Blackening Kettle Bottoms

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ChemE

ChemE

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I looked around a little and didn't find any threads on this so I thought I'd share in case this is new to Homebrewers. Ultralight backpackers use this trick to get faster boils with less fuel...

Painting the bottom surface of your boil kettle with flat back engine paint will drastically increase the emissivity of the surface which allows it to absorb heat much more readily as compared to a surface with a very low emissivity. A little painters tape and some newspaper makes the finished product look nice and neat.

Freshly blackened pot bottoms
P1000743.JPG

I really should polish up the aluminum sides of the pots to further drop their emissivity so that less energy is radiated to my garage but its such a hassle I'm not sure I'm up to the task. Anyone have a good/fast/easy way of getting a nice polish on such a large aluminum surface?
 
I want to get this going on my BK too. Do you think that you could use similar techniques to shine up aluminum as you can on stainless?
 
ChemE said:
I looked around a little and didn't find any threads on this so I thought I'd share in case this is new to Homebrewers. Ultralight backpackers use this trick to get faster boils with less fuel...

Painting the bottom surface of your boil kettle with flat back engine paint will drastically increase the emissivity of the surface which allows it to absorb heat much more readily as compared to a surface with a very low emissivity. A little painters tape and some newspaper makes the finished product look nice and neat.

Freshly blackened pot bottoms
P1000743.JPG

I really should polish up the aluminum sides of the pots to further drop their emissivity so that less energy is radiated to my garage but its such a hassle I'm not sure I'm up to the task. Anyone have a good/fast/easy way of getting a nice polish on such a large aluminum surface?
Click to expand...

Do you know how much of a difference this makes in practical terms? IOW, if you brought 5 gallons to a boil as a test with and without the blackened bottom what do you think a reasonable estimate of the time difference required for each? This is interesting, but I'm not convinced there would much of a benefit on our scale.

The best thing to use for roughing up an aluminum kettle would be a wire brush on an offset grinder. Some coarse emery paper would also do the job.
 
Catt22 said:
Do you know how much of a difference this makes in practical terms? IOW, if you brought 5 gallons to a boil as a test with and without the blackened bottom what do you think a reasonable estimate of the time difference required for each? This is interesting, but I'm not convinced there would much of a benefit on our scale.
Click to expand...

The calculation itself is pretty straight forward to make using the Stefan-Boltzmann law but we have to make some assumptions. I assume that the emissivity of the original aluminum bottom of my boil kettle is 0.07 which is the reported literature value for aluminum. I further assume that with the black engine paint in place the emissivity is 0.9 which also seems reasonable based on literature values (http://www.infrared-thermography.com/material-1.htm). Finally we have to assume the temperature of the propane flame beneath the kettle. I'm less sure of the true average value here so I'll use the peak flame temperature to give a best case scenario. Realistically we know that the average temperature below the kettle's surface is less than this so the difference in blackened vs. silvered is less. Also, my boil kettle has an outside diameter of 13.5" which is what I'll use for my calculations.

Given all the above, the original surface experiences a radiative heat gain of 9,611 watts. When blackened the same surface experiences a radiative heat gain of 123,576 watts. My burner is rated as capable of putting out 185,000 BTU/hr which is only 54,000 watts. Clearly my assumption about the average temperature below the boil kettle is way off but you get the idea. You get drastically better heat transfer to the kettle which should give you significantly shorter rise times.
 
I haven't brewed since blackening my beer making pots. I did a large number of tests on my backpacking pot (1.5 quarts so WAY smaller) on an alcohol stove. In those tests I was able to move the boil time from 12 minutes down to 10.5 minutes. I did five boils each way and though there was some variation in the data the findings were statistically significant meaning it wasn't just random chance, there is a measurable difference in the boil times.
 
