Mini oven roaster project... - Page 2

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

In a regular oven the temperature is very stabile because the temp. probe is inside the oven.
Again, I could be wrong, but I think the temp inside a "regular" oven only appears to be stable because the mechanical thermometer is not quick to respond to temperature fluctuations. If you install a proper thermocouple and measuring device, you will likely see a wider temperature swing. Large ovens also have the advantage of containing heat better than their smaller counterparts which helps the situation.

Another issue you might come across once you fire it up, is that the very high temps inside the roast chamber are going to be transmitted down the drum shaft to the motor drive shaft and then into the motor. You might need some kind of insulating device in the motor drive shaft. Maybe a ceramic coupling of some sort.

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Vakse Viggo (original poster)
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#12: Post by Vakse Viggo (original poster) »

Almico wrote:Another issue you might come across once you fire it up, is that the very high temps inside the roast chamber are going to be transmitted down the drum shaft to the motor drive shaft and then into the motor. You might need some kind of insulating device in the motor drive shaft. Maybe a ceramic coupling of some sort.
Yes, I have to isolate around the hole where the motor is, so the only place where the heat comes through will be the holes into the thermostat.
Then I will run another test and see if the temp. gets a bit more stabile. If not, I will have to look at another option.
I have used some oven sealant all around the oven, inside and out, so when I also put insulation on, the whole thing will be very nice and sealed.

Larz
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#13: Post by Larz »

Almico's point is that heat will be transmitted down the motor shaft, not (just) through any openings in the oven wall themselves, thus the recommendation for a ceramic coupling to isolate the motor somewhat.

fun project; good luck.

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Vakse Viggo (original poster)
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#14: Post by Vakse Viggo (original poster) »

This is how it looks like.


Larz
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#15: Post by Larz »

Looks like you've done a fine job of isolating the oven environment from the motor environment, Vakse. Not to beat this to death but the point made earlier referred to the thermal conductivity of the steel shaft extending into the oven which will approach 250°C. While the insulation may hinder radiant heat from entering the motor chamber, no amount of insulation between the oven wall and the body of the motor will stop that heat from transferring conductively along the shaft of the motor to the motor itself, and that is what we were trying to get across. To stop that transfer, the steel motor shaft MAY ( don't know the properties/maximum operating temperature environment, of the motor you are using) have be thermally isolated from the shaft entering the oven and I believe this was Almico's point. You can test this by firing up your oven to maximum heat, running it there for 15 minutes and then reaching in and touching the motor shaft right where it enters the motor. If upon removing your finger from the shaft you find skin stuck to the shaft, you will know you probably need to put a relatively non-thermally conductive coupling between drum and motor shaft as Alan suggested. :lol:

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Vakse Viggo (original poster)
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#16: Post by Vakse Viggo (original poster) »

Heh...I 100% get what you both are saying. :D
Someone else, who is also roasting with a mini oven, told me to buy this very motor and he does not have any problem with overheating, but thanks for the tip.
Tomorrow I will run another test to see if the cover around the thermostat did a better job.




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Vakse Viggo (original poster)
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#17: Post by Vakse Viggo (original poster) »

Well...I am happy to announce that the trick with the cover around the thermostat worked really well. The temp. is very, very stabile right on 190 c. Celsius.
Before the temperature would go way past 250, but now it is almost rock solid. :D


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Vakse Viggo (original poster)
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#18: Post by Vakse Viggo (original poster) »

Okay, so here is my first two roasts. I have a bit of trouble getting the temp over 200 c. The trouble starts when I am placing the cold drum inside, and then it takes a long, too long, time to get up to over 200 c. again.
And also when the door opens, a lot of warm air comes out. Anyway, I have a plan. I still need to build a pipe so I can load the drum, without opening the door.
I hope that will do the trick.
I also put an extra pair of aluminum list's, so the beans will mix better and I think it has helped looking at the difference of nr. 1 and nr. 2 roast.
Next time I will try to preheat the drum and hopefully see a better result.

Here is the Time and Temp. on the two roasts:

Nr. 1: 00:00 180, 04:50 170, 07:55 180, 11:08 190, 16:00 190, 22:40 195, 25:00 dump.

Nr. 2: 00:00 210, 04:30 155, 07:00 170, 09:30 180, 12:50 190, 17:30 195, 20:30 200, 23:00 200, 27:00 dump.

The temp. have to come up and the time down. That is also why I could not hear any cracks, but nr. two roast is pretty even. :)

First roast.


Second roast.



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Vakse Viggo (original poster)
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#19: Post by Vakse Viggo (original poster) »

I had a bit of trouble getting the temp. up past 200. But with this little fix I manage to get a steady temp. 210 or 220 c. Celsius. The hole was a bit too big, so I narrowed it down to these two smaller holes. :)


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Vakse Viggo (original poster)
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#20: Post by Vakse Viggo (original poster) »

I also tasted the first and second roast and was very surprised how good it tasted. Two light roasts, but a LOT of sweetness and flavor. :D