Possible Electric build with silicone heat mats

I'm in the begining stages of planning a electric build and I got two Wattco Slicone rubber sheets that can reach 450 degrees for free. So I thought I would use one for the MLT and one for the boil kettle. The plan is to wrap the heaters around the keggles so the entire walls will be heated. I was thinking it would be more efficient then using regular elements and I wouldn't need a RIMS or HERMS to keep the mash at temp.

Has anyone seen a system like this or can think of any issues with this plan?

This is a link to the heaters I will be using:
http://www.wattco.com/silicone-heaters.html

I used those in a former job. Never heard of them being used in brewing but it seems like it would work. Would be easy to test, so give it a go. Would love to hear the results.

It can be done, I know a guy that has a 30qt kettle that uses 3 silicone heating pads from McMaster. If I remember correctly, it'll bring 5 gal to a boil in about 1/2 hour. He did have it listed on ebay, but IDK if its still there.

I guess it depends on the voltage and wattage of the particular mats you have. The spec says up to 480V, which is not going to be available in your home. And if they are rated at 480V, running them at 240V will decrease the wattage by 75%. But like fifelee said, easy enough to test! Let us know.

If they don't work for the actual brew, they'd certainly make great warming belts for fermenters.

Here's the link at Mcmaster Carr:

http://www.mcmaster.com/#heat-blankets/=bnozzp

These are interesting. The highest wattage is 2160W so you could use it to boil but only a small (possibly 5 gallon) kettle. $584 however which makes them somewhat expensive compared to $50-75 or so to integrate a 5500W hot water heater element (as is done by many electric brewers).

Since yours were free, why not try them of course.

Kal

The guy I got the mats from uses them to heat a press that runs from 165 to 150 degrees 8 hours a day. He said if I ran them at 120v they could hit about 175 and at 220v they would easily go over 200.

Would the heat source coming from the sides change the physics of the boil?

Any ideas of what kind of insulation to put around the outside?
I'll wrap the heater around the kettle, then wrap a metal strip around the mat (completely covering the mat) to hold the mat tight against the kettle. Put insulation around that and seal it up. I need an insulation that can handle the direct heat without melting.

Shane23 said:

Would the heat source coming from the sides change the physics of the boil?

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Possibly, but not in any bad way. Boiling in commercial boil kettles is often with steam jackets that provides heat through the sides in the same manner. Just make sure that the heat source has wort on the other side of the kettle wall instead of air (ie: place the heat mats low enough).

Kal

Seems like a bit of an old thread... Curious if you tried these out... If so how did they work out???

Well unfortunately I haven't gotten to that stage of my brew stand build. Finding a keg for my mlt is next on the list so hopefully I will have it going soon.I've done some tests and I know the mat will work for the mlt running on just 120v. I don't think I'll end up trying it on a kettle for a while. As soon as I get it going I'll post soon pics tho

Interested to this thread...
Trying to understand if you have enough power with 220V power supply to bring to boil my single vessel all in one (BIAB) 5gal system.
The idea behind is to have uniform heating in all the phases

I have a 35cm diameter kettle with 30cm height so I have a rectangular surface of around 100cm x 30cm plus a circular surface on the bottom of 35cm so you have In total 3000-3500cmq of heating surface
Is this enough with the power density of silicon rubber heater for mashing/boiling ?

Regards
Davide

If it didn't work for boiling, that would probably make a great HEX tube setup for either instant-on mash water or HERMS. They'd also beat the piss out of reptile heaters wrt to a germ chamber build. The only thing I'd be worried about is overheating and fire I the OP's kettle water level got too low.

So... how did it go?

Would be great for wrapping around a RIMS tube I reckon.

