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Old 01-16-2007, 11:35 PM   #1
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Default Convection, stirring and stainless steel

I have been reading "Brewing" by Michael Lewis. It is a good but very heavy read, full of molecular chemistry and solid backbone science for all this brewing voodoo we do.

Among the things that has caught my eye is a piece in his discussion of heat transfer and convection. The long and the short is that Stainless has a tendency to hold onto its heat, and is much more reluctant to let it go into wort then say copper. This is called wetting and stainless is not so good at it. He points out that mechanical convection or stirring greatly increases wetting, thus decreasing stainless' shortcoming and making for a much more rapid transfer of heat into the liquid and rapid boiling.

What this leads me to is wondering how much gain I would have by stirring my wort as it is coming to a boil. My larger batches (30+ gallons) can be slow to heat and anything I can do to speed up the process is welcome. I am aware of the dangers of hotside aeration but this can be avoided. I have in mind the use of one of my pumps for example.

My question is if any of you know anything more about this. I assume I will find the answer further on in the book, but that is a couple hundred pages away and so I don't know when I will get there.

Thoughts?

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Old 01-17-2007, 11:57 AM   #2
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I never got that far into the science to learn about wetting...but just scanning some topics I did see an interesting experiment using ultrasound to decrease the contact angle of the droplets. Interesting. I would be interested to hear about anything you try.

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Old 01-17-2007, 03:50 PM   #3
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I am not sure yet what I am going to do and will want to do it in a controlled way so I can gauge its value. I will post what I do and what I come up with.

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Old 01-17-2007, 04:00 PM   #4
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Why not just use one of your pumps to circulate the boiling wort? It'll be easy to gauge the advantage since you can just shut it off next time.

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Old 01-17-2007, 11:52 PM   #5
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In theory I suppose so. In a tightly controlled system too, like if you were worried about every second, or every last cost for Propane er sumpthin. In your brewshed, tell me, how hot is the gas after it gets above the kettle? It can be surprisingly cool. If not, that is wasted heat. Who cares? Anheiser Busch, not me.

I know they make copper plated cook pots, but I wonder if they would bother if it wasn't also pretty?

Copper plating keggles, anyone?

Now, if welders want to talk about stainless vs aluminum heat transfer in welding, you are getting practical.

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Old 01-18-2007, 12:06 AM   #6
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If you are looking for a way to get more heat for a quicker boil, I would lean towards a Blend of oxygen/ propane. But for god's sake use a regulator on the O2. Set for about 4 P.S.I of outlet press to main gas line. Propane has a heating quantity of 2500 btu's / cubic ft. And as a general rule..Air which is 20% O2. 100btu's will be unleashed from any 10 c.f. of air. so, boost your O2 and vola, more and more heat. Too much heat will melt metal of course. Plain steel will melt @ 1300* ish. steel will appear cherry red @ 1200*. Stainless steel will appear cherry red the same as regular steel but the stainless will hold together better. As long as you have liquid on the other side of that flame you will almost never make that metal overheat to the point of mlelting. A propane flame in normal air is around 950* so a small amount of O2 will boost you if you are that inclined to rig something up

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Old 01-18-2007, 12:17 AM   #7
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Wetting and conduction are different things. Stainless is lousy for both of these. Water doesn't wet stainless as well as it does copper. Stainless is also a poor heat conductor, compared to copper. The sum of these two phenomena is that heat transfer from a stainless steel (SS) surface is lots worse than from copper. The wort or water doesn't spend as much time in contact with the surface, and the heat that is present isn't conducted efficiently through the SS. I believe what does happen is that the stainless gets so hot that a microlayer of steam forms at the interface of the SS and the water, further reducing heat transfer. Stirring will help because it sweeps the water past the surface and helps to wet it and moves cooler water to the SS surface. Since the bigger the temperature difference between the heat source and the heated liquid, the better the transfer of heat, stirring helps.

In the end, I don't think we notice much, except broen stains on the heated surface of our keggles. A copper bottom would make a differerence, so if someone knows how to weld a copper sheet in place of the keggle bottom, go for it.

I think the reason high price pots have copper plated on them (Revere ware) or a slab of aluminum bonded (megapots) is to help spread the heat from the flame over a larger surface area compared to an unadulterated SS bottom. This minimizes areas of overheating near flames or other heat sources.

It all sounds impressive, hope it makes some sense.

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Old 01-18-2007, 02:02 AM   #8
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This is much of what Lewis says in his book. The question I have is how much difference are we talking. But also I see it as an opportunity to play around and see what happens.

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Old 01-18-2007, 04:46 PM   #9
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I make big pots of lots of things, and it has always been my intuition that stuff cooked in a steel pot (spaghetti sauce, chili, wort, whatever) required a lot more stirring when you used stainless rather than copper. The explanation, given in cook's terms, was that copper heats more evenly, whatever that means. However I've always found that food cooked in copper clad pots tended to scorch much less than those cooked in non-clad pots, and I've always assumed that the same dynamics took place with doing my boils.

So since my pot is steel, I always stir...but I would have anyway, even if it was copper clad.

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Old 01-18-2007, 04:47 PM   #10
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Quote:
Originally Posted by Brewpastor
This is much of what Lewis says in his book. The question I have is how much difference are we talking. But also I see it as an opportunity to play around and see what happens.

On both accounts...I'd agree. I mean you might make a slight difference in time and energy used, but would it be justifiable? And on the other hand, if you like to mess around it is a good opportunity to optimize. But since you are operating on a larger scale when compared with many homebrewers, it just might add up to something feasible. I still think the idea of introducing vibration might be interesting to look at. Maybe time a batch with and without something producing a vibration on the vessel. Just a thought.
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