Patrick, what about my experience that I've gotten many good roasts with the flame touching the drum as shown, with enough airflow to allow oxygen for the flame, relatively high charge so there are a lot of beans to absorb heat, and higher drum speed, so the beans are tumbling well? There is minimal and usually no scorching and there's never tipping. Added: Generally I'm following your suggestion so that higher heat may be applied with large loads during part of the drying stage only.
Also your sketch differs from my roasting chamber, where there's much more room between the back end of the drum and the rear firewall.
Here are photos of the flame in my drum from an earlier post. The first photo is with the flame set at 2.5 kPa.
I listed it as an example of the worst case scenario, and it doesn't necessarily apply to a particular model of any machine. However, increasing airflow in this case will result in the increased temperature difference between the hot and cold regions. The burners maintain the same even BTUs, but the high velocity airflow region has lower temperature comparing to the slow airflow region, due to more air is been heated up by the same BTU.
Above is a picture of mini-500 with perforated drum I found on internet. (It could be only of this particular machine at the high flame setting.) There is a blue flame from the burner at the right side touching the drum. The person who takes the picture describes in his blogger that he prefers his bigger production roaster, a Fuji Royal 3kg which is also a perforated drum roaster, because the roaster produces more even roast. Note the burner in the Fuji Royal (picture below) is far away from the drum.
It could be more complicated when charging full load or half load. When charging at lower % capacity (the black arrow, 10 seconds after charge), the beans may localize at the exit (black oval). When charging at full load (the blue arrow, 10 seconds after charge), the beans may be more evenly distributed in the drum (blue oval). It is related to the vane design.
Not exactly the flame height. It's the non-linear feature of the perforated drum roaster.
In a solid drum the hot air has relatively more time & space to pre-mix, and the subsequent more-or-less laminar-flow through the drum. It has very broad linearly adjustable airflow and heat intensity range.
In a perforated drum the response to the variable airflow is non-linear. Assuming in the diagram of the first post, the drum is rotating clockwise, the bean pile would be accumulated at the hotter air region. Or in the Fuji Royal case the cold air inlet is feeding from the bottom of the burner and has more pre-mix space. It may increase the linear component.
Not good or bad, just its feature. Find the sweet spot for operator-adjustable parameters and stay within the boundary of the design of the machine for optimum result.
9Sbeans wrote:Find the sweet spot for operator-adjustable parameters and stay within the boundary of the design of the machine for optimum result.
I'm totally with you on this. I think the discussion you've started here is very useful for helping people understand how to find the sweet spot. The photo you posted of the Mini 500 perforated drum roaster with uneven heat is an instance where that's obviously not the optimum setting. Yet Yang Chia makes a fine roaster, so what are its capabilities if used within its sweet spot? And how would you tell? I would think observing the interaction of flame and air, trying different profiles and cupping -- just like I tried high air flow in my roaster and found a really good way to mummify coffee.
Sorry, can someone post a pic of the back wall of any of these solid drum roasters so I have an idea of how much ventilation is coming through the back? Difficult to pin down the airflow... my experience is with only perf drum roasters of ~half pound capacity, and fluid bed roasters. New to solid drums.
Question, whether it is a perforated drum or a solid drum, wouldn'y the flame height have a direct relationship to heat change in the Air and the Drum temps?
I have my roaster working on Natural Gas after a month on Propane and when I switched to Natural Gas I was getting very hot temps and the roast was pretty uncontrollable for me (Iam a newbie to a gas roaster). I noticed that I could turn the gas valve so low using Natural Gas that Propane would have died out much sooner. I had to lower the burners to almost their lowest setting/placement.
I run the roaster mostly in the .5kPa range, maybe going to .7 or down to .4. Just to idle the roaster I can run it at .15kPa. Propane would quit at about .3kPa.
Lowest setting.
I have 5 burners in a row with this solid drum, but I do know that Bella Taiwan makes a perforated drum roaster as well but I am not sure the sizes of them.
Front of drum is on the left in the lower photo. I ended up lowering the burner bar further using Natural Gas, but if I was using Propane I would raise it back up closer to the drum. I like this little roaster's adjust-ability with air, temp settings and drum speed.
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walt_in_hawaii wrote:Sorry, can someone post a pic of the back wall of any of these solid drum roasters so I have an idea of how much ventilation is coming through the back? Difficult to pin down the airflow... my experience is with only perf drum roasters of ~half pound capacity, and fluid bed roasters. New to solid drums.
aloha, walt
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