It’s an idea that I have since long time ago: use external silicon flexible heater band in order to mange the mashing and boiling process

The advantages are more than one, have uniform heating without scorching the worth with immersion heater, have an internal surface without any interference for whirpool, easy cleaning etc…

I don't konw if there have been progress so far but now the idea is a reality and prototype, I have requested some custome silicone heater to a specialized company and I have mounted on my BIAB pot
The bottom pud is around 800W @ 220V while the lateral band is 1700W
They are self-adhesive with special 3M glue and easy to install







First test are promising , I would like to run a full test from cold water to boiling logging time/power/temperature and share the result

stay tuned
Davide

Good luck with your experiment! The idea with elements is that you are heating the water directly, therefore more efficient and quicker heating. That is the major benefit (as well as enhanced temperature control) versus external heat sources where you have to heat the vessel prior to heating the water/wort.

Your design should certainly work for mashing, but you have a watt shortage for boil applications (consider most electric setups use a 4500 - 5500 watt element). From 45 degree water, it takes my 4500 watt BoilCoil around 45 minutes to reach boiling. For my 5500 watt ripple elements it takes about 25 - 30 minutes to do the same. My pots are 15 gallon SS, typically boiling a volume of 7 or 8 gallons.

For the mash you'd need to make sure you recirculate to ensure an even temperature.

Since you mentione BIAB, I'm assuming you're doing 5 gallon batches at most? Unfortunately the 2500W you have for the boil kettle isn't probably going to be enough, especially since you do not have all of the heat going directly into the kettle as the previous poster mentioned. Much of the heat will simply escape in the other direction.

The other thing I'd be concerned about is the fragility of the connection points when cleaning the kettles (the wires look easy to break off) not to mention getting the pads wet/difficulty in cleaning (a boil over would make a mess of the pads), and issues with having the glue eventually no longer hold/drying out.

Using a HERMS setup with indirect heating of the mash never causes scorching, nor is scorching a problem in the boil kettle with ULWD elements (many use higher density elements as well without issues). If scorching was a big issue, then we wouldn't have the tens of thousands of brewers using electric heating elements that we do today. Some of the professional brewers I've talked to (ex: WingMan Brewers, Tacoma WA) even tell that they have less caramelization since they switched to electric heating elements.

You can still get a whirlpool going with an element in the way from what I understand.

I'm still curious to hear your results however. Please do post! Good luck!

What did the pads cost you for one kettle?

Kal

trboyden said:

Good luck with your experiment! The idea with elements is that you are heating the water directly, therefore more efficient and quicker heating.

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You are right the best efficnecy can be achived by immersione heating

That is the major benefit (as well as enhanced temperature control) versus external heat sources where you have to heat the vessel prior to heating the water/wort.

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Not clear to me this point , in term of control you can apply the same strategy and systems (Typically PID/PWM with SSR)

Your design should certainly work for mashing, but you have a watt shortage for boil applications (consider most electric setups use a 4500 - 5500 watt element). From 45 degree water, it takes my 4500 watt BoilCoil around 45 minutes to reach boiling. For my 5500 watt ripple elements it takes about 25 - 30 minutes to do the same. My pots are 15 gallon SS, typically boiling a volume of 7 or 8 gallons.

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Maybe in US everything is "big" but here in italy the maximum household power is 3KW and off course you can't use all for the electrical brewing pot.
Average power of EU brewing system is between 1.8 to 2.3 KW (braumeister, bielmeier..) for 20L batches

Indeed also a insulation of the external part could help a lot to increase efficiency and heat transfer to internal

I let you know my experimet result

Davide

I've toiled over external electric elements as well. I use a 940W thermal wire to maintain mash temps, but its very inefficient for heating water to strike temps or boiling. A wire wrapped tight on a copper tube van work well for RIMS, and can be enough to step mash. But you have to remember that external elements are only going to be 33-50% efficient. You'll need 4500W+ just to get 2000W of heating power to the wort. It can be done but you'll be heating the room more than the beer. Your setup is 1500W, so your wort is seeing 500-750W.

kal said:

For the mash you'd need to make sure you recirculate to ensure an even temperature.

