Possible Electric build with silicone heat mats

Doh! My bad. I'll be the first to admit I made a math error. My calculator was set to a 40 gallon pot size and I didn't change that value. So re-checking my calculations based on the formula at the Physics Classroom, the real math is...

1. Calculate the delta between start and goal temp
   • Start temp = 20*C
   • Goal temp = 100*C
   • Delta = 80*C
2. Calculate the volume in Grams
   • Grams = Gallons * a conversion factor of 3785.4118
   • Total Grams for 5 gallons = 18927.059
3. Multiply Delta * Volume * Energy Constant (4.186 Joules)
   • Total energy required to boil = 6338293.518 Joules
4. Divide Total Joules Required by Energy Applied (2200 Watts)
   • 6338293.518 / 2200 = 2881.042508 seconds
   • Total seconds / 60 = 48.01737514 minutes

So 48 minutes based on 100% thermal transfer and no losses.

So now the results makes more sense to me. At 120 minutes he had a 40% efficiency. That would make more sense given the external heat pads, the absorption of the pot, and the losses to the air.

That's something worth noting on my Excel calculator. The fields in yellow require manual entry. So if you use the unit convertors at the bottom, make sure you type those values into the relevant fields. When I changed the volume value from 40 gallons to 5 gallons, I didn't do that.

-Tim

dyqik said:

The post above shows a 70 C change in 60 minutes for 25l of water. This is a total energy of 4200 J/litre/C x 25 litres x 70C = 7.35 MJ supplied to the wort in 3600 seconds, for an average power of 2041 W supplied to the wort. With an average power supplied of 2200W, this is an efficiency of 93%. This seems very good, but not implausible, and certainly not unphysical. I'd like to be sure that the volumes and temperatures were properly calibrated, have an accurate specific heat and know that the power draw was constant before being more precise than estimating that at 90-95% efficiency.


I don't know where this figure of 38 MJ required to boil 5 gallons comes from. It's wrong by a factor of 4 or so.

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He has something like 90-95% efficiency for 20C to 90C, which seems about right, given the relative thermal conductivities. At higher wort temperatures, the heat loss to the air increases so those losses will go up as the temperature rises. Above 90C, the evaporative losses become more and more important, and as boiling starts, you start losing significant amounts of heat to evaporation. At boiling, you lose 2700W for every gallon boiled off per hour. To get over that last hump from about 90C to boiling, in a reasonable time, you need quite a bit more power than you do to get from 20C to 90C.

Augie, to answer your other question about thermal transfer from silicone to steel, the thermal conductivity of thermal interface silicone is about 1.2 W/m/K, about 3 times that of water. The thermal conductivity of glues e.g. epoxies is a bit better still, and the layer will be very thin if applied correctly. As long as a bit more than about 50% of the surface of the silicone is making contact with the glue or the steel, the thermal conductivity is limited by that of the water, not the glue or silicone.

Not sure why you keep cherry picking numbers. What does it matter what the efficiency is for 20C to 90C? The goal is 100C. The overall efficiency of hitting the goal is what matters, not some cherry picked number to make the efficiency look better than it was. If his efficiency went down considerably over 90C, which in all indications based on his photos of temperature shows, than he's not going to have 90%+ efficiency. 194*F is not boiling.

dyqik said:

He has something like 90-95% efficiency for 20C to 90C, which seems about right, given the relative thermal conductivities. At higher wort temperatures, the heat loss to the air increases so those losses will go up as the temperature rises. Above 90C, the evaporative losses become more and more important, and as boiling starts, you start losing significant amounts of heat to evaporation. At boiling, you lose 2700W for every gallon boiled off per hour. To get over that last hump from about 90C to boiling, in a reasonable time, you need quite a bit more power than you do to get from 20C to 90C.

Augie, to answer your other question about thermal transfer from silicone to steel, the thermal conductivity of thermal interface silicone is about 1.2 W/m/K, about 3 times that of water. The thermal conductivity of glues e.g. epoxies is a bit better still, and the layer will be very thin if applied correctly. As long as a bit more than about 50% of the surface of the silicone is making contact with the glue or the steel, the thermal conductivity is limited by that of the water, not the glue or silicone.

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trboyden said:

Not sure why you keep cherry picking numbers. What does it matter what the efficiency is for 20C to 90C? The goal is 100C. The overall efficiency of hitting the goal is what matters, not some cherry picked number to make the efficiency look better than it was. If his efficiency went down considerably over 90C, which in all indications based on his photos of temperature shows, than he's not going to have 90%+ efficiency. 194*F is not boiling.

