You're right Lawrence
We have the three-layered drum, and I also did a few dozen roasts on a Ghibli with the standard stainless drum last year.
To our beloved admins: if this boarders on the non-commercial policy, please advise! I find the questions very interesting, but since Dan and I both have commercial interests I find it difficult to to balance.
lsjms wrote:Dan, I would assume if you do not have the power to scorch or tip then the roaster would be incapable of running a very aggressive profile. I am a huge fan of my ' barrel on flames' and wonder if your guys Ghibli can do a 9 min roast with control? 17 mins +- is quite a long way from acceptable. I assume you can reduce the time by dropping batch size, a lot of roasters seem to way over quote the capacity and then the temps are hard to control because of the massive thermal mass with barely any beans inside to reign the temps in.
The flame definitly has the power to scorch the beans beyond recognition, when it did that on the first few roasts it turned out we just got the settings wrong
. It's quite a capable burner and has to be set carefully. What the drum does is a very good job of applying the heat to the beans. During the roast, we have air temperatures within the drum of up to 270degC, yet a 12kg roast of my beloved Yirgacheffe takes ca. 22min. And no, the coffee is in no way baked
, explanation below. I am not sure anymore if it is a reasonable goal to achieve a 9-minute-roast on the ghibli at all. The only coffee I ever roasted this fast on my little Quest was a Kenyan Gatomboya imported by Trabocca that was brilliant. I would assume this translates to a ca. 15 minute roast on the ghibli?
Dan, correct me if I'm wrong please:
What you need to keep in mind when comparing overall roast times and what is often overlooked in the forums is the method of heat application. In normal cast-iron drum roasters ala Probat/Giesen/Toper, you have a certain ratio of convective vs. conductive vs. radiative heat transfer. As a general rule of thumb, the more convection is involved, the shorter the roast times can be, and the more coduction and/or radiation is involved, the longer the roast times have to be. Every manufacturer praises it's own ratio as the best
. I believe that coffee-tech tries to maximise the conductive heat transfer and minimise the convective while allowing enough airflow to remove excess heat/chaff/smoke. The 3-layered drum also enhances the radiative heat transfer, but I can only speculate on the effects of that.
Trying to force really short roasts on a roaster that minimizes convection will result in not so tasty coffee, and trying to do 25 minute roasts on a sivetz will certainly bake the coffee to something unrecognizable.
When coffee-tech first informed me about the chimney requirements for the ghibli I thought they must be nuts - 22cm diameter for my setup seemed a little gross, and what volumes of air would the ghibli produce to need such a big chimney? Turns out my thinking was a little flawed: The Ghibli has a very controlled, and I believe quite low airflow when compared to other roasters that needs quite a resistance-free chimney - hence the bigger chimney, especially if cooling and drum air share the same chimney. We have a 25cm diameter chimney now and are quite happy with the results.
Edit: granted, our Chimney is nearly 10m long. That adds to the required diameter as well I believe.
I do need to do a lot more roasts to get confident controlling the machine, though - even though the coffee generally tastes good I still feel a little like roasting in free flight mode every time I drop in a batch.
Of course, Toms' roastlogger helps a lot!