Building a Custom Roaster: Heat Source Debate


New member
Mar 22, 2014
Austin, TX
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Hi all,

I'm in the process of piecing together a little 5-10lb drum roaster for my fledgling microroasting business. I've read alot of literature on the internet about heating methods for drum roasters and haven't found anything too decisive, so I'd appreciate all the input I can get!

So we know there are three ways of heating coffee in drum roasters: convection, conduction, and radiation. Convection is air-to-bean. Conduction is drum-wall-to-bean (and minimal bean-to-bean). Radiation is direct energy from the heat source (less than 2% of total energy absorbed by the bean). A majority of the information "out there" puts a favorable emphasis on convection, as it is the most even heating method. Conduction is helpful in small amounts, but can be tricky: with coffee beans being round on one side (only one point receives conductive heat), using too much conductive heat can lead to tipping and scorching.

The two primary heating systems I've researched are infrared (like the good old Diedrichs I've worked on) and open atmospheric flame (Probats, etc.). According to Diedrich, infrared allows for more gentle manipulation and faster bean-temp response time... but that would only be the case if you are heating beans through alot of conduction. (We know the reason why atmospheric roasters have slower temp response times... you are essentially waiting for a column of air to heat up/cool off.)

Probat and other atmospheric roasters had to worry about too much conduction also, until someone had the bright idea to put a diffusion plate between the flame and the drum. But it seems like this diffusion plate would then become a secondary conductive heat source, as it would be radiating heat directly to the drum, much like an infrared burner. Aside from producing a large amount of heated air for potential convection, can anyone tell me what would be preferable in an atmospheric system?

Thinking about the radiating heat sources (the Probat diffusion plate and the Diedrich infrared burner), wouldn't both of them play a role in heating the air in-line-of-sight, and thus create an appropriate medium for convection (as air insulates quite well)?

Thanks so much for the input. If you want to follow my roaster construction, I have a forthcoming blog attached to my roasting page: Freytag Coffee Roasters | Microroaster in Austin, TX
I use an Ambex, direct flames hitting the drum. It's never a problem unless the drum stops and the beans stop tumbling. I've heard the opposite you have regarding Diedrichs, that the infrared tiles are slower to respond than direct flames.

There's some debate on how much a drum uses conduction vs. convection. I don't know how much time any given bean spends touching the drum, but think it's less than we think. And since air is being pulled through the drum, there's plenty of convection heating going on as the beans tumble.
I believe drum roasters use about 80% convection, about 18% conduction, and 2% more or less radiation.
Ambex and roasters like Probat are generally considered convection heating roasters.
Then again you have stainless steel drums and cast iron(both different animals).
If I do small roasts on my YM10(6lb. and under), I lose the steady blue flame that is present with the larger batch roasts.
Even with 100% airflow to drum I get too much heat, so I lower flame rather than stalling the roast.
So as you stated, maybe the infrared burner, may be a gentler way to control heat manipulation.
Sounds like an interesting project. Keep us updated. Pics would be great too.
I have read the whole thing and I normally do not like reading tech stuff on here because of the headache it gives me. lol

But here is my take on it.

I really don't think there is clearly better choice or better method of roasting.
It really depends on your comfort level of different method. I have talk to many different roasters out there. And their main concerns is not the how the heating units work but more to do with consistency of the machine.
If you are the commercial roaster, you want to have consistent product for your customers. And I would worry about the durability, consistency and simplicity.

Good Luck and I hope you are get what you are after and, yes, please keep us updated and result of your finding.