I have no experience with the KKTO but have used two different drum roasters, so apologies for the really basic questions. First question is why 400 g is the smallest charge you can use? When I was reading about your problems it immediately sounded like they should be fixable by lowering charge weight. The very slow move to dry (I consider 6 minutes as slow as I'd want and with smaller charges I get there in 4 minutes) and the inability to get the system temp through 1C and beyond sound exactly like charge weight is too high.
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Roasting slow-fast profile in the KKTO
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Previously my drying was only 5 or so minutes, but that's with a preheat of 265 degrees. The suggestion above was that my drying was (proportionally) too fast then. It just doesn't seem I can get much momentum in this roaster without terribly high temperatures. Probably cause it does such a good job of distributing that heat evenly throughout the system. Maybe I'll dare to tweak how the fan works to get more direct heat onto my beans.
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So the problem is that despite getting ET close to the limit (I find that ET greater than about 270 C causes ashiness and scorching) that you still can't get enough energy into the system. How is airflow controlled in a KKTO? If I had the same problems on a drum roaster I'd increase airflow and raise gas to get same ET but higher system energy.
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So there's just a central fan that sits above the whole system, not behind the heat source, but "inside" it (hard to describe) pushing it out to the perimeter of the chamber and circling it round. It's indirect, so ultimately probably terrible airflow in traditional terms. I've been convinced for some time that the airflow is significantly ineffective.Originally posted by kwantfm View PostSo the problem is that despite getting ET close to the limit (I find that ET greater than about 270 C causes ashiness and scorching) that you still can't get enough energy into the system. How is airflow controlled in a KKTO? If I had the same problems on a drum roaster I'd increase airflow and raise gas to get same ET but higher system energy.
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I think I'll be looking at converting this into a "glorified breadmaker" and go with a heat gun, and ditch my electronic control via my roasting software. I suspect it's the only way I'm going to get the kind of airflow that I really want. Just a bummer that so many seemed satisfied with the KKTO over the years. Maybe I have much higher expectations!
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Also I may use "ET" incorrectly, as I'm referring to the general environment temperature, rather than an exhaust temp. That's the other downside with this whole setup is not being able to create negative air pressure with an exhaust to draw the hot air through. It (generally) is fairly static air. The definition of baking. Given turbo ovens are... ovens... that's what it's really designed to do!Originally posted by kwantfm View PostSo the problem is that despite getting ET close to the limit (I find that ET greater than about 270 C causes ashiness and scorching) that you still can't get enough energy into the system. How is airflow controlled in a KKTO? If I had the same problems on a drum roaster I'd increase airflow and raise gas to get same ET but higher system energy.
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Have never used a KKTO so cant comment on the device, however I have been using a Coretto for over 10 years with great success, my setup really is quite basic, Breville single loaf bread maker, Bosch variable speed heat gun, DMM with temp probe and timing with wrist watch.Originally posted by readeral View PostI think I'll be looking at converting this into a "glorified breadmaker" and go with a heat gun, and ditch my electronic control via my roasting software. I suspect it's the only way I'm going to get the kind of airflow that I really want. Just a bummer that so many seemed satisfied with the KKTO over the years. Maybe I have much higher expectations!
I roast 725 grams weekly, over cooked one batch slightly in the early days, since then have not had a failure, perhaps I'm not as fussy or as scientific as some, however i do know and enjoy good coffee and consider my efforts as well above average.
The one disadvantage with a Coretto is uncontrolled chaff, I'm sure this could be controlled, not a problem for me as I roast in a shed and set up a pedestal fan to blow the chaff into the garden.
Not attempting to denigrate the KKTO, simply explaining a setup that works very well for me.
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There are some interesting observations in this thread, and clearly a lot of collective roasting experience, but there also appear to be some misconceptions about the fundamentals of heat transfer.Originally posted by readeral View PostI'm assuming that by delaying my ramp up, I can get better momentum into first crack for a more rigorous crack, as well as avoiding those high environment temperature negatives, damaged beans from earlier high heat input and hopefully carrying it into post-FC development a lot better.
Forget about 'momentum' - it's a misleading analogy. What you are describing is a product of the temperature gradient in the bean (i.e. that the middle of the bean is at a lower temperature that the outer surface).
