Drum material: what is the best

[RubensGardelli]

I would like to hear your opinion about the best material to use for a drum roaster.

I quote from a roaster manufacture document:
"..... we use carbon steel because it contains the best of these metallurgical properties, is durable longterm
and cost effective. Many people think stainless steel or cast iron is better however this is not the
case in regards to "evenness" of retaining and disbursing heat across the entire drum and transferring
that evenly inside the drum to the bean. This is also a fallacy as stainless steel is not as effective because
it is a reflective metal that dissipates the heat differently and cast-iron is a porous metal that has uneven
density.
Since cast iron is "poured' in to a mold, the density is not consistent throughout and therefore the
thermal dynamics of heat transfer is not even. Carbon steel is rolled and the density is consistent in its
thickness - the result is "evenness" in the thermal dynamics of heat transfer. Please note however, the
steel used and the engineered design of the drum is only one element of consistency in a ... roaster."

Thanks for your opinion.

[lsjms]

There are many ways to skin a cat.

I can think of awesome performing roasters using a variety of methods including cast, rolled and stainless. Remember an rolled drum will have a weld seam and welds for the vanes, so the "evenness" will never be perfect.

[orrinoconnor]

It all sounds like marketing crap to me!

Ray

[bruceb]

It is undoubtedly a matter of scale. On a small, home roaster the material is probably relatively unimportant, but for a large roaster where the mass of the drum and the mass of the beans are both very large I can imagine there being definite advantages and disadvantages to different materials. I would think that stainless steel is a very poor choice since it has a thermal conductivity at 200°C of less than 20 (compared to 60 for iron), assuming of course that conductivity is an important parameter. As mentioned above, "evenness" of heating will depend much more upon manufacture, welding, etc.

[lsjms]

Heat conductivity is certainly important but it depends on what kind of roaster you are building. If you want more heat applied to the beans via conduction from the drum go with the iron but why not copper?
If you prefer a back ventilated drum style and want the beans to roast in the flowing hot air SS could be preferable. I think Ambex do this.
On some drum roasters the flame does not even contact the drum and they perform like fluid bed roasters.

Harold Mcgee did some tests in "On food and cooking" and noted polymerized oils stick to stainless much easier than iron.

[bruceb]

I think anyone who has seriously roasted with an Ambex will be somewhat leery of their philosophy.

Edit: I'm not sure, but pure copper would probably be too soft to use as a drum and the alloys do not conduct anywhere near as well as the pure metal. Also, at the present price of copper you'd have to have a police guard around the roaster all the time.

[lsjms]

When collecting roaster quotes one company offered stainless or copper drums- the copper added about 400eur to a 1kg.

I found this interesting so I asked what the advantage of copper was, I got the following response;

"Copper is a natural element which pours taste and vitamins into the coffee"

Pure genius.

[bruceb]

"Copper is a natural element which pours taste and vitamins into the coffee"

Pure genius.

Damn! I always knew iron was a Hollywood invention!

[RubensGardelli]

Harold Mcgee did some tests in "On food and cooking" and noted polymerized oils stick to stainless much easier than iron.

What are the consequences of those tests result regarding to coffee roasting?

"Copper is a natural element which pours taste and vitamins into the coffee"

Sorry, I didn't get if that statement has a bad or good connotation regarding roasting with a copper drum.

I think anyone who has seriously roasted with an Ambex will be somewhat leery of their philosophy.

Would you mean that the reason they choose the SS drum is not supported by "a back ventilated drum style and want the beans to roast in the flowing hot air" ??? Is you this what you mean?

[dr.chris]

To simplify (or maybe not) the copper reference its also worth noting that arsenic is also a natural element that has just as many vitamins as copper.