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This is for sure Kal..the system is a BIAB with recirculation from bottom to top

Since you mentione BIAB, I'm assuming you're doing 5 gallon batches at most? Unfortunately the 2500W you have for the boil kettle isn't probably going to be enough, especially since you do not have all of the heat going directly into the kettle as the previous poster mentioned. Much of the heat will simply escape in the other direction.

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Yes you are right the idea is to mange 5gal batches, not sure that 2,5KW is not enough for boiling…I had in the past (long time ago &#61514 may brewday with my glorious bielmeier pot that is 1.8Kw and indeed also the well known 20L speidel braumeister is not much more powerfull with heating coil of 2KW and no insulation !

The other thing I'd be concerned about is the fragility of the connection points when cleaning the kettles (the wires look easy to break off) not to mention getting the pads wet/difficulty in cleaning (a boil over would make a mess of the pads), and issues with having the glue eventually no longer hold/drying out.

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This is good point ..indeed the wires are very flexible and well anchored and my idea is to use a power connector to the control unit in order to plug and unplug for vessel removal and cleaning. About wetting the external surface you are right and you must be very conscious unless you use some insulation/cover water proof (neoprene??)

Using a HERMS setup with indirect heating of the mash never causes scorching, nor is scorching a problem in the boil kettle with ULWD elements (many use higher density elements as well without issues). If scorching was a big issue, then we wouldn't have the tens of thousands of brewers using electric heating elements that we do today. Some of the professional brewers I've talked to (ex: WingMan Brewers, Tacoma WA) even tell that they have less caramelization since they switched to electric heating elements.

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This is a matter of philosophy ..In HERMES systems you need at least two vessels and this is NOT my project requirements that is strictly all in one single vessel compact/light/easy to use and store. About scorching I can partially agree with you…is not a very big issue but the idea behind my solution is to have an ALTERNATIVE to the traditional brewing sustem (single vessel) with external heating helmet…I mention again Speidel Brumeister and Bielmeier that has 2Kw resistor on the bottom. Compared to that reference sytesm I assume that my heating is better…with some cons discussed above..but at the end of the day better.

You can still get a whirlpool going with an element in the way from what I understand.

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Depending of the heating element type and placement whirpool and cleaning can be more difficult than a completely empty and clean vessel

I'm still curious to hear your results however. Please do post! Good luck!

What did the pads cost you for one kettle?

Kal

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Sure I will keep you updated !

Davide

Weezy said:

I've toiled over external electric elements as well. I use a 940W thermal wire to maintain mash temps, but its very inefficient for heating water to strike temps or boiling. A wire wrapped tight on a copper tube van work well for RIMS, and can be enough to step mash. But you have to remember that external elements are only going to be 33-50% efficient. You'll need 4500W+ just to get 2000W of heating power to the wort. It can be done but you'll be heating the room more than the beer. Your setup is 1500W, so your wort is seeing 500-750W.

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same answer than to other people complaining with the low efficiency

how does the two widespread systems work ??

BIELMEIER
http://www.bielmeier-hausgeraete.com/bielmeier-maische-und-sudkessel-edelstahl-040001-1

BRAUMEISTER
http://speidels-braumeister.de/shop/en/20-50-Litre-Braumeister/20-litre-Braumeister


they have maximum 2Kw heating coil on the bottom without any insulation and are used by thousands of HB

Davide

You seem to think the debate is that we are saying it can't be done with 2000 watts. But no one is saying that. We're just saying it won't be efficient. You have to factor in all the math; starting water temperature, the differential between that temp and boiling temp, the volume of wort, how many watts you have, and time.

Also you are heating externally - those devices you mention use an element in the pot, therefore they are more efficient because they are just heating the water. They are also insulated. So they can get away with only 2000 watts. But it probably takes them at least 2 hours to get to boiling (and the instructions say they aren't necessarily able to get to 100*C - see page 4 in the manual). They are also designed for exactly 20L. Most US home brewers make a compromise and use 15 gallon pots so they can do 20L or 50L batches, so they size their elements for the larger batch size. Which means they can do a 20L batch in a shorter period of time.