ChemE said:
The calculation itself is pretty straight forward to make using the Stefan-Boltzmann law but we have to make some assumptions. I assume that the emissivity of the original aluminum bottom of my boil kettle is 0.07 which is the reported literature value for aluminum. I further assume that with the black engine paint in place the emissivity is 0.9 which also seems reasonable based on literature values (http://www.infrared-thermography.com/material-1.htm). Finally we have to assume the temperature of the propane flame beneath the kettle. I'm less sure of the true average value here so I'll use the peak flame temperature to give a best case scenario. Realistically we know that the average temperature below the kettle's surface is less than this so the difference in blackened vs. silvered is less. Also, my boil kettle has an outside diameter of 13.5" which is what I'll use for my calculations.

Given all the above, the original surface experiences a radiative heat gain of 9,611 watts. When blackened the same surface experiences a radiative heat gain of 123,576 watts. My burner is rated as capable of putting out 185,000 BTU/hr which is only 54,000 watts. Clearly my assumption about the average temperature below the boil kettle is way off but you get the idea. You get drastically better heat transfer to the kettle which should give you significantly shorter rise times.
Click to expand...

God I love Chemical Engineers... :tank:
 
ChemE said:
Given all the above, the original surface experiences a radiative heat gain of 9,611 watts. When blackened the same surface experiences a radiative heat gain of 123,576 watts. My burner is rated as capable of putting out 185,000 BTU/hr which is only 54,000 watts. Clearly my assumption about the average temperature below the boil kettle is way off but you get the idea. You get drastically better heat transfer to the kettle which should give you significantly shorter rise times.
Click to expand...

So, if I'm understanding this correctly, there would be something like a 13 to 1 increase in the efficiency. (123,576/9,611=12.86). All I can say is that's totally unbelievable. I'd be impressed if it were only 1/10 of that improvement and I'm very skeptical that it would be even that much. I don't think the tiny 1.5 quart test boil translates well to the large 7-14 gallon kettles. It's a problem of the kettle surface area to volume being nowhere close to the same ratio. Perhaps I'm missing something. I'm not well versed in thermodynamics.
 
bja said:
My guess is that the 123,576 is a typo and should be 12,576.
Click to expand...

Yeah, that would explain a lot and then it would be only about a 31% improvement, but I'm having trouble even buying that number.

Additionally, I have yet to see any paint withstand the heat generated by a propane burner. Even the high temp paint intended for such applications as exhaust headers or BBQ's can't handle it. There may be some very exotic paints available that could take it, but they are likely to be very costly and would probably require special surface treatment and application beyond the average DIY home brewer. I'm wondering if you could just intentionally soot up the bottom of the kettle and leave it that way. Lamp black, so to speak.
 
It is not a typo, it is an artifact of a bad assumption. The Stefan-Boltzman equation is a function of temperature to the fourth power. When I rerun with 1000F (found that value on Bayou Classic's website) the difference is much smaller.
 
propan12.jpg


The 1000 degrees F seems to be way low according to the information found here: http://www.propanecarbs.com/propane.html

Those figures are supposed to be peak flame temperatures, so I would guess that they are maximums for ideal conditions which I am fairly sure that our burners fall substantially short of. OTOH, I would guess that the temps could reach 75% of those listed without too much trouble and that would be about 2700 deg F.
 
With 1000F I get 151 watts silver and 1950 watts black
With 2700F I get 3,500 watts silver and 45,000 watts black
 
So, would that be 1950/151=12.91 times more efficient with a 1000F flame temp
and 45,000/3500=12.88 times more efficient with a flame temp of 2700 F? Very nearly the same, but still not reasonable numbers IMO. I would need to see some actual real time test data using full scale equipment before I would be convinced. Very interesting though, I must admit and I would love for this to prove out to something that could result in some significant fuel and time savings. It may well be worth looking into further.
 
I'm waiting for the subsequent post of "how do you remove engine paint from the bottom of a kettle"

It will be interesting though if this will work to decrease propane usage.

When I first read it, I couldn't help but think about how it smelled when the burner was running.
 
Keep in mind that what I'm discussing here is radiative heat transfer. Conduction will be ever so slightly diminished by the addition of the engine paint but otherwise the same in both cases. There is absolutely no way that there will be a 13x increase in heat gain but there may be that increase in radiative heat gain. The two are extremely different.
 
