Air Flow Path in Diedrich Design

December 5th, 2014, 7:06 pm

Can someone who knows please walk me through the specific airflow path in a Diedrich roaster like the IR 12? Apparently the air is drawn through the bean chute, but its somehow heated before entering the drum? Is it only fresh air entering the drum and no combustion gas?

#2: Post by **TomC** » December 5th, 2014, 8:45 pm

Essentially most gas fired drum roasters (Loring likely excepted) have all of the products of the combustion gas entering the drum. If only fresh air entered the drum, they'd never roast (that's not meant to come across insulting or duh

) and it suggests that you'd be wasting gas energy if you did. There's a rather humorous back and forth discussion between Jim Schulman and Andy Schecter about this very topic here, back in 2010. Andy cited some pretty strong evidence from Marty Curtis, and I'm only paraphrasing what Curtis himself said.

And here's a quote specifically to the Diedrich IR-12, that should answer your question from CoffeeForums.com

"I just joined this forum and see that no one answered your question about how air gets into the roasting drum. I also made the same assumptions you did until I got educated by Jason at Diedrich. On an IR-12, most of the air enters the roasting drum through the front hopper. If you remove the front hopper you will see that there are 2 holes in the front plate. The upper hole is above the drum and the lower hole leads into the drum. Air enters the roaster and is heated by the heat sinks around the drum it then travels out the front plat and into the hopper. From there it reenters the roaster into the drum."

December 5th, 2014, 11:52 pm

Thanks for this. I think where I'm mostly unclear is exactly what path the air takes between the point where it enters the roaster through the first hole in the bean chute and the second hole where it enters the drum, as well as how hot this air is when it enters the drum.

December 6th, 2014, 12:03 am

TomC wrote:If only fresh air entered the drum, they'd never roast (that's not meant to come across insulting or duh ) and it suggests that you'd be wasting gas energy if you did.

Only fresh air enters the drum of certain probat open barrel sample roasters, and they roast coffee, right? (that's not meant to come across insulting or duh

)

Are you saying this approach is very inefficient?

#5: Post by **TomC** » December 6th, 2014, 1:27 am

I don't have an expert level of understanding of all of Probat's sample roasters individually, but I have played with a few, gas and electric. I was keeping my train of thought along the lines of your post, not a small sample roaster. I know their air flow is more than adequate, yet difficult to control. The air damper is more to let heat out, not to draw air in.

You could finish a roast in 6 minutes with one if you choose. Granted, a significantly larger portion of the heat transfer in one of those is likely due to conduction versus convection due to the lack of massive amounts of airflow early on. The air damper valve acts like a chimney and lets hot air draft out, slowing the roast when opened. The air entering the front end is likely rather trivial in how the coffee roasts.

And in terms of a sample roaster like that, efficiency isn't even relevant. It's designed to allow the person doing the roasting the ability to frequently pull samples and visually inspect their color development quickly.

December 6th, 2014, 2:18 pm

I doubt anyone wants to consume any more fuel than necessary, especially on 100 grams of coffee. My point is that designs that draw in fresh air from the front are viable, and furthermore it appears that the IR 12 shares some similarities (albeit perhaps distant ones) to the design ideology you describe here; adding more airflow as the roast progresses in order to slow it down. I understand as well that this is interpreted on the Diedrich IR12 as progressively increasing convection in the roast. However, I recently spent a couple of hours observing an IR 12 in use, and the roast master commented that he begins with minimum airflow, increases airflow at yellow, then again increases airflow nearing first crack, in order to slow the roast down, as well as clear chaff. Unfortunately we couldn't nail down the exact airflow path.
My original question that remains yet unanswered is about the specific details surrounding the airflow path in the IR 12 (or similar), namely what happens to it between the bean chute inlet and the inlet to the roasting drum. And now that the conversation has progressed a bit, I would again add the question of how hot the air entering the drum actually is, and what specific factors (excluding the obvious difference in scale) differentiate this method from what you describe above about using the airflow control as a way to dump heat and slow down the roast, as opposed to pulling hot roasting air into the drum.

#7: Post by **TomC** » December 6th, 2014, 7:04 pm

I'm not sure what similarities you're referring to, but maybe you can elaborate? I think they're quite different actually..."between the bean chute inlet and the inlet to the roasting drum" per your reply,-there is no ambient air entrainment happening there, so it's a moot point, the lever for the hopper holds it closed. On most of the large Diedrich roasters, you'd have to be manually holding the hopper lever handle open in order to have any continuity from drum to the environment at that location. If you're not, then its closed and the airflow is going out of the drum from back to front and makes a 180 degree u turn along the top. So fixating on the holes on the top of the faceplate isn't going to help you understand how air is getting entrained into the drum, because it's not happening there. Going back to the sample roaster analogy for contrast, you're just letting trapped heat escape by opening the air valve acting like a passive chimney, whereas on these larger units, your choice of where you're venting the air thru (drum or cooling tray) and percentage of each, determine the convective nature. They're rather different, fundamentally.

Air entry into a Diedrich comes from below and mostly thru the cooling tray and even fully "closed" it's apparently still 80/20, so it's never truly fully "off". It's heated as it wraps around the atmospheric burners on it's way into the drum.

December 10th, 2014, 5:08 pm

I think its from this quote you posted earlier this is where I'm confused(Ive read that thread previously, and it was partly why I started this thread to find some clarification):

"I just joined this forum and see that no one answered your question about how air gets into the roasting drum. I also made the same assumptions you did until I got educated by Jason at Diedrich. On an IR-12, most of the air enters the roasting drum through the front hopper. If you remove the front hopper you will see that there are 2 holes in the front plate. The upper hole is above the drum and the lower hole leads into the drum. Air enters the roaster and is heated by the heat sinks around the drum it then travels out the front plat and into the hopper. From there it reenters the roaster into the drum."

I interpreted that as fresh air being drawn in at or near the hopper, channeled through heat exchangers above the drum, then back out to the bean hopper area and then into the drum.

That's the basis for my comparison to an open-faced sample roaster, the difference being the fresh air would get heated a bit before entering the drum.

But If I understand you correctly, on an IR12 the cool fresh air is drawn in from below, similar to most drum roasters, wraps up around the burners and the drum, then does a sort of an exit/u-turn at the top front near the bean hopper and enters the roasting drum. Is that correct?

#9: Post by **amhas** » December 13th, 2014, 7:51 pm

I got this from Diedrich not much more than a month ago when I asked them this question.