Temperature Control in Heat Gun Roasting?

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
vze26m98
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#1: Post by vze26m98 »

Greetings-

I was hoping someone might help me with my Gedanken experiment concerning heat source regulation. FWIW, I'm thinking in terms of HGDB roasting, which might have different heat dispersal characteristics than a closed drum.

Let's say I have an ideal heat gun that produces an exact output of 400f. With this heat source, will the temperature of the bean mass ever go higher than 400f?

In other words, is heat added to the roasting process, so the temp of the bean mass will go above 400f? Or do the beans reach some sort of steady state with regard to temperature (although not necessarily in regard to roast)?

Thanks! Charles

billsey
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#2: Post by billsey »

Ignoring what might happen if something ignites, the bean temperature will never exceed the heater temperature. In fact the bean temperature will slow it's climb as the differential between bean temp and heat source narrows. Unfortunately, this falling ROR isn't the optimal falling ROR everyone looks for, or else we'd use it. :)

vze26m98 (original poster)
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#3: Post by vze26m98 (original poster) »

Thanks for your comment Bill.

My understanding of the physical world would suggest that things can't get hotter than the heat source, but given how roasters talk, it's not always obvious. I should go back to some of the physical descriptions of drum roasters and try to understand heating elements and heat control better.

Your mention of a "natural" roasting curve versus a flavorful one was very useful, thanks again!

Charles

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[creative nickname]
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#4: Post by [creative nickname] »

I think the confusion comes in because people forget that measured temperature is always an approximation. Watching the numbers on my sample roaster, it is typical to see measured BT climb past measured ET, but that is because measured ET runs low when measured via the exhaust port. The true heat input of the system coming from the gas burners will be much higher, but it isn't worth measuring directly because what matters is the variance between roasts, not the absolute temperatures themselves.

When I used a HG/BM set-up, measured ET from the heatgun was always higher than measured BT, because I put the ET probe in the airstream above the beans. With that design, one could adjust the heatgun manually over time to keep the measured ET constant, and the BT would approach it asymptotically over time, so the measurements were much closer to the reality.
LMWDP #435

vze26m98 (original poster)
Posts: 264
Joined: 10 years ago

#5: Post by vze26m98 (original poster) »

When I used a HG/BM set-up, measured ET from the heatgun was always higher than measured BT, because I put the ET probe in the airstream above the beans. With that design, one could adjust the heatgun manually over time to keep the measured ET constant, and the BT would approach it asymptotically over time, so the measurements were much closer to the reality.
Thanks for this, very helpful for my thinking ATM.

If one is trying to hit a final BT of say 415F, it would make sense to limit ET from the heat gun to that temperature, then BT and ET would eventually meet. With HGDB roasting, something that would help position the heat gun at that final temperature sounds like a good idea. But it sounds like earlier in the roast, you'd want a higher ET, so positioning the heat gun should be flexible.

I had luck with HGDB roasting for a while with lighter roasts, but working with Indonesians and darker roasts has shown me it was just luck that was getting me good results. So I'm trying to evolve a simple form of temperature control for my setup.

I'm committed to the dog bowl. I know I could tack-weld a sensor on the bottom, but would rather consider positioning of the heat gun as a control for now.

Best wishes! Charles

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ripcityman
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#6: Post by ripcityman »

Get a hotter heat gun. I designed a stainless pot coffee roaster and I spin it over a 10,000 BTU camp stove. Propane burns at about 1550F, so I have no issues with getting beans to temp. I was interested in designing a roaster using a heat gun, but I can refill the 1 lb propane cylinders with a valve I got from Cabelas for about 50 cents. I get 10 roasts per cylinder.

Variable Setting, 1500W Digital Heat Gun, Model HT3500, 250-1350 Degrees Fahrenheit, 2 Fan Speeds, 12 Selectable Temperature Settings.
These specs are for a Wagner Heat Gun.