Kettle geometry, Is my pot too wide? Options

[cmh]

I found a good price on an aluminum pot which I am thinking of using as a kettle so I can boil full batches. It's a bit bigger (12.5 gal) than I need to do full 5 gallon batches, but the price is right!

It's wide and low, about 17 inches in diameter by about 15 inches high, give or take. Do you think this will be too wide for a 5 gallon batch?

This post has been edited by cmh: Jun 29 2009, 11:47 PM

[SkiWithG]

Not a problem, it actually a good size as you will lessen your risk of boiling over. I used to use a 32 qt turkey fryer and was always right on the brink of boiling over.

But you will likely get a lot more boil-off than you're used to with that much surface area. I use a 62 qt pot with a 16" diameter and see about 1.5 to 1.6 gallons per hour boil-off with a low boil. It's not a problem as long as you plan for it in your volume calculations.

Cheers,
Glenn

[cmh]

But you will likely get a lot more boil-off than you're used to with that much surface area. I use a 62 qt pot with a 16" diameter and see about 1.5 to 1.6 gallons per hour boil-off with a low boil. It's not a problem as long as you plan for it in your volume calculations.

Thanks Glenn. I'll give it a try!

[jwjeep]

I have a couple of friends with those huge wide polarware pots that sound similar in shape. They evaporate a lot more than my keggle, but once you get used to it it's not bad. It's all a part of getting used to your system and equipment if you ask me.

[Oldfart]

Hijack Alert! Well, sorta...

How, physically, does pot surface area cause more or less boil-off? To my way of thinking, the amount of boil-off is caused by the amount of BTU's delivered to the wort per unit time; wort surface area per se doesn't seem to enter this equation. I suspect that a wider pot leads to more efficient heating of the wort, as it presents more pot surface area to the direct flame, relatively less heat is wasted. Of course, pot surface area and pot bottom area are about the same, so that whether you call it "more wort surface area" or "more bottom heating area" does not affect the amount of boil-off, only the accuracy of its description...

Any thoughts?

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[jwjeep]

No, the bottom/heat transfer doesn't affect it much, you're still boiling at the same temperature. Now, if you have more efficient transfer it'll lower the amount of propane you're using, but that's not the point. More surface area of the wort allows more high energy molecules to escape in a given period of time, therefore higher boiloff.

[Oldfart]

More surface area of the wort allows more high energy molecules to escape in a given period of time, therefore higher boiloff.

What do you figure is the mechanizm by which the ability of high energy molecules to escape is related to surface area? There's nothing at the surface to hold back the steam, if there were the wort pressure (and temperature) would have to increase, which it doesn't because of the free surface. Hmmm?

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[jwjeep]

You're seriously arguing that woder and shallower evaporates at the same rate as tall and narrow? How about this, if I took 1 gallon and put it in a 2" tube long enough to hold the whole galon, and then took 1 gallon and put it in a big wide pan so the water was only 1/2" deep, that the two would evaporate at the same rate? I guarantee you that it's not even close. Same thing with brewing kettles, just sped up.

It's actually fairly complex to calculate, and you've got part of it already. IF there is greater heat transfer into the vessel, that should translate into greater heat transfer into the water (or wort), causing there to be more higher energy molecules capable of escaping as steam. Likewise, the greater surface area of the water surface allows more molecules to escape as steam, which transfers some of the heat out to the environment at a greater rate. There is also the fact that a wider kettle containing the same volume as a skinnier kettle has less depth of the liquid. This exposes more of the water (or wort) to the heat at a given time, and less high pressure distance to travel through the water to reach the surface and evaporate. The reason you get convection and bubbling in your pot is that there is very hot liquid near the bottom and the sides which expands and becomes slightly lighter. This rises (usually along the sides of the pot because there is heat being transferred into it through the sides as well) until it meats the surface where some energy escapes with the steam. As this is happening the cooler liquid in the middle sinks to take the place of what left, becomes heated, and rises itself in a continuing cycle. The greater the surface area exposed to air, the more energy can escape at a time, and thus the more heat you have to put in to it to maintain a boil. The shallower liquid allows a higher percentage of the liquid to be heated at a time, gaining energy faster and releasing it to the environment faster. Is it making sense now why wider shallower pots boil off more than taller narrow pots?

It would be interesting to see which required more energy to maintain a boil, the larger pot sheds heat faster through evaporation, and thus requires more heat to maintain that boil. The smaller pot has less efficient heat transfer into the liquid, assuming we're talking about a direct fired relatively flat bottomed pot that's not being heated from the sides. I have a feeling that the material of the pot would have a drastic effect on this experiment, especially considering that stainless is such a poor conductor.