Awhile ago I made up a calculator using Microsoft Excel to calculate how much time it would take to get wort to boiling. It's ballpark because it doesn't take into account external influences such as ambient temperature or the material of your pot/insulation factors, but I have found it to be pretty accurate. Here is the link to the file via my OneDrive account:

Electric Brewing Calculator

Cheers,

Tim

mysobry said:

same answer than to other people complaining with the low efficiency

how does the two widespread systems work ??

BIELMEIER
http://www.bielmeier-hausgeraete.com/bielmeier-maische-und-sudkessel-edelstahl-040001-1

BRAUMEISTER
http://speidels-braumeister.de/shop/en/20-50-Litre-Braumeister/20-litre-Braumeister


they have maximum 2Kw heating coil on the bottom without any insulation and are used by thousands of HB

Davide

Click to expand...

trboyden said:

You seem to think the debate is that we are saying it can't be done with 2000 watts. But no one is saying that.

Click to expand...

I am saying that....because:

trboyden said:

Also you are heating externally - those devices you mention use an element in the pot, therefore they are more efficient because they are just heating the water.

Click to expand...

Mysobry, you need to recognize the significance of the difference between immersed heating elements and external heating elements. The Braumeister has a 2000W immersed coil, so its transferring ~90% of its energy right to the liquid. So the wort sees about 1800W, and people have said they need insulation to keep a rolling boil in those. External elements, just like an electric stove, are very inefficient because a lot of the heat is just lost to the air and the rest is transferred to the pot, then some of that finally transfers to the wort. They are 33-50% efficient typically. Your 1500W is maybe getting 750W to the wort. You can maybe boil 8 liters.

Play with Tim's spreadsheet. He did a nice job on that.

But enough of the number crunching. Plug the sucker in and let's see that boil!

Yup. Math be damned!

I'd also be curious to hear how much these pads cost to do a typical ~6-8 gallon kettle as shown in the pictures.

Kal

kal said:

Yup. Math be damned!

I'd also be curious to hear how much these pads cost to do a typical ~6-8 gallon kettle as shown in the pictures.

Kal

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Over in the automation section Trimixidiver1 uses band heaters on his mashtun, looking on ebay you can get one relatively inexpensively to wrap around a keggle

https://www.homebrewtalk.com/f235/bucks-electric-brewery-479895/index4.html

you can see what he uses 1/2 way down the page

Las Sunday I have performed as promised some tests with current configutation:

25L of water , silicone heater direct connected to power supply (220V AC) without any controller

Recirculating pump always on, digital temperature probe and a wattmeter on the main supply



Every 10 minutes I have a temperature sample as you can see in the picture



The rump up is quite good with more than 1C per minutes

After 1H 20’ the water start boiling and even if it is not a roll boiling like gas the evaporation was quite good.
The video shows you the result

http://youtu.be/_kmqVdywYiI

The power absorption was around 2200W



I can conclude that the technology is promising and adding a good thermal insulation can be used effectively for small volume and compact systems
Next steps adding the ArBir controller and complete insulation

Stay tuned

That's 80% efficiency. That's quite remarkable for external elements. Manufacturers I've spoken to have never rated their stuff over 50%.

congrats! i'm glad it worked. Nice simple and clean setup.

Not sure where you are coming up with your efficiency factor or what you are basing it on, but heating in general with electricity tends to be highly efficient compared to other heat sources. That is because it has a low energy content (3,412 BTU) per unit of measure (kilowatt) versus a fuel source such as propane which has 92,000 BTU per gallon. So while a high efficiency rating sounds good, it's not the whole picture.

It was a good test and I am glad the OP had success. But I look at it from the perspective of the typical home brewer:

• How easy is it to clean?
• How much does it cost?
• How long does it take versus other methods?
• How hard is it to put together/use versus other methods?
• Does it overcome any other environmental issues I have that prevent me from using another method?