Okay, all you disbelievers pushed me into it. I just took the smaller of my two pots (the one that came with my Bayou Classic) and put 5 gallons of tap water in it, put my propane tank on a bathroom scale, and went for it. First and foremost the engine paint took 20 minutes of FULL flame with absolutely no problems. No foul odors, no smoke, no nothing. So I don't think there will be any "how do I remove engine paint from my boil kettle?" threads.

Engine Paint After 20 Minutes of Full Fire
P1000747.JPG


Now the results. I got from 84F to boiling in 20 minutes which for me is significantly faster that usual. Below is a plot of the actual data. I didn't bother recording seconds so the data jumps around a little as I was just recording minutes.

Temperature Evolution as a Function of Time
Blackened_Rise_Time.png


Usually heating five gallons of sparge water to 180 (not boiling as I did here) takes me more like 30 minutes though I've never measured it terribly accurately. Also the bathroom scale (not the most precise instrument) didn't move during the 20 minute trial. It displayed 30 pounds before and after.
 
this was a fun thread, but while I appreciate a good theory, I'm not sure that there is ANYTHING that can make my boil times SEEM to be shorter.


'...the waiting is the hardest part ...' -T. Petty
 
In actual measurements of ejaculation of small-particle matter inside gas enclosed fecal particles shows negligible increase of fecal velocity inside gas encased ejaculation modules. When small particle ejaculation occurs at higher transfer rates under pressure during 5,000+ degree environments, sticking often occurs.
 
passedpawn said:
Nice test. Now scrape all the paint off and repeat:D
Click to expand...

Hehe, nice try. I think the engine paint will be staying since it did do exactly what I put it there to do. I started this thread mostly because I was genuinely surprised that no homebrewers (we're a scientific and crafty bunch) were doing this and wanted to sound out the craftiest and most scientific of us (the DIY boards). Clearly this is new to the community so perhaps someone with a silver pot will repeat my experiment and then add the paint. If not no biggie, I just wanted to feel out the community and perhaps contribute something new.

Oh and most importantly during the trial I relaxed, had a homebrew, and didn't worry about it. :mug:
 
ChemE said:
Hehe, nice try. I think the engine paint will be staying since it did do exactly what I put it there to do. I started this thread mostly because I was genuinely surprised that no homebrewers (we're a scientific and crafty bunch) were doing this and wanted to sound out the craftiest and most scientific of us (the DIY boards). Clearly this is new to the community so perhaps someone with a silver pot will repeat my experiment and then add the paint. If not no biggie, I just wanted to feel out the community and perhaps contribute something new.

Oh and most importantly during the trial I relaxed, had a homebrew, and didn't worry about it. :mug:
Click to expand...

IMO, lacking a comparative test on identical equipment without the paint, this test is at best inconclusive. I may have to give it a try myself. I think I have an aluminum kettle out on loan somewhere that I may be able to retrieve. Do you guys think that we could skip weighing the propane tank. So long as it doesn't run empty, the gas output should be relatively constant. I'm thinking that simply measuring the time from start temp to ending temp would do it while running the burner wide open, given that the starting temps were the same and the burner cooled down to ambient each time. I'll run the test in my garage and out of the wind and weather. The only hitch that I can see might be having to run the tests on two different days as I'm sure the high temp paint would require some time to cure before exposing it to the direct flame of the burner. I could probably get pretty close to equal start temps on different days. After all, what we are looking for here is a significant improvement. Some slight gain won't much interest me. I plan to go with five gallons from tap water temp of about 70F to boiling and I will plot the temps as done previously. Might be few days until I can er done though, so stand by for updates.
 
Curing of the engine paint isn't too onerous or time consuming and can absolutely be done with household equipment. I used my oven since both my pots fit inside (though not at the same time).

1) Apply
2) Let cure 1-2 hours
3) Bake @ 200F for 30 minutes
4) Let cool 30 minutes
5) Bake @ 400F for 30 minutes
6) Let cool 30 minutes
7) Bake @ 600F for 30 minutes (here I only did 550F since my oven only goes that high)
8) Let cool 30 minutes

I was a little worried that the paint wouldn't take the heat but the blue flame was absolutely blasting on it and there was no damage.

