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Old 04-01-2009, 02:25 PM   #21
The Pol
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Well if you are determined to use a dimmer, then do it. Let us know how it goes. No one has any experience here with that sort of thing.

If the SSR does recognize it, I am not sure it will be able to switch that quickly. In a normal system the SSR will be running a 1 second duty cycle... since you dont see your lights flicker in your house with a dimmer, it must be running hundreds of duty cycles per second, not sure what that will do to an SSR.

Id recommend getting an oversized SSR and heat sink, switching 100's of cycles per second is definately going to create some heat and wear on the SSR.

Do it, let us know how it works.

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Old 04-01-2009, 02:33 PM   #22
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Thanks! 3,500 watts is a very believable number since conduction and radiation losses over such a large area and thermal gradient are not going to be negligible. If I get some free time this evening I might make a first pass approximation to see if these values calculate out to be around 1,500 watts.

On a side note, when I switch my boiler to electric this fall I'm going to use some sweet heat blankets from McMaster Carr and some serious insulation in order to try to minimize the heat loss through conduction and radiation. The goal is to do it all for 2,500 watts as it is very easy to place this much heat around my boil kettle. Plus, why pay for the electricity if I don't need to? This would drop the cost of fuel for each batch down to less than $0.50. Its always fun to do more with less and it will pay out after around 4,000 batches! Good thing this is a hobby and not a business venture!

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Old 04-02-2009, 04:11 PM   #23
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So I did a little boning up on heat transfer through radiation and finally got the equations producing reasonable numbers. Below is the radiative heat loss for my boiler based on a 70F room, a highly polished aluminum boil kettle, and an assumed skin temperature of 190F. I was very surprised to see that the vast majority of radiated energy is coming from the wort rather than the boil kettle itself.



If anyone truly wanted to minimize heat loss while not risking DMS suspend an inverted shallow cone made of aluminum flashing a few inches over the rim of the boiler. This will allow condensing DMS to drip out but a lot of the radiated energy will bounce back into the boiler.

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Old 04-02-2009, 04:16 PM   #24
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That really surprised me! I thought that all of these ppl insulating thier SS tanks were mitigating the problems with insulation... you are saying that they are putting a bandaid on a gunshot wound.... ? Wow... cool.

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Old 04-02-2009, 04:28 PM   #25
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So I did a little boning up on heat transfer through radiation and finally got the equations producing reasonable numbers. Below is the radiative heat loss for my boiler based on a 70F room, a highly polished aluminum boil kettle, and an assumed skin temperature of 190F. I was very surprised to see that the vast majority of radiated energy is coming from the wort rather than the boil kettle itself.
Interesting. That seems really low though. I don't know the calculations myself but I think the skin temp (especially in an aluminum kettle) will be rather close to boiling since it is such a good conductor. Still doesn't explain the 1500W difference between the original (just boiling off water) calc and The Pol's experience. I've run my 5500W element as low as 50% but the boil does pulse a little bit. I haven't run enough batches to nail down how much I boil off though so it could be The Pol boils off more water.
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Old 04-02-2009, 04:41 PM   #26
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That really surprised me! I thought that all of these ppl insulating thier SS tanks were mitigating the problems with insulation... you are saying that they are putting a bandaid on a gunshot wound.... ? Wow... cool.
My textbooks claim that at temperatures around boiling radiative heat loss and natural convection are roughly equal so one might expect convection to be good for another 500 watts or so since polished aluminum has an incredibly low emissivity.

Insulating the boil kettle will tremendously reduce convection losses since rather than an outer skin temperature of 190F to 212F you can get a skin temperature very close to room temperature. Then your biggest loss becomes radiation from the surface of the wort itself (well after mass loss through boiling water of course!).
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Old 04-02-2009, 04:47 PM   #27
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Interesting. That seems really low though. I don't know the calculations myself but I think the skin temp (especially in an aluminum kettle) will be rather close to boiling since it is such a good conductor. Still doesn't explain the 1500W difference between the original (just boiling off water) calc and The Pol's experience. I've run my 5500W element as low as 50% but the boil does pulse a little bit. I haven't run enough batches to nail down how much I boil off though so it could be The Pol boils off more water.
I run my 5500W element in the keggle at about 60-65%... 3300W to 3575W.

I get 1.5-1.6 gal/hr boil off
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Old 04-02-2009, 04:50 PM   #28
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Interesting. That seems really low though. I don't know the calculations myself but I think the skin temp (especially in an aluminum kettle) will be rather close to boiling since it is such a good conductor. Still doesn't explain the 1500W difference between the original (just boiling off water) calc and The Pol's experience.
But The Pol doesn't have a means of measuring how many watts are required other than 2,200 isn't enough but 3,300 is. There is a lot of gray area there!
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Old 04-02-2009, 04:51 PM   #29
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I run my 5500W element in the keggle at about 60-65%... 3300W to 3575W.

I get 1.5-1.6 gal/hr boil off
You're boiler is also stainless steel and much larger than mine, so you would produce a decent amount of convective and radiative heat loss compared to me. Just for fun, what is the ID, OD, and height of your boiler? Let me see what I get for you.

EDIT: I missed that 1.5-1.6 gal/hr boil off rate. It takes 3,643.7 watts just to accomplish that rate of mass transfer out of the boiler. I'd say theory and experience are jiving pretty damn well now!
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Old 04-02-2009, 04:53 PM   #30
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Let me meaure... BRB

OKAY...

15.75 OD
15.5625 ID

20" deep...

I cant measure how many watts it takes, but I DO know that below 60%, the boil is pretty anemic.

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