Drums with metal finishes are a poor choice for thermal radiation

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
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Arpi
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#1: Post by Arpi »

Hi all.

Thermal radiation (electromagnetic waves like light but in infrared region) occurs at various wavelengths between .7 and 50+ microns (wavelength of a millionth of a meter). In order for an object like a drum to absorb or emit thermal radiation, the surface needs _not_ to be a shiny metal surface. Otherwise it acts like a mirror as thermal radiation is mostly reflected in metals. For example metal shiny surfaces have an emissivity of only .02 (shiny copper), which translates in being a 99.08% insulator of thermal radiation (~ a perfect mirror). Non shiny metals also have very low emissivity, making metal drums a poor choice because they reflect most of the energy or they don't emit it either. While metals are excellent heat conductors (have excellent heat capacitance), they are terrible radiators-absorvers of thermal radiation. Thermal radiation has to do only with the top layer surface. For example if a metal drum were painted then it would transfer (absorb - emit) about 90% more radiation. If a metal had an external layer of oxide then it would have about 68% emissivity (much better when compared to a ~1%). So even if a electric roaster had a very powerful heater element, most of its radiation power would be wasted if the drum has a metal finish, as it would be reflected away. If the inside of the drum had also a metal shiny surface it would only conduct most of its energy by conduction only.

An efficient drum would need to have a non metal finish in the outside wall (to absorb the radiation) and also in the inside wall (to emit the radiation to the beans).

A solution is not easy. Maybe it could be to treat a metal drum with a top layer of a known safe substance to make it more radiation friendly. For example, a drum could be treated with an acid to darken it. Doing this would make the roaster more powerful while maybe using a smaller heater element, as a lot more energy would reach the beans (faster roast using less energy).

Cheers

germantown rob
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#2: Post by germantown rob »

The inside of a drum after seasoning will no longer be shiny. Even the outside of a SS perforated drum like the Hottop only is shiny for a few roasts.

Edit: I just peered into my Diedrich IR-1's drum and after 100lbs+ it is still shiny however the outside of the drum which sits above the IR burner is a flat black or dark grey coated.

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Arpi (original poster)
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#3: Post by Arpi (original poster) »

That's a good point. Just remember that the crud is your friend :) The more different than metal the surface, the better. A non metal surface is better than shiny but not very good either. It would be like 50% better but not 100%. Painting the drum would jumped the emissivity by a lot (in the 90s).

What would you do about the outer surface?

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farmroast
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#4: Post by farmroast »

I don't see where shinny or not matters inside the drum. Though some say a seasoned coating limits metal taste. My jury is still out on that one. Might matter with some formulas of metal. Outside it does matter somewhat but can't reflected heat be picked up and utilized in convection?
LMWDP #167 "with coffee we create with wine we celebrate"

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Arpi (original poster)
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#5: Post by Arpi (original poster) »

Hi Farm.

Paint half of a metal surface. Then heat the surface and put (but don't touch) your hand comparing the painted and unpainted surfaces. You will feel a huge difference, guaranteed. The painted surface emits ~ 90% more heat.

Cheers

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farmroast
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#6: Post by farmroast replying to Arpi »

What I'm saying is if you paint half of the outside surface(exposed to IR) yes the painted half will emit more heat inside but if you painted the whole outside and half the inside my guess is you would not feel any difference inside.
LMWDP #167 "with coffee we create with wine we celebrate"

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another_jim
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#7: Post by another_jim »

I'm not sure the drum surface is the only factor, there is also airflow and roaster insulation.

The merit in a dull surface would be if the same ET inside the drum could be maintained at a lower drum temperature, since it is radiating more heat. But if the airflow is very high, the drum temperature will be regulated by the airflow. Any lack of radiation would be compensated by the airstream picking up the extra heat. So this line of thought leads to the conclusion that the drum surface may be more critical in low ventilation designs.

The roaster insulation and heat gradients are another factor. If it is poor, the higher amounts of heat will be required to maintain the interior of the roaster, and heat gradients from source to sink will be higher. In this case, high radiation from the surfaces close to the heat source may be counterproductive -- they will scorch the beans. On the other hand, far away from the heat sources, where the surfaces are cool, a high radiation surface may be a good idea. For instance, it may be best for the drum exterior, close to the heat soruce, to be shiny, to keep it cool and redirect the source heat to the airflow, and dull in the interior, to radiate whatever heat it has to the beans.
Jim Schulman

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Arpi (original poster)
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#8: Post by Arpi (original poster) »

Hi

I agree that there are two other ways to transfer heat (convection and conduction). But maybe if we were to increase the radiation type, then we could maybe add more weight or shorten the total time.

The heat needs to first be absorbed by the drum, then the drum needs to transfer the heat to the beans. All the three types of heat transfer methods are usually present. The beans should be very good absorbing radiation. For example, when the beans are not touching the drum, they should still be getting hot.
The merit in a dull surface would be if the same ET inside the drum could be maintained at a lower drum temperature, since it is radiating more heat.
In the planck's curves of long wave infrared light, as the temperature increases, the spectrum radiated widens. That is, a higher temperature emits more different frequencies spread over a wider range (even visible light) but that does not lessen the infrared heat. It is like a growing bell curve.

I don't now if the beans would burn up. I would think that it would be controllable by maybe using less power to the heaters.

I would like to try this in practice, but I want to first find a food safe method to change the surface of my stainless steel drum.

Cheers

germantown rob
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#9: Post by germantown rob »

I have been wondering all day why my drum is so shiny inside after so many roasts. I am guessing that 1-2.2lbs of green beans are scrubbing it with their friction for the first few minutes of every roast. I am also thinking if I where to get much more heat transfer from the drum to the beans I would end up with scorching. Since my roaster has 20% airflow through it at all times the beans are benefitting from convection heat throughout the roast.

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DJR
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#10: Post by DJR »

This is an interesting thread. It also validates my disinclination to clean my drum. Just today I was thinking about sand blasting my new rusty hood. But now, I can tell people I left it rusty for valid thermodynamic reasons, which is great!


The hood is so efficient, I put a handle and tracks on it so after 1st crack I can easily remove it. Otherwise the roast progresses too fast. I may oxidize the outside of my stainless drum as well. Seems like a good idea. Less propane. A nice thing about building your own roaster is you can let it evolve and learn welding at the same time.

I suppose people who polish the bottoms of their copper pans are also making them less conductive, right? I have a copper jam pot which I'll never polish thanks to you!

dan