Double wall drum

[Axel]

From how I read it... a double wall drum configuration has to do with there being two layers that make up the drum.
from the very vague and confusing descriptions(with seemingly more grammatical errors than even my writing has) on manufacturers websites, I feel led to believe there is something like a single cast Iron outer layer surrounding a steel inner layer.
according to one manufacturer the cast Iron distributes the heat evenly while the steel allows for rapid changes in roast profile...

..back to the question:
What I'm trying to figure out is how the two layers("double wall") are physically set in relation to each other.
Are they in full contact with each other?
Or is there a space between the two layers for air circulation?

Or.. does the "double wall" really just refer to the front and rear face plate, which I noticed on the related machines are made of cast Iron.
seems like making the drum itself out of two separate layers would make very little sense if you want the inner drum layer to get decent heating action.
unless it's something like a copper coated frying pan...

I guess a simpler way of asking my question may be:
Are the flames from the burners basically just heating the carbon steel drum directly, like having a carbon steel frying pan on a stove?
or is there a cast Iron drum layer encapsulating the carbon steel layer, like having a smaller carbon steel frying pan inside of a cast iron pan which is on the stove?


Is there a diagram or photo of a double wall drum out there?

 

[peterjschmidt]

If there are in fact double-walled drums, they are simply a way for a manufacturer to distinguish themselves from the rest of the industry. Don't buy into the hype. Flames heat the drum directly - flame is in contact with the metal - and the row of flames produces a uniform area of heat, and because the beans are agitating constantly and uniformly they are not in contact with any one portion of the drum even if it did have hot spots.

I repeat, don't buy into the hype. Even the cast vs mild steel debate is pointless. Of course, I admit I could be wrong, but common sense tells me I'm not.

I also repeat, buy a roaster. Sorry if I sound like I'm trying to tell you what to do; I'm not, but I think you'd have a much easier time. Either a cheap, dilapidated unit that doesn't work so you can simply copy some aspects. Or, something that is operable, that you can use to get your business up and running, that you can copy while using it. Most things I make (and I make a lot of stuff) I make a prototype first and then tweak it for the final 'whatever it is'... sometimes, the prototype is pretty close and I'm too lazy to go over a flaw, so it just goes into use. But there's almost always something about what I made that I would change. What you're trying to do is in effect to reinvent the wheel by figuring out how to make a roaster. But that's already been done for you. I can almost guarantee, if you make it from scratch, even though it will be satisfying to say 'I made that' there will be things you would have done differently. After you settle on a drum material and dimensions, you'll be wondering about blower speeds and airflow control, and the dump chute, and this or that. I admire your energy, and when I was younger and had access to the tools I would have done the same thing. I'm only saying you'll pull out less of your hair if you have a roaster in front of you to work off of.

One last point... and since I'm not a moderator this carries no weight at all... you might consider having just one thread for your project once it gets going, or even now. It'll make it easier for people to search if they want to follow your footsteps.

 

[Axel]

Thank you peterjschmidt.

I too have built various prototypes and have had to deal with things I would change. seems to be a part of the creative process...
basically I think I understand what you are saying.

However, a roaster to base my prototype on is something I do not have the luxury of owning or operating at this time(nor can afford$$$), and yet I would like to get my roasting/brewing business off the ground ASAP.

Seems to me like having a well proportioned drum on a heavy shaft powered by a properly geared motor and heavy bearings is the foundation.
having good blower control that draws air past the burner(with decent flame control), in through the back of the drum, out the front top, and back to a chaff collection bin before going out the chimney, is the next step.
Also a good sight glass and bean tester along side a thermometer is elemental for feedback.
and finally a powerful blower to quickly cool the beans in a large diameter perforated tray using appropriately angled apposing fins rotating from the center of the tray pushing the beans so that they eventually can all go out the trap door when ready.
Did I miss any of the basic components?

Unless I'm building a cheap scrap prototype, I generally overbuild and design with the various inevitable(and unpredictable..) modifications/adjustments in mind.
Which is not to say that there will not be any "could have made that part different" moments, but hopefully they will be few and not serious enough to teardown the whole roaster and rebuild any major components.

I will take your wise suggestion of consolidating my project to ease information sharing under advisement.
however, as I have not yet begun to build and am still only in the first stages of design, my strategy so far has only been to seek answers to questions by implementing a broad net of inquiry..

 

[peterjschmidt]

Do you at least have some roasters not too far away, that you could go see (and maybe watch in operation)?

 

[Axel]

Do you at least have some roasters not too far away, that you could go see (and maybe watch in operation)?

Yes maybe one or two.

seems like there is very little info about commercial roaster drums online...