[Oldfart]

You're seriously arguing that woder and shallower evaporates at the same rate as tall and narrow? How about this, if I took 1 gallon and put it in a 2" tube long enough to hold the whole galon, and then took 1 gallon and put it in a big wide pan so the water was only 1/2" deep, that the two would evaporate at the same rate? I guarantee you that it's not even close. Same thing with brewing kettles, just sped up.

Likewise, the greater surface area of the water surface allows more molecules to escape as steam, which transfers some of the heat out to the environment at a greater rate. .... The greater the surface area exposed to air, the more energy can escape at a time, and thus the more heat you have to put in to it to maintain a boil. .... Is it making sense now why wider shallower pots boil off more than taller narrow pots?

Jw -- in polite discussion, it is considered impolite to answer a quetion with a question. So to begin, how about answering my previous question first, the one about what relates escape of steam to surface area? You rely upon the supposition that more surface area somehow allows more release of energy -- you refer to this twice in the above paragraph -- but how or by what forces the water at the surface can somehow block or limit release of energy remains unexplained.

Your comparison of the wide pot with the two inch tube: The tube "pot" will have a bottom area of a little over three square inches, the big pot maybe 200 square inches. So it's pretty obvious that a lot more BTU"s will be transferred into the wide pot, and it will thus boil-off at a more rapid rate. This follows my original argument in this thread -- BTU's into the wort will drive the amount of boil-off, not surface area.

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[RecklessDeck]

I think the broader the pot, the greater the surface area of the liquid, that is more water molecules are near the surface of the liquid and in contact with the atmosphere. This means more molecules are in a position to be knocked out of the liquid by high-energy molecules and escape as gas.

P.S. I am not a scientist, I got this info from the encyclopedia

[Dave F]

I think that OF and Jeep might both have parts of the answer...there's a subtle difference between boiling and evaporation, although they're similar concepts. Check out this thread...it is asking, remarkably, the same *exact* question, only in the context of a physics discussion board.

The question of whether volume loss would occur at a faster rate once *boiling* was underway wasn't specifically answered...it seems like the volume loss *should* be more given more surface area, but perhaps not...

ON a separate but related note...this is one of the reasons I love brewing! It feeds my inner science geek...the practical application of science (chemistry, physics, biology, biochemistry, engineering) as a means to a (tasty) end...

This post has been edited by Dave F: Jul 2 2009, 10:09 PM

[sdeweese]

I hate it when I get interested in a discussion I don't fully understand and am dying to ask a potentially stupid...so here it is-Isn't there empirical evidence to support one case or the other?

[Oldfart]

Dave -- Thanks, understanding the differences between "boiling" and "evaporation" goes a long way to resolving the issues being raised here.

RD, I agree with your post, with the reservation that it applies to evaporation. Boiling is sort of a different story, it doesn't rely for most of it's activity upon molecules being knocked of the surface, but rather gets ~99.9% (I think) of boil-off energy from heat delivered from below, from the propane.

There are certainly secondary effects that influence the wide Vs. narrow pot boil-off, such as heat radiation losses from the sides of the pot, but I didn't mean to get into all these details here.

Dave, yes, brewing certainly invites a lot of possibilities for digging into science and engineering in interesting ways. Or to put it differently, to ignore the sciences and engineering related to brewing probably won't help the taste of your beer...

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[jwjeep]

That thread actually did a good job of explaining it. I think OFs problem is understanding that boiloff rate is the rate of evaporation due to a boiling fluid, but it takes the same energy for the same volume to reach a boil. Your water boils at the same temperature no matter what pot you put it in (provided it is open at all and not a pressure vessel). You're not going to somehow heat your wort to 400 degrees, because it will just turn into steam faster and the liquid still in the pot will still be just over 200, depending on your atmospheric pressure. The evaporation rate increases with surface area because more steam and heat can escape in a given period of time than with a small surface area. All other things being equal, boiloff rate is directly proportional to surface area, more area, more evaporation. This really isn't a difficult concept, not sure why this is so difficult.

[Oldfart]

I hate it when I get interested in a discussion I don't fully understand and am dying to ask a potentially stupid...so here it is-Isn't there empirical evidence to support one case or the other?

I doubt it. We have simply been fussing about how to explain why a wider pot will produce more boil-off -- due to more surface area or due to improved propane utilization (heating efficiency)? It's really just a nit-pick, prompted by nerdism. Go back to sleep...

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