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I'm really not cherry picking 90C, I'm referring to actual numbers from the experiment above, which are also relevant to brewing. I chose 90C because it gave a longer time to average over than e.g. 75C. Those numbers give the actual efficiency for heating strike or sparge water, or maintaining mash temperatures. And it's much higher than the 50% that a number of posters naively guessed would be the case at those temperatures. If you'd prefer to stop at 75C for sparge or strike water, then you'd get similar efficiencies for those processes.

The reason for calculating the heat transfer efficiency in this range is that you don't have to factor in any significant changes in heat losses from the wort. Above this range as you approach boiling, the heat transfer efficiency from heating pad to steel to wort won't change very much. However, above about 90-95C, the heat losses from the wort change rapidly, regardless of the heating method, because the losses increase due to evaporation. So I'm stopping at 90C because the losses from the wort become seriously nonlinear somewhere between 90C and 100C. Immersion elements won't get anywhere near 95% efficiency in that range, if you measure the heat into the wort just by the temperature change. Certainly my propane burner and 9000 BTU/hr gas stove burner lose a lot of efficiency there, with the final 5 C to boiling taking much longer to achieve than the 5 C before that, and if you look at the "my element won't reach boil" thread, you'll see that a 1500W immersion element can have zero efficiency by this measure. You have to be very careful by what you mean by efficiency in this temperature range, and any differences could be down to the geometry of the pot and other things that affect the losses.

If you really wanted to do a good job on this, you should use similar figures for a 2000-2500 W immersion element all the way to boiling to compare to these figures to see how much of the increase in time to boiling is attributable to the nature of the element. I'll bet that a 2000W immersion element isn't that much faster, if at all. I should get my hot-rod components this week, so I could do that test if you'd like.

This is dumb. im ready to pull out the thermogoddammics book and do some math but I have better things to do. Dygik, by inspection, his efficiency can't be as close to a fully immersed heating element as you're suggesting. There is going to to be double digit loss to the air and poor conductivity to the pot.

Weezy said:

This is dumb. im ready to pull out the thermogoddammics book and do some math but I have better things to do. Dygik, by inspection, his efficiency can't be as close to a fully immersed heating element as you're suggesting. There is going to to be double digit loss to the air and poor conductivity to the pot.

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I'd love to know why both my estimates and the experiment above are wrong. Please do the maths.

BTW, it's 1h20m or 80 minutes to boiling in the reported results, not 120 minutes. That gives a total efficiency of 77% for the entire heating process to boiling based on wort temperature (although if you look at the video, he's actually drawing 2240W at that point in time).

You are correct, I misread that. Still, 48 / 80 = 60% but I digress at this point. We don't know the final time because he didn't snap a photo of when it hit 97 - 99*C, if at all, which is the range I usually get a roiling boil in. Could have taken another half an hour or more for all we know. Time is an important factor in the efficiency calculation. So we have an important missing data point and as a result, no one here can be fully factual. at best there is a fudge factor due to the missing info.

Regardless of the math/science, I do think the experiment was a cool one and props to the OP. Do I think it's practical as a regular brew rig? no. That thing would be a ***** to clean and the pads cost too much compared to other heating options. Still an interesting project and I definitely see the uses in an apartment setting with electrical restrictions. I think a heat stick would be more practical in those circumstances though.

dyqik said:

I'd love to know why both my estimates and the experiment above are wrong. Please do the maths.

BTW, it's 1h20m or 80 minutes to boiling in the reported results, not 120 minutes. That gives a total efficiency of 77% for the entire heating process to boiling based on wort temperature (although if you look at the video, he's actually drawing 2240W at that point in time).

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As anticipated as follow up of my tests I'm trying to insulate the pot in order to increase efficiency and heat transfer

I have selected a very special material with incredible insulation power in a small thick : Aerogel

I got some foils 5mm from UK company

The blanket seems easy to cut ad adapt but is very powdery so don't know how to wrap and insulate

Any suggestions ?

BR
Davide

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I Davide
I'm thinking building a similar mash tun inspired by yours. Are you satisfied with the results? Is it feasible to apply the same solution based on silicone pads only to a dedicated 20 or 30 gallon mash tun? What do you think?

Thanks
Hugo