Blackening Kettle Bottoms

[PHBalanced]

I'm still curious on the original intent of this thread. Has anyone got data to go with blackened bottoms?

 

[ChemE]

Inquiring minds want to know...

 

[shortyjacobs]

I probably won't bother covering the holes. I kind of like the convenience of being able to easily view the flame level without standing on my head. It looks like I may be using a little more propane, but i think I can just turn up the burner some and it won't take any longer to reach a boil. One odd thing I noticed when running this test. The rate of heat gain decreased as the water temperature increased. The rate was quite stable on the first run. I logged the time and temp at five minute intervals for both tests. I have no explanation for this difference.

This is a TOTAL guess, but maybe convective currents? Without vents, the side of the keggle stay cool, so you will have a strong heat gradient from the bottom up, allowing water to rise in the center and fall on the sides. With the vents, the sides of the keggle will heat up too, meaning the water won't want to fall down on the sides as much. You'll get a hotter outside and a cooler core in your keggle, perhaps, leading to less even heating and a longer heating time.

Again, total guess.

 

[Catt22]

This is a TOTAL guess, but maybe convective currents? Without vents, the side of the keggle stay cool, so you will have a strong heat gradient from the bottom up, allowing water to rise in the center and fall on the sides. With the vents, the sides of the keggle will heat up too, meaning the water won't want to fall down on the sides as much. You'll get a hotter outside and a cooler core in your keggle, perhaps, leading to less even heating and a longer heating time.

Again, total guess.

Maybe, but the vent holes are only on the back side and span about 10". Just bases on my casual observation, there seems to be considerable convection turbulence happening while heating. I doubt that the gasses escaping through the vents actually heat up the side of the kettle much as they don't really hug the side much. Might even heat the sides less than the usual hot gasses escaping more uniformly from the bottom of the non-vented skirt. I'm guessing a this too. The change in the rate was substantial. Much more than I would have ever expected. The rate was about 4 degrees per minute and dropped to about half of that as the boiling point was approached. The tests were both run in my garage so there was no wind or breeze either.

 

[Catt22]

I'm still curious on the original intent of this thread. Has anyone got data to go with blackened bottoms?

Not yet, but I plan to test the blackened bottom theory soon. I will be using a four gallon SS kettle for the experiment. It should provide some useful data. I plan to run the test about the same way as I did with the keg.

 

[shortyjacobs]

Maybe, but the vent holes are only on the back side and span about 10". Just bases on my casual observation, there seems to be considerable convection turbulence happening while heating. I doubt that the gasses escaping through the vents actually heat up the side of the kettle much as they don't really hug the side much. Might even heat the sides less than the usual hot gasses escaping more uniformly from the bottom of the non-vented skirt. I'm guessing a this too. The change in the rate was substantial. Much more than I would have ever expected. The rate was about 4 degrees per minute and dropped to about half of that as the boiling point was approached. The tests were both run in my garage so there was no wind or breeze either.

How much time in between boils? If the first one chilled your propane tank, it could have dropped the supply pressure for the second one.

Edit: the more I think about this, the more I think this idea is wrong too....from your timestamps, it was at least a few hours in between boils...and it would take a bigassed freeze on your propane tank to drop output enough to bump up heating time by that much....

 

[Catt22]

How much time in between boils? If the first one chilled your propane tank, it could have dropped the supply pressure for the second one.

Edit: the more I think about this, the more I think this idea is wrong too....from your timestamps, it was at least a few hours in between boils...and it would take a bigassed freeze on your propane tank to drop output enough to bump up heating time by that much....

The two tests were many hours apart. The first was run about 5:30 PM yesterday and the second this afternoon about 2:00 PM IIRC. The pressure to the burner should have remained constant as that is what the regulator does. The actual internal tank vapor pressure should not matter so long as it was at least 10 psi and it would have to be extremely cold for it to drop that low. I don't think the tank temp was a factor at all. The propane tank was about 3/4 full and showed no indication of being excessively cold. The ambient temperature was around 82 or so. Both tests were done from a cold start (meaning that the burner and stand were at ambient temps). I weighed the water for each run, so the volumes were the same and I double checked this by measuring the depths. The burner flame height appeared to be identical for each test.

 

[ChemE]

Since the test to be will be on stainless steel rather than aluminum I'm going to predict that the difference will not be that pronounced. The emissivity of polished 316 climbs from 0.28 at 75F to 0.57 at 450F which is a believable skin temperature during a vigorous boil. Aluminum on the other hand begins around 0.03 at 75F and climbs to only 0.10 at elevated temperatures. Thus, non-blackened stainless steel accepts significantly more radiation than does non-blackened aluminum.

Blackening alters the emissivity of 316 only 40% while it moves aluminum 90%; big difference. Still, I'm curious to see if the difference on stainless will be measurable. I have little doubt that those replicating the experiment on aluminum will appreciate a difference.