Catt, I look forward to seeing your data. I do not think that the propane weight matters one iota I was just being thorough.
 
No real negative thoughts here. Its just idle thoughts of what might happen. I don't have any preconceived ideas on the actual output or odors. Its one thing to wonder and another to do. I salute you.
 
Catt22 said:
Do you guys think that we could skip weighing the propane tank. So long as it doesn't run empty, the gas output should be relatively constant.
Click to expand...

Hmmm. I think this will be a waste of a test. As soon as you are done, no matter the result, the obvious dropping pressure in the tank will be questioned, and it will be all for naught. Hate to be the naysayer, but it's inevitable, and rightfully so.
 
do you think that the water staying at 212 gives the paint a "cool side" to help it not blister off?
you are having better luck with it than I did (not on a kettle).
let me know how the first few hours go, thanks.
 
brewmonk said:
do you think that the water staying at 212 gives the paint a "cool side" to help it not blister off?
you are having better luck with it than I did (not on a kettle).
let me know how the first few hours go, thanks.
Click to expand...

Absolutely I do. Sure, I'll keep this thread updated on that score.
 
This wouldn't help if you're using a heat stick. You need fire under your kettle for it to make a difference. In your case it would hurt since your boil kettle would radiate more heat to the room by having its emissivity increased.
 
Catt22 said:
Do you guys think that we could skip weighing the propane tank. So long as it doesn't run empty, the gas output should be relatively constant. I'm thinking that simply measuring the time from start temp to ending temp would do it while running the burner wide open, given that the starting temps were the same and the burner cooled down to ambient each time.
Click to expand...

Catt, since you're planning on running the tests on different days anyway, why not just top off the propane tank between the two runs so they both start at the same pressure? One more variable out of the equation.

I'm in favor of this mod overall - if nothing else, it makes me feel clever about not religiously cleaning soot off the bottom of my keggles. :)
 
ChemE said:
Curing of the engine paint isn't too onerous or time consuming and can absolutely be done with household equipment. I used my oven since both my pots fit inside (though not at the same time).

1) Apply
2) Let cure 1-2 hours
3) Bake @ 200F for 30 minutes
4) Let cool 30 minutes
5) Bake @ 400F for 30 minutes
6) Let cool 30 minutes
7) Bake @ 600F for 30 minutes (here I only did 550F since my oven only goes that high)
8) Let cool 30 minutes

I was a little worried that the paint wouldn't take the heat but the blue flame was absolutely blasting on it and there was no damage.

Catt, I look forward to seeing your data. I do not think that the propane weight matters one iota I was just being thorough.
Click to expand...

Damn. Anyone have a keggle sized oven? :drunk:
 
passedpawn said:
Hmmm. I think this will be a waste of a test. As soon as you are done, no matter the result, the obvious dropping pressure in the tank will be questioned, and it will be all for naught. Hate to be the naysayer, but it's inevitable, and rightfully so.
Click to expand...

Not true. The vapor pressure will remain relatively constant so long as there is some liquid propane remaining in the tank. The vapor pressure of propane is a function of it's temperature. I will check the tank temperature and take measures to insure that it's reasonably close for each run. I will probably keep the tank in a tub of tap water before and during each test run. I plan to start with a full tank.

I just remembered that I have a four gallon SS kettle that should be large enough to give us some usable data. It will fit in my oven for the paint cure whereas my aluminum kettle would not. I also don't have to hunt it down as I would the aluminum one. The smaller kettle will shorten the test time too which I like a lot.
 
shortyjacobs said:
Damn. Anyone have a keggle sized oven? :drunk:
Click to expand...

I would think that you could do the cure using a heat lamp or even a halogen shop light at close range. Might have to increase the exposure time some, but I bet it would work. You might also be able to turn the keg upside down on a propane burner to heat it up for the cure. Use an infrared thermometer to check the temperature.
 

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