 

[Hankua]

15 kilo: Joper Cast Iron Shop Batch Roaster
Joper and Probat have double drums according to CoffeeTec

 

[wwcove]

Actually the double wall drum is really not just a bunch of hype... Read Scott Rao's book: The Coffee Roasters companion and you will see from his decades of experience he recommends the double walled drum... Unfortunately they are very expensive... I do not own a unit with one. I do have several with very thick stainless steel drums (310S seems the the best for minimal expansion and heat endurance) and I own two cast iron drums which I prefer over stainless ( Both are good). A double drum is 2 layers of steel separated by several mm of air between them. Direct flame hits the outer drum while inner drum remains cooler and produces more even heating. A double drum reduces conductive heat and reduces risk of scorching beans... With that said I personally have never scorched beans in a stainless or cast iron single layer drum...:lol:

 

[Axel]

A double drum is 2 layers of steel separated by several mm of air between them.

indeed there seems to be such a thing,
here is a page by Scott Rao that I found on google:

do all of the new Probats have double walls?
any idea what thickness the individual walls are?

 

[Hankua]

I'm curious how your planning on drum fabrication? I've noticed some brands of roasters have a steel band welded around the front of the drum where it contacts the faceplate. My little cast iron drum has this feature and it was trued on a lathe, for a very precise fit. The faceplate was also machined with a recess where the dump door fits it. As a result it drops very little or almost no chaff into the collection tray beneath the drum.

 

[Axel]

a steel band welded to the faceplate may be a good idea.
extra chaff in a collection tray is ok by me if it means the build is much simpler to complete.
I do not have a lathe at the moment.
I do have a 4'x8' cnc/router table which I built for producing a series of original aluminum folding chairs.
It will pierce mild steel up to 1/2" thick.

 

[MillCityRoasters]

If you want to see how a Probat drum is put together, watch this:
https://instagram.com/p/5UqhvyD45i/

Double wall drums are useful for production roasting because they allow slightly higher drop temps without tipping. They are also slightly slower to react than a single walled drum. This means they are marginally more limiting for some artisian roasting profiles. Drum construction is only 1 piece of the equation. Air flow, burner diffusion, drum speed, mechanical fluidization, heat output, etc. all come into play. There's no "best" idea, only successful or unsuccessful integration/ imitation.

You can do this and I have no doubt you'll turn green beans brown, but its going to take some time and refinement to put a machine together that gives you the control a good commercial drum roaster allows.

I pm'd you with my phone number. If you have any questions, you can call me.

Steve

Whoops. Instagram cut the video short. What you're watching is Paul beat the spider out from the end of the inner drum. You can see the darker outer drum on the other end.

 

[Axel]

Nice! thank you Steve.
Any way of seeing the rest of the video?

 

[ellatas]

The theory of the double walled (or insulated) drum is the following: Most of the heat applied to the coffee is done through forced convection via the air. At the last third of the roast, many like to slow the "rate of rise" (ROR) or slope on the time vs temperature curve. This is especially the case when you are in the exothermic phase and you want to prevent run-away. The fastest way to moderate the heat to the coffee is to reduce the heat in the air (by either reducing the air temperature or air flowrate). If the drum was not insulated, the high drum temperature would become a more dominant heat transfer pathway and impede the reduced heat applied to the coffee (potentially causing an uneven roast and tipping). The double walled drum allows for the drum temperature to be a more stable and lower temperature allowing the air to be the dominant heat transfer pathway.
That being said, many produce great coffees on single walled drums

 

[Axel]

The theory of the double walled (or insulated) drum is the following: Most of the heat applied to the coffee is done through forced convection via the air. At the last third of the roast, many like to slow the "rate of rise" (ROR) or slope on the time vs temperature curve. This is especially the case when you are in the exothermic phase and you want to prevent run-away. The fastest way to moderate the heat to the coffee is to reduce the heat in the air (by either reducing the air temperature or air flowrate). If the drum was not insulated, the high drum temperature would become a more dominant heat transfer pathway and impede the reduced heat applied to the coffee (potentially causing an uneven roast and tipping). The double walled drum allows for the drum temperature to be a more stable and lower temperature allowing the air to be the dominant heat transfer pathway.
That being said, many produce great coffees on single walled drums

What about the theory that roasting primarily using convection produces a roast with less body?
Would the single walled drum produce more body verses the double wall drum highlighting more of the floral and high notes?

doesn't radiant heat off the drum wall also play a major role?

 

[ellatas]

The high airflows on a fluid bed roaster are generally thought to have less complexity, but not necessarily less body. For a drum roaster, the airflows can be much lower since the mixing of the coffee is done mechanically by the rotating drum as opposed to the air. Also, on most commercially made drum roasters, air is passed through the drum to not only roast the coffee, but to remove chaff. If chaff remains in the drum, it will burn and impart a smokey flavour to the coffee.
The heat transfer to each coffee bean within a drum is approximately 80% convection from the air, ~10% conduction from bean to bean, ~5% conduction from drum to bean, and less than 5% radiant from drum to bean. If there is no airflow at all, the other pathways dominate more and the drum needs to be a lot hotter to roast in the same time. The flavour profile will be different.
Specific flavour profiles (such as floral and high notes) depend more on the coffee type and the time/temperature profile it is subjected while roasting. All roasters will produce their own unique flavour profile and it can be difficult to replicate from one equipment type to another. In building your drum roaster, accept that it won't roast the same way as other roasters, but that doesn't mean it won't be capable of producing great roasted coffee.