 

[Catt22]

Blackening alters the emissivity of 316 only 40% while it moves aluminum 90%; big difference. Still, I'm curious to see if the difference on stainless will be measurable. I have little doubt that those replicating the experiment on aluminum will appreciate a difference.

I still think that the difference in the radiant heat energy absorbed will be negligible relative to the amount of heat transferred by conduction, so I don't expect to see a measurable difference. I'll probably be just as wrong on this as I was on the venting mod, but we shall soon find out.

 

[ChemE]

We could always work out the steady-state heat conduction across an aluminum bottom assuming a constant temperature on one side but no one would believe the result anyway. Better just to wait for the test data.

 

[jasonsargo]

Has anyone tested this yet? Stainless keggle untouched versus stainless keggle blackened?

 

[Catt22]

Not yet that I am aware of. I'm still planning to give it a go as soon as I have some time to spare. I'll probably run the un-blackened bottom test tomorrow and the blackened sometime over the weekend. I won't be using a converted keg as I cannot fit one in my oven in order to properly cure the heat resitant paint. I have a smaller 4 or 5 gallon SS kettle that I plan to use for the testing.

 

[robtotten]

So you're saying I have a chance?

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.

 

[ChemE]

So you're saying I have a chance?

I have no doubt.

 

[Catt22]

So you're saying I have a chance?

A chance at what?

 

[Catt22]

ChemE,

I'm skeptical about the 185,000 btu's per hour output for a propane burner. The gas association states that a gallon of liquid propane can generate 91,600 btu's. At approximately 4.24 lbs/gallon, a full 20 lb propane tank would hold 4.72 gallons. At the 185,000 btu rate, a 20 tank would be depleted in approximately 2-1/2 hours running at full output. That doesn't sound right to me and leads me to suspect that the btu output claims for these burners are often highly exaggerated. How would a lower assumed output affect the calculations?

 

[Catt22]

Update:

Ran Part I of the blackened bottom test.

5 gallon SS kettle filled to the brim

SQ-14 burner at medium-high setting (will use same setting for both tests)

Time from 80 F to full boil = 41 minutes

Interestingly, the 41 minutes to bring 5 gallons to a boil is the same time required to bring 6 gallons to a boil in the converted keg with vented skirt. I attribute this mostly due to the smaller diameter of the 5 gallon kettle vs the keg. Less surface area on the bottom of the smaller kettle and that's where the vast majority of the heat transfer occurs.

Next step is to paint the kettle bottom and cure it properly. May be a couple of days before I can get to this part, but it will happen.

 

[ChemE]

The burner output is not included anywhere in the calculations. I merely make an assumption about the average temperature of the surroundings radiating to the kettle bottom. I assume that the kettle skin temperature is 212F (which may well be too low) and then make a guess as to what the temperature below the kettle is. This lets the Stefan-Boltzman equation do its thing and then I switch the emmisivity between 0.07 and 0.9 to see with that set of assumptions how much more radiative heat transfer would occur to a blackened kettle. Below is a chart generated with various average temperatures.

Radiative Heat Transfer to Polished/Blackened Kettles

It is worth noting that it takes around 3,000,000 Joules to bring 6.25 gallons of wort and a 35 quart aluminum kettle from 160F to boiling (160 chosen as the average wort temperature after a one hour fly sparge). If you batch maybe that initial temperature is more like 170F.

 

[Catt22]

The burner output is not included anywhere in the calculations. I merely make an assumption about the average temperature of the surroundings radiating to the kettle bottom. I assume that the kettle skin temperature is 212F (which may well be too low) and then make a guess as to what the temperature below the kettle is. This lets the Stefan-Boltzman equation do its thing and then I switch the emmisivity between 0.07 and 0.9 to see with that set of assumptions how much more radiative heat transfer would occur to a blackened kettle. Below is a chart generated with various average temperatures.

Radiative Heat Transfer to Polished/Blackened Kettles

It is worth noting that it takes around 3,000,000 Joules to bring 6.25 gallons of wort and a 35 quart aluminum kettle from 160F to boiling (160 chosen as the average wort temperature after a one hour fly sparge). If you batch maybe that initial temperature is more like 170F.

So, care to predict the time difference to bring the 5 gallons to a boil in the SS kettle polished vs blackened, based on your calculations? That should be doable and then we can compare your prediction to the actual Part II test run. I think it would be interesting to see how well the calculations correlate with a real time test.