I think if you step back from the novelty of this method; cleaning, cost, and time are big negatives. But for someone that has restrictions like apartment dwellers, this definitely has some promise versus not brewing at all.

Weezy said:

That's 80% efficiency. That's quite remarkable for external elements. Manufacturers I've spoken to have never rated their stuff over 50%.

congrats! i'm glad it worked. Nice simple and clean setup.

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Eh? How does the size of the unit of measure affect the efficiency? That's just nonsense. Efficiency is heat energy into wort/energy used. Nothing more or less.

Even when calculating cost, the size of the unit doesn't matter, because units are divisible in both cases. The cost per unit is a factor, but not the size of the unit.

Shane23 said:

Would the heat source coming from the sides change the physics of the boil?

Click to expand...

I participated in a yeast experiment with a local brewery. They brewed, gave us a 5.5 gallon keg and a different white labs yeast. I think about 20 or 25 different ones total. They kept all these.
In the end there was left over wort, and a bunch of home brewers with empty carboys, they all got filled!:rockin:
^


Anyway... Their kettle was copper and steam fired. Aka, a copper pot, with a jacket around the outside where steam was pumped in to. We got to climb up and peek in the hatch, it boiled from the sides and not the bottom, it was pretty cool.

I think a side boil will be fine! also this is a pretty cool idea!

trboyden said:

Not sure where you are coming up with your efficiency factor or what you are basing it on, but heating in general with electricity tends to be highly efficient compared to other heat sources. That is because it has a low energy content (3,412 BTU) per unit of measure (kilowatt) versus a fuel source such as propane which has 92,000 BTU per gallon. So while a high efficiency rating sounds good, it's not the whole picture.

Click to expand...

You're misunderstanding the efficiency we're talking about. You're thinking about how efficient electric elements are at converting electric energy to thermal energy. The efficiency is how effective the element is at transferring that energy to what you want heated. In this case it's the transfer of energy from the heat pad, through the metal pot, to the wort inside. There are energy losses as the heat energy much move through and between each material (generally referred to as losses 'to the environment'). An immersed water heater element is 90%+ efficient because it is directly heating the wort and is completely immersed in what we want heated. Here's a silly example to explain the differences. Say you have a small, electric, ceramic, space heater with a fan in it. You use it to heat a small bathroom. It works well enough since the room is small. It's working at, lets guess 75% efficiency. Now put that heater into a cardboard box inside that room and see how well it warms the room, through the box. That box is going to get really hot and it will warm the room somewhat, but the efficiency of this system is going to be much, much lower than the unhindered space heater. Maybe the efficiency is now 20%.

Weezy said:

You're misunderstanding the efficiency we're talking about. You're thinking about how efficient electric elements are at converting electric energy to thermal energy. The efficiency is how effective the element is at transferring that energy to what you want heated. In this case it's the transfer of energy from the heat pad, through the metal pot, to the wort inside. There are energy losses as the heat energy much move through and between each material (generally referred to as losses 'to the environment'). An immersed water heater element is 90%+ efficient because it is directly heating the wort and is completely immersed in what we want heated. Here's a silly example to explain the differences. Say you have a small, electric, ceramic, space heater with a fan in it. You use it to heat a small bathroom. It works well enough since the room is small. It's working at, lets guess 75% efficiency. Now put that heater into a cardboard box inside that room and see how well it warms the room, through the box. That box is going to get really hot and it will warm the room somewhat, but the efficiency of this system is going to be much, much lower than the unhindered space heater. Maybe the efficiency is now 20%.

Click to expand...

Actually, it'll be exactly as efficient in the steady state. Where do you think the rest of the energy is going? All of the net energy going into the cardboard box must end up as heat in the room.

Space heaters in closed rooms operate at 100% efficiency. Every bit of energy that enters the room and doesn't leave as electrical energy is converted to heat in the room.

