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caffeinegeek

When a DIY roast goes wrong!

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I'm happy to share my bad experiences as well as good to help others.

I modified a toaster oven for home roasting. I uprated heater elements, fitted a barbecue spit spindle and motor and mesh peanut roasting drum. The heaters are PID controlled with programmable ramp sequences. I kept all the original front panel controls and over heat protection. Finally, I fitted a huge fan on the rear panel. The fan is the type used for domestic gas boilers and moves a LOT of air.

I started off test roasting cheap Santos beans at around 150gm per load then upped the load to 250gm. The oven fan exhaust looped out of my kitchen window and i kept a CO2 fire extinguisher close by. First attempts with air oven roasting gave me an under roasted result so I started tweaking the PID ramps, duration and temperature which aren't anything like the published profiles for professional roast ovens, although the Gene roaster seems close to mine, based on results from their users? By now I hade done 5 test roasts but still needed a darker result. I adjusted the final level temperature to 235C. I found listening for cracks hard because I'm not sure if first crack means the first bean that cracks or hold the temperature and wait for several beans to crack?

I was getting more confident now and loaded up the mesh drum with 500gm of Guatemala SHB. I started the routine and watched the bean colour as the oven (NOT bean) temperature was held at 235. The cracks started to occur but I left the temperature set for another minute before starting the cooling phase. I fitted a speed controller on the fan so I could adjust the cooling rate from very fast to something slower. This is fairly easy to do manually by looking at the PID controller present value.

THEN DISASTER. The exhaust fan switch/speed controller had failed and the beans were getting darker very quickly. I cut power to the oven heaters but with no exhaust fan I couldn't get much convection cooling through such a small oven cavity. The oven was filling with smoke and its temperature was RISING with heaters turned off. I could see the bean load was now in its exothermic stage and thermal runaway which I wasn't prepared for and the fire extinguisher was last resort. By this time and after I opened the oven door, the house smoke alarms had gone off creating a racket and adding to the confusion.

I grabbed a fan heater, switched it to cool and held it in front of the open oven. More smoke everywhere, but the bean load was cooling down at last. When the drum and beans were cool enought to remove, they were as black as black having been self consumed of all oil and were no good for anything but charcoal.

And so I learned that whatever fancy PID controller you may have on the heat side, a strong working exhaust fan (and standby fire extinguisher) is an important component to control temperature around the crack point and stop beans going into thermal runaway. I also learned the bigger the bean load in a small cavity oven, the harder it can be to stop thermal runaway! All this is unsurprising when one considers the oil content held trapped inside green beans.

 

Edited by caffeinegeek
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Scary!! I remember my Hottop once wouldn’t eject the beans and suddenly in what seemed like seconds they turned very dark and smoke was pouring out. I always stood by with a fire blanket and working fire extinguisher within reach (even before that happened)!


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1 hour ago, caffeinegeek said:

I found listening for cracks hard because I'm not sure if first crack means the first bean that cracks or hold the temperature and wait for several beans to crack?

Depends on the results you want and how fast the roast is progressing (your ability to control the roast). If it's rapidly getting darker and you hear a pop you might want to drop the temp if you want to stretch out first crack or just prevent the roast from taking off and hitting second immediately. If it's slowly moving along and you hear a crack and drop the temp you might stall it or it might mean you'd have to roast for a very long time to get it to the level you want. 


ACS Minima (Beta) -- Reskinned Ceado E8, Niche Zero --- Gene Cafe CBR101 with Dimmer Mod and Bean Mass Probe

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Thanks for the tips on crack. I'm looking at what mods I can do to the roaster. At the moment I'm more confident looking visually at the roast and listening for cracks than just relying on PID automation. I'd built in manual overide for heating and fan cooling (which failed on this event) and using manual intervention to control near the crack point has so far given the best results. The issue I've not seen much discussion on is most PID controllers including those with ramp programming are only single channel. Some make misleadingly claims they are more than this by using relay alarm outputs, but this isn't the same as proportional control of temperature for both heating and cooling. The only true dual channel PID controller I've found is Arduino based, using an addon board from the States which isn't easy to get here at an affordable price.

With small roasters inluding the popcorn poppers, you rely on air temperature measurements which we know are not the same as measuring bean temperature, but I've not yet worked out how to measure bean temperature with my small drum roaster. My next issue is the drum itself. Peanut roasting drums are a wire drum with lots of open hole area and very little mass. The drum itself isn't going to retain much heat so I'm making a larger drum using thicker perforated stainless sheet. That will allow the drum to be pre-heated and retain more heat before the beans go in. I can easily measure drum surface temperature which I think will be closer to outside surface bean temperature than oven air. But if there's an easy way I'd like to measure bean temperature?

PS: Do you think a 500gm bean load is too risky in a small roaster? Clearly the larger the bean mass, the more heat they will store up to be cooled and this increases the risk of a thermal runaway?

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Edited by caffeinegeek
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