 

[ChemE]

Wow, that looks like a trap if I've seen one. Still, I'll put my foot in and see if I get lucky. During my test a brought five gallons of tap water from 84F to 212F which should require 5,832,830 Joules to do. I did it in 20 minutes and observed an extremely constant temperature increase which means I was putting 4,861 watts (NET) into the kettle/wort system (a Joule per second is a watt for those non-scientists following along. Joule is a unit of work, watt is a power which is the rate at which work gets done). This makes me think strongly that the average temperature below my kettle is 1,250F or lower so I'll pick 1,000F.

If we pick this temperature, blackening should net us an extra 1,800 watts. So 4,861 watts net blackened and presumably 3,065 watts net polished. This would increase the time to boil to 32 minutes as opposed to the 20 minutes I experienced. This washes with my limited experience so at least as a first approximation it feels believable.

 

[Catt22]

Wow, that looks like a trap if I've seen one. Still, I'll put my foot in and see if I get lucky. During my test a brought five gallons of tap water from 84F to 212F which should require 5,832,830 Joules to do. I did it in 20 minutes and observed an extremely constant temperature increase which means I was putting 4,861 watts (NET) into the kettle/wort system (a Joule per second is a watt for those non-scientists following along. Joule is a unit of work, watt is a power which is the rate at which work gets done). This makes me think strongly that the average temperature below my kettle is 1,250F or lower so I'll pick 1,000F.

If we pick this temperature, blackening should net us an extra 1,800 watts. So 4,861 watts net blackened and presumably 3,065 watts net polished. This would increase the time to boil to 32 minutes as opposed to the 20 minutes I experienced. This washes with my limited experience so at least as a first approximation it feels believable.

I don't understand why you would view it as a trap. All I was after was an estimate to find out if we should expect a significant difference or not. There certainly should be nothing to fear. The worst that could possibly happen is that the actual test might show there is not much of an advantage. Your calcs suggest that it would take approximately 60% longer to boil with a non-blackened bottom. That's a huge improvement and would certainly make this mod worth doing. Even have of that would make it well worthwhile. We shall soon see if this jives with reality or not.

 

[ChemE]

Nah, don't worry I was just kidding around a little. Making light of the fact that this is half calculation and half assumption!

 

[Catt22]

It's cool. I think we were all aware of that.

 

[CrAzYmOuSe]

Why hasn't anyone mentioned the 1.21 gigawatts needed to make the flux capacitor work? I don't have to fall off my toilet to say, YAWN...

 

[Almighty]

I think this is the best thread going right now. Homebrewers are always going through great lengths and cash to increase their brewing efficiency. Also I know there are not too many direct heat breweries out there, but I could see them interested, too.
The next step if this works well is how I'm going to cure the high temp paint on my keggle. But I'm sure we can figure it out.

 

[Catt22]

I think this is the best thread going right now. Homebrewers are always going through great lengths and cash to increase their brewing efficiency. Also I know there are not too many direct heat breweries out there, but I could see them interested, too.
The next step if this works well is how I'm going to cure the high temp paint on my keggle. But I'm sure we can figure it out.

Think you might be able to cure the high temp paint on a keggle by inverting the keg and placing it on a propane burner. This would be rather tedious and require constant monitoring and adjustments I am sure, but without a big oven of some kind it may be the best alternative. I'm also thinking that a heat lamp close up may do the job or even big halogen shop light.

 

[BrewMoreBeers]

i vote for making a steel box (with some vent holes) and putting it on the driveway with a keggle and burner in the box. Like a ghetto oven you will be able to cure the paint.

But, the box may cost more than the saved propane from increased efficiency.

 

[Catt22]

OK fellas, I just painted the kettle bottom with Rustoleum High Temp Flat Black paint. Cleaned and prepped the kettle bottom as per the instructions on the can and gave the bottom two coats. I should be able to do the oven curing later this evening after a two hour room temp drying time. Then it's onward through the fog to the actual test sometime tomorrow maybe. Gosh, I'm so excited. I'm betting it will take some time to recover the cost of the paint in fuel savings, but we shall see. Place your bets now gentlemen, while there is still time. It would really suck if the test showed it actually takes longer to reach a boil with the blackened bottom, but that's essentially what happened with the skirt venting fiasco. I highly doubt that will happen, but it may well turn out to be a push or nearly so. The high temp paint is pricey at about $9 a can from Autozone.

 

[CaptainCoconut]

Don't think of it as something just to save money on propane. That's an added benefit, but I think the real value would be in time saved and having to get propane refills less often. If you think of it that way, you're saving money on car trips, too.

 

[Catt22]

Don't think of it as something just to save money on propane. That's an added benefit, but I think the real value would be in time saved and having to get propane refills less often. If you think of it that way, you're saving money on car trips, too.

I'm fortunate to have a propane filling station near my home and I fill about six tanks at a time, so I don't make many trips, but I'm sure not everyone has that advantage, so I would agree. OTOH, if it turns out that there is a substantial improvement it would save both brewing time and fuel. I'm about to begin the curing process.