The heater issue the cardboard box will reach equilibrium more slowly, but it will be 100% efficient.

The efficiency losses in immersed elements are due to the base of the element being heated and losing heat energy that doesn't go via the wort. If you could put the element on a cable into the wort and immerse the base, it would be practically 100% efficient (only losses would be resistance in the cable before it enters the wort). For silicone heating mats, the losses are due to heat being lost from the outside of the mats directly to the air and table. The overall kettle efficiency in both cases needs to also include the heat losses from the walls and top of the kettle as distinct from the heat energy that goes into increasing the temperature of the wort.

No, I understood the efficiency you meant, which is why I questioned the 80%. If an immersed element is 90%, how can an external one be 80% that has to heat both the pot and the water. That doesn't pass the smell test. I also question the results. The amount of energy needed to boil water/wort is measured in Joules. For 5 gallons, you'd need 38029761.11 Joules to bring the 5 gallons to a boil. If the OP really is only using 2200 watts of energy from his mats, there is no way that could be done in less than 288 minutes (Joules required/2200 watts = seconds / 60 = minutes). And that's assuming 100% thermal transfer to the water/wort. So 360% efficiency? No. There is missing variable(s) that is skewing the results.

It's a thermal physics equation. http://www.physicsclassroom.com/class/thermalP/Lesson-2/Measuring-the-Quantity-of-Heat

Ugh. Yes the cardboard box example was bad. A straw man argument doesn't change the fact that external elements experience energy loss to the environment. Period. An electric hot plate, heating a can of soup, sends more heat into the room than into the pot of soup. Period. How well an external element is attached to what you want heated is critical in how efficient the thermal energy transfer will be. These silicone pads obviously experience energy loss to the air....I'd bet $100 that they're too hot to touch on the outside when running, yes? That's energy loss to the environment. That's energy not going to heating the pot or wort. That's a reduction in the efficiency of the system.

The only real difference between the silicone pad case and the immersed element case is in the difference in temperatures between the outside of the pot in the immersion element case and the outside of the heating mats in the silicone pad case, and the relative heat transfer coefficients to the air for the two materials. Everything else is the approximately the same.

If the silicone pads have a relatively high heat transfer coefficient to the wort (they are glued to the pot over a large surface area, and a recirculation pump is used to stir the wort), then they could approach the efficiency of an immersed element - the temperature of the pads would be approximately the same as that of the wort. The losses from the pot in the immersion element case would then be about the same as the losses from the pads in the silicone heating pad case. 80% efficiency doesn't seem unreasonable at all to me, and 90% or higher would also be plausible in still air (which would lower the heat transfer coefficient to the air).

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.

trboyden said:

For 5 gallons, you'd need 38029761.11 Joules to bring the 5 gallons to a boil.

Click to expand...

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.

dyqik said:

The only real difference between the silicone pad case and the immersed element case is in the difference in temperatures between the outside of the pot in the immersion element case and the outside of the heating mats in the silicone pad case, and the relative heat transfer coefficients to the air for the two materials. Everything else is the approximately the same.

If the silicone pads have a relatively high heat transfer coefficient to the wort (they are glued to the pot over a large surface area, and a recirculation pump is used to stir the wort), then they could approach the efficiency of an immersed element - the temperature of the pads would be approximately the same as that of the wort. The losses from the pot in the immersion element case would then be about the same as the losses from the pads in the silicone heating pad case. 80% efficiency doesn't seem unreasonable at all to me, and 90% or higher would also be plausible in still air (which would lower the heat transfer coefficient to the air).

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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Not to interrupt your scientific argument but
In reality, isnt half the heating pad or element surface exposed to air and therefore being wasted by transferring /releasing half the heat energy to the air outside the pot?

