Something I've been playing with and not sure has been discussed is the Airflow path through a drum roaster.

I have both a Santoker Rev 500 and Mill City North Coffee TJ-067 drum roaster.

Mill City installed an incoming air temp probe on the back of the roasting drum chamber near the drum perforations/grid which allow air to be sucked into the drum via the variable speed exhaust fan on top of the cyclone. So not only do you have an ET and BT temp probe, but an incoming air temp probe displayed in Artisan.

One difference I noticed between the Santoker and Mill City roasters configuration is the "gap" between the back of the drum and roasting chamber wall ... it is only a half inch or less on the Santoker and a full 2 1/4 inches on the Mill City. The amount of "unheated" air that can be sucked into the roasting chamber seems to be quite different between the two based on where the air intake slots on the side panels of the roaster are in relation to the burner location and the back of the drum. I have played with the intake slots by blocking some off in order to try to "force" the air to have more contact time with the heated drum and/or burners before it enters the roasting drum through the back drum air intake perforations/grid.

In my Mill City 1k roaster there are 5 vertical slots/vents on both sides which allow air to enter and feed the burners and pass over the heated drum to the back perforations and enter the drum. If you think about it, air entering through the most rearward slots can simply be sucked into the perforations with very little heating of the air. I have played with blocking off some of the air intake slots and have significantly increased my incoming air temp as registered on the "incoming air" probe in Artisan. I blocked the heat so much one time that I had to turn the gas down to slow the roast. Here is a picture of my current "slot configuration"





I have also done something similar on my Santoker but don't have an incoming air probe.
The newer Mill City roasters have a horizontal slot instead of the 5 vertical ones that are present on mine. but this can also be modified.
I think if you block off too much air intake to the point where you start to notice more yellow flame from the burners you have gone too far and this is undesirable.

I think by increasing the air temp going into the drum, I am increasing my convective air heating to drum surface contact ratio of heat application to the bean. I am not saying this is better but I do believe my roasts are a bit more even and I am conserving a bit of fuel as it seems to increase the efficiency of heating the beans.

Just something else to think about and play with for those who have drum roasters which are amenable to this modification and want to delve into things a bit.

Steve Egge

Interesting post. So does Mill City add a probe for the air intake to all of their TJ-067s? Not sure I understand the practical value of a probe there.

But I wonder if by blocking the air intake you are mainly just reducing the total airflow volume. And in turn a reduced volume of air results in the roaster heating up that air more. If so, then isn't blocking the vents essentially the same as turning the air down? Or is the thinking that the vents closer to the drum perforations don't allow as much opportunity for the air to be heated, so the path the air takes through the roaster influences the thermodynamics? If the latter, then I wonder if covering the vents might hinder cooling ability later in the roast.

In any case I'd be curious to hear how you think this affects control of the roaster and quality of your roasts.

Did the same thing on our Ambex YM-2 a couple years ago. Taped up the 'A' on one side of the roaster and the 'x' on the other side near the rear of the drum to prevent cool air from entering the back of the drum. Worked great. Tried to tape up one more letter on each side, but the amount of airflow was reduced too much.

keno wrote:Interesting post. So does Mill City add a probe for the air intake to all of their TJ-067s? Not sure I understand the practical value of a probe there.

As I understand ... they all do if you buy them probed. And I believe they added it after talking to Scott Rao. Dave doesn't much pay attention to it but does think it is helpful to remind you that you left the "bean chute" vent open (thus reducing air flow and bypassing the drum) or the bean dump door open (same result) ...

keno wrote:But I wonder if by blocking the air intake you are mainly just reducing the total airflow volume. And in turn a reduced volume of air results in the roaster heating up that air more. If so, then isn't blocking the vents essentially the same as turning the air down? Or is the thinking that the vents closer to the drum perforations don't allow as much opportunity for the air to be heated, so the path the air takes through the roaster influences the thermodynamics? If the latter, then I wonder if covering the vents might hinder cooling ability later in the roast.

In any case I'd be curious to hear how you think this affects control of the roaster and quality of your roasts.

No I the fan is drawing the same amount of air through the roaster, I'm quite sure of that. Blocking the vents is redirecting the airflow closer to the burner flames (there are 4 burners and I now see the flames of the one closest to the rear being drawn towards the rear of the drum by the airflow).
Visually I do think my roast is more even. It will take me some time before I can really tell the difference in the cup. I am using a bit less propane than I used to. My Mill City is up at a vacation home and I can't seem to spend enough time there to really nail the difference in taste profiles if any. But I posted as I thought it was food for thought. I'd really like to be able to construct a metal insert to put below the drum to better direct the air down the flame channel ... but that is a bit above my skill and time level at the moment. I'm not trying to turn my drum roaster into an air roaster ... just trying to boost efficiency and uniformity of the roast and it is just fun to tinker.

Steve

Clint Orchuk wrote:Did the same thing on our Ambex YM-2 a couple years ago. Taped up the 'A' on one side of the roaster and the 'x' on the other side near the rear of the drum to prevent cool air from entering the back of the drum. Worked great. Tried to tape up one more letter on each side, but the amount of airflow was reduced too much.

Way ahead of me! Glad to see someone else thinking along these lines.

Are you folks having capacity issues with your roasters to necessitate these mods? The roasters mentioned hear have atmospheric burners, so they explicitly heat the air, where most of that heat does get drawn into the back of the drum. If you start blocking vents, that may affect the burners themselves, distorting the flame coverage and making it less affective at low power. If you are worried about cold air coming in -- since these are solid drum roasters, then just increase the gas output with increased fan speed, making the roast more convective.

The roaster has plenty of power as you know since your tagline says you own one. I had just observed that I could increase the incoming air temp to the drum by redirecting the airflow by occluding some of the vents which would allow unheated air to enter the drum. In looking at the burners, there is no shortage of oxygen or air to the burners which remain a nice blue flame. Increasing the air speed is not what I want to do with my roasts.

If you go to the Mill City site and look at the current gas TJ-067 roaster:
https://millcityroasters.com/shop/coffe ... e-roaster/
... they now have a totally different "airflow" panel in the side, two horizontal slots. Totally different from mine (5 vertical slots) and you can see both if you scroll through the pictures.
The aluminum grid panel is removable and eventually I may replace my tape job with a perforated plate that performs the same function. In this roaster you can modify fuel delivery, airflow speed through the drum, drum rotation speed, and also apparently the airflow path into the roaster.
Just tasting an Ethiopian I roasted 2 days ago with my airflow modification ... one of the best I've roasted ... can't say it is due to my mods but certainly didn't hurt the roast.

Steve