[Dan_Urieli]

Hi there,
New to this forum (and site) so hello all...
Just wanted to add my two cents - we (as in Coffee-Tech Engineering) have three types of drums for our commercial roasters and after several years of R&D we came to a very unsurprising conclusion - there's no such thing as better
What I mean is this - we have a drum which is two layers of mild steel with a third layer of copper in between. I'm in love with this drum and with the roasts it produces... so?
So when we have people over, obviously they have a cup of coffee, and some of them - including veteran customers - prefer much, much more coffee from one of the other drums. So I can talk all day long about thermal treatment, heat dispersion and so on until people start yawning - the best material for a drum is the material that makes the coffee taste best - and that differs from one to another.
That being said, I can add that the reason I'm in love with the drum I wrote about is that it's design makes scorching and burning (aka tipping) impossible - and that's good news for many a roasters.
Was that a bit more than two cents? I got carried away...

[GreenBean]

Coffee just seems to be like that. People strive for a particular temperature and pressure profile from their Espresso machine and the best heat dispersion in their roaster drum but the coffee just refuses to conform to the best theories of what should taste best.

It makes it so much more interesting as a hobby but must be frustrating for equipment manufacturers.

[motoman]

Now you know why most people (except us nuts), always let other people do the roasting.

[lsjms]

"several years of R&D"

Got any jobs? Took me about a year to conclude(see above)

"There are many ways to skin a cat"

Dan, I would assume if you do not have the power to scorch or tip then the roaster would be incapable of running a very aggressive profile. I am a huge fan of my ' barrel on flames' and wonder if your guys Ghibli can do a 9 min roast with control? 17 mins +- is quite a long way from acceptable. I assume you can reduce the time by dropping batch size, a lot of roasters seem to way over quote the capacity and then the temps are hard to control because of the massive thermal mass with barely any beans inside to reign the temps in.

So does the Ghibli come with a choice of three drum specs? What are the other two?

[Dan_Urieli]

Well... since you asked (an excerpt from one of our documents):

Our standard drum is built using solid mild steel, 3 mm thick. This drum is used in our Ghibli R-15 roaster, with 5 mm thick steel for larger models up to 90Kg batch machine.

The drum was designed on 3D animation software and welded with crossing agitation blades for perfecting key features such as smooth motion, heat conduction, chaff evacuation and rapid beans evacuation for the cooling.

Both material and configuration were thoroughly studied for obtaining high conduction roasting, allowing wide aromatic spectrum and high level of sugar to develop during roasting. This drum is expected to produce superb roasts in most roasting styles, including blends roasted for espresso.

There are two variants of back plates for this type drum:

Perimeter Air Passage Holes Back Plate:
This plate causes air to work at the contact points of the drum surface and the beans. The result is milder and finer heat conduction and enhanced heat distribution.

Centered Air Passage Holes Back Plate:
This plate restricts the amount of air flow, increases its velocity and enhances convection inside the drum. This will perform well in machines equipped with drum blower speed controller.


Coffee-Tech Engineering’s Infrared Drum
This drum is 4 mm thick, made of two separate layer of perforated mild steel.
The perforated drum utilizes infrared radiation for roasting. The radiation is produced by the flame which is positioned horizontally, parallel and underneath the drum inside the ceramic combustion chamber. This drum is indifferent to the type of rare plate as air flows through the drum walls and around, this drum also requires low airflow to prevent excessive conveying heat by the air.

The perforated drum produces stronger taste and higher rates of caramelization in sugars. This much desired outcome occurs without running the risk of producing roasts with burnt marks on the surface of the beans as may happen while caramelizing with solid drums. Using this drum may prove favorable for filter coffee roasters or those who seek to obtain intense tastes for their espresso roasts.

Coffee-Tech Engineering’s Thermodynamic Drum
This drum is 5 mm thick in total, made by 2 different layers of mild steel and a third layer of 99.9% copper in located between both layers.

This drum has the best thermal absorption and conduction characters; the copper layer evenly distributes the heat all around and conveys it to the perforated, rough surface “in drum beans contact layer”. This structure allows better conduction and better beans agitation (anti sliding). The 2mm gauge between the copper and the beans allows mild heat reflected through the holes on the surface, rather than by direct contact.
Holes also create bens rest above the reflecting spots, with greater surface than solid drums.

This drum allows higher temperature roasting, faster process and excels in batch uniformity. It will suit anyone who wishes to roast blended or non sorted coffee, or just for those who wish to get the best as it is the most promising configuration of all.