Airflow Experiment

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
aabud

#1: Post by aabud »

Did a little experiment with some cheaper beans I didn't care for... did 4 1lb batches using a temperature similar to what I might use for a roast, varying only the airflow (which I kept static for the entire roast after soak). As I kinda expected, at a really low airflow, increasing the airflow will speed up the roast, but you eventually reach a point (I'll call that "air max transfer") where more airflow slows down the roast.




I'm not sure I've read anywhere a suggestion for where on this curve you want to set your air... since I've read that, if anything, maybe air should be increased a notch toward the end of maillard... and assuming you don't want heat to increase at that point... the implication is you should be on the right of "air max transfer", so you're not adding heat when you increase air.

On my BC-2, it would appear for a 1lb batch "max air transfer" is around 5pa, so perhaps this suggests fix the air at that level, at least into maillard.

What doesn't quite reconcile with all of this is, I have much better luck controlling crash/flick when I'm at 4pa with higher gas levels, than when I'm at 5pa and somewhat lower gas. Admittedly, I haven't done that many roasts at 5pa, so maybe my gas profile just needs more work there.

Capuchin Monk

#2: Post by Capuchin Monk »

I'm not sure if it's common or not but in some of the Mill City roast along videos I've watched on youtube, they increase the air flow as the roasting progresses.

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Almico
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#3: Post by Almico »

It's nice to know where your "max heat transfer" point is...so you can ensure setting your airflow well below it. As long as the cigarette lighter trick indicates that enough air is moving through the roaster to evacuate smoke and chaff, you're fine. Bringing the air to the "max air transfer" level makes controlling a roast much harder.

I haven't found much advantage to increasing airflow towards the end of a roast, unless I missed a gas adjustment and need to cool down the roast to prevent a flick. But that's dancing with the devil and not a desired roast plan MO.
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aabud (original poster)

#4: Post by aabud (original poster) »

Almico wrote: dancing with the devil
definitely... :D

Just trying to rationalize something of what I'm seeing (at the risk of just making stuff up)... maybe when you're on the peak with air at "max heat transfer", it oscillates a bit between taking heat and giving heat, so stability is hard and it's easy to fall of the peak (crash/flick). Whereas if I stay solidly a bit behind "max heat transfer" things are more stable and there's less variation in heat going on.

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Almico
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#5: Post by Almico »

aabud wrote:Just trying to rationalize something of what I'm seeing (at the risk of just making stuff up)... maybe when you're on the peak with air at "max heat transfer", it oscillates a bit between taking heat and giving heat, so stability is hard and it's easy to fall of the peak (crash/flick). Whereas if I stay solidly a bit behind "max heat transfer" things are more stable and there's less variation in heat going on.
Yep

On my 5kg roaster I put a third probe behind the drum to measure the air coming in. It taught me a lot about airflow and when increasing it would raise or lower drum temp.

Basically, early in the roast when the gas is high, more air will result in more heat to the beans. As the gas goes down, so does the available heat outside the drum. By the time gas is all the way down to finish the roast, more air will cool the coffee.

Without that incoming air third probe, you're just guessing what will happen when you open/close the air baffle.

It's better to cool the roast with less gas than more air.
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EddyQ

#6: Post by EddyQ »

Almico wrote: Basically, early in the roast when the gas is high, more air will result in more heat to the beans. As the gas goes down, so does the available heat outside the drum. By the time gas is all the way down to finish the roast, more air will cool the coffee.

Without that incoming air third probe, you're just guessing what will happen when you open/close the air baffle.

It's better to cool the roast with less gas than more air.
Seems to reflect my experience with my 1K where I have air in, air out and bean probes and an IR sensor measuring my drum. During development the air is definitely doing some cooling. And to minimize, I drop air when high heating is no longer needed. It does make it harder to control crash/flicks since the drum heat is what it is and you cannot change that quickly(and really shouldn't anyway). But IMO with low airflow during development there is the most stable flow of heat possible.

But if you are going dark, you should have enough air to remove any smoke. I personally haven't experienced any smoke flavors due to too low of air, but some have reported such things.
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Brewzologist
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#7: Post by Brewzologist »

This thread reminds me of a test I did on my roaster some years ago where I tried to maintain stable BT/ET temps in an empty drum at different gas settings by varying just airflow. My roaster has low thermal mass and is much more responsive to gas than air, so my goal was to see if I could find one optimal airflow level for any gas setting that I could set-and-forget during a roast.

Much like everyone here, I found that for a given gas setting, increasing airflow increased temperature to a certain point, after which it began to cool the drum. Of course, higher gas settings tolerated higher airflow before cooling happened than lower gas settings. And another caveat is that selecting an optimal airflow level assumed I could maintain a consistent operating enviroment too.

After all my experimenting, I settled on a low airflow level because I didn't want the drum to start cooling when I dropped to low gas settings during the development phase. In short, after all my playing around I ended up arriving at the equivalent of the typical "lighter trick" to set airflow; just enough to evacuate smoke and chaff without introducing temperature changes. I do often crank up airflow just before drop to evacuate smoke, but otherwise I leave it alone.