbles, but it seems like a waste of time. My next addition is to use some high temp insulation to keep the heat where it belongs. Hope this helps, good luck...

We all thought a double burner would be needed.
As soon as the first runnings got to about 2 gallons we fired up the single burner and kept the new wort at near 200 degs so when done sparging, near ready to roll.
Insulation no doubt is a good idea.
At minimum, a shroud of some sort to force the hot gases around the outside of the kettle.
Was hoping to have brewed this weekend to evaluate the continuos agiatation but too many graduation open houses, etc.
Will keep you all posted. And if and when I figure how to post some photos, will certainly do so.
FSR 402-Location is North Muskegon, actaully closer to Whitehall. 1 Mile from Lake Michigan shore.
Come by for a brew if you are in the area. I'll buy.
rick

aa8jzdial said:

FSR 402-Location is North Muskegon, actaully closer to Whitehall. 1 Mile from Lake Michigan shore.
Come by for a brew if you are in the area. I'll buy.
rick

Click to expand...

I'm near GR and I know Muskegon well.

You need a bigger burner. As the water boils the convection heat transfer coefficent increases, causing more heat to be dispated causing the boiling to recede. The burner is just barley enough to boil, but not hot enough to maintain it.

Really want to see your flash heater.