If you start with a cold bean and pass hot air over it, heat (energy) is transferred from the air to the bean surface. The rate of convective heat transfer depends primarily on the difference in temperature between the air and the bean, and on the velocity of the air.
Meanwhile, heat is being conducted away from the surface of the bean towards the centre. It takes time for this to happen (and from memory the dynamics are primarily a function of conductivity and heat capacity).
If conduction through the bean is rapid relative to convection then there won't be a significant temperature gradient (i.e. if heat is transferred away from the surface nearly as fast as is is transferred to it, then the temperature at the inside of the bean will be close to that of the surface).
If, on the other hand, the rate of convection is very high relative to conduction (through the bean) then the surface temperature will rise faster than the centre temperature (resulting in a temperature gradient).
This effect is amplified when the air temperature is increasing (because the surface temperature will rise with it faster than the centre).
Now, first crack occurs once a particular temperature is reached. The slower the temperature rise of the bean surface in the lead up to FC, the closer the centre of the bean will be to FC temperature - perhaps resulting in the observed "tighter" first crack.
This of course is somewhat simplified - chemical reactions and physical changes in the bean as it expands complicate the situation beyond simple heat transfer - but it is still useful for picturing what is happening.
A picture is worth a thousand words: https://youtu.be/NwpJDV5vzm8Last edited by MrJack; 20 August 2016, 03:21 AM.
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MrJack, most of this I understood from many sources I've read, thanks for a concise summary. I might be using the term 'momentum' unhelpfully, but I suppose I have meant 'ongoing heat transfer' especially as the roast has progressed. It's a minor point in a broader discussion that essentially is determining: 'effective heat transfer to the bean, and through the bean is essential' and as the whole roast is progressing very slowly, due to poor transfer, the beans are subject to high temperature for a long time. Couple this with the likely evaporative cooling due to poor exhaust/air flow, and we've got ourselves a complex problem.
The salient comment you made is "The rate of convective heat transfer depends primarily on the difference in temperature between the air and the bean, and on the velocity of the air." What I'm finding is that although this is a 'convection oven' unit, I'm getting very little convective transfer of heat given how it's now being used. The challenge I'm trying to solve is how to improve energy transfer to and through the bean for a successful roast within reasonable parameters. The success of the suggestion by kwantfm of lowering the batch is contingent on good roaster design to have good, direct convection (and, lesser so conduction/radiation), which I personally feel it does not have. I obviously have no instruments to actually measure this, and chaff flying around the roaster seems to contradict my hunch, but I think the problem may lie in the issue of air pressure (which, if I have understood correctly, means with negative pressure outside the system will produce air flow through the roaster) - rather than just air movement.
The reason - I suspect, and honestly all of this is just hunch given I am a musician not an engineer - that lowering the batch will not help much anyway is that: given the size of the roaster, the amount of energy lost to the increasingly exposed metal surfaces of the roaster (to be conducted outward to the ambient air), will offset the desired gains from a lower batch size. Creating better convection directly to the beans, rather than to the surfaces of the roaster, should help a lot more.
I'm sure I've got some science wrong - feel free to correct me, but in the context of solving this problem of effectively executing a slow-fast profile in a KKTO.
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I think the profile you are trying to achieve is not particularly suited to the KKTO, all my profiles have a slightly decreasing RoR to FC, it starts around 11deg/m and at first crack is around 6.5deg/m, then I continue to control a slowly falling RoR from FC to SC - from 6.5deg/m down to around 3.5deg/m at the end of the roast. I aim for a Rao style profile with 20-25% of the time to FC for the phase between FC & finishing.
I roast 1.2kg batches in my KKTO, i bypass the thermo couple so the heat is always on, i lift the TO off to 'burp' when I need to control RoR.
I actually found over the years that bigger batches are easier to manage in the KKTO - as long as you bypass the cycling of the element.
Temps and thermocouples are something I use relatively - they do vary and its more important to observe whats happening (FC, SC, etc) than relying on what you think should be happening at a given temp. Different beans vary a lot too. I dont bother any more with ET, I think its more important in drum type roasters, I couldnt find any correlation in the KKTO.
My load temp is 250 deg.
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