How can this possibly be compared to the efficiency of an immersion element where 100% of the element surface area and heat is in direct contact with the liquid being heated? id say 96% or more of the heat generated is directed to the liquid vs 50% in the heating pad and thats before you take into effect the cooling properties of the ambient air against the pad that the element must overcome?

augiedoggy said:

Isnt half the heating pad or element surface exposed to air and therefore being wasted by transferring /releasing half the heat energy to the air outside the pot?

How can this possibly be compared to the efficiency of an immersion element where 100% of the element surface area and heat is in direct contact with the liquid being heated?

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No, the heat energy losses from each side of the heating pad are not equal, because the heat transfer coefficients are different. Heat transfer by conduction to a well stirred high heat capacity medium like wort is much much more rapid than heat transfer to a low heat capacity medium like air that is stirred only by convection.

dyqik said:

Eh? How does the size of the unit of measure affect the efficiency? That's just nonsense. Efficiency is heat energy into wort/energy used. Nothing more or less.

Even when calculating cost, the size of the unit doesn't matter, because units are divisible in both cases. The cost per unit is a factor, but not the size of the unit.

Click to expand...

From what I understand size can have a lot to do with efficiency.... how efficient would it be to heat your house with a small undersized furnace vs the correctly sized one?

dyqik said:

No, the heat energy losses from each side of the heating pad are not equal, because the heat transfer coefficients are different. Heat transfer by conduction to a well stirred high heat capacity medium like wort is much much more rapid than heat transfer to a low heat capacity medium like air that is stirred only by convection.

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If that were soo true than insulating a kettle would have little effect on efficiency... all one has to do is wrap a towel around a heating kettle to see it does in fact have a huge impact.... The same principal with a cooler of ice... what am I missing here? what your saying does make sense but I think how much efficiency/ energy is lost is still greater than I believe you are insinuating.

The liquid itself is a better conductor and if all the energy is with in it...

plus how much energy is lost by just the poor surface tranfer from the rough silicone surface on the mat and the surface of the pot? we all know how much difference even thermal paste makes in transferring energy from one medium to another? (yes because of your point about air being a poor medium)

augiedoggy said:

if that were soo true than insulating a kettle would have little effect on efficiency... all one has to do is wrap a towel around a heating kettle to see it does in fact have a huge impact....

The same principal with a cooler of ice... what am I missing here? what your saying does make sense but I think how much efficiency/ energy is lost is still greater than I believe you are insinuating.

The liquid itself is a better conductor and if all the energy is with in it...

plus how much energy is lost by just the poor surface tranfer from the rough silicone surface on the mat and the surface of the pot? we all know how much difference even thermal paste makes in transferring energy from one medium to another? (yes because of your point about air being a poor medium)

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Sorry, I have to completely edit my post because of your changed questions.

The "half the power going each way" thing is just plain wrong as a starting point. It would be right for a heating pad with similar volumes of wort or similar volumes of air on each side, but that isn't the case here at all.

Taking still water and air on either side of a heated plate as the starting point, the thermal conductivity of water is 20 times higher than that of air. The heat lost to the water will be at least 20 times greater than that to air, and actually much more because water also has a much higher heat capacity. You know this - if you drop a piece of hot metal into water it will cool much much more rapidly than if you leave it hanging by a thread in mid air. Even if you have only small pot of water and a much bigger volume of air so that they have equal heat capacities.

You need to do the calculations properly, and you'll need the equations here. Note that the heat transfer to the wort is given by the "Forced convection, external flow, vertical plane" equation, while I think the heat transfer to still air is governed by the "Internal flow, laminar flow" equation, modified for an approximately flat plate, and by something like "External flow, vertical plane" for the blown air case. The Prandtl and Rayleigh numbers will be different for air and water as their densities, viscosities, speed, thermal conductivities, etc. are all different so the heat transfer coefficients will be different.

In particular, the Prandtl numbers of air and water are different by a factor of 10 (and the difference will be higher for a more viscous wort). These are raised to something like the 2nd power in the empirical equations for the heat transfer coefficient. There are other differences in other parts of the equations as well.