(Mod Note: Split off from the Wood roasted coffee thread)

another_jim wrote:Most modern shop roasters have the Probat design -- a solid drum inside an air chamber and completely separate flame chamber beneath that. The design was for peat-coal fired European roasters where you absolutely do not want combustion gases getting into the beans. Firing these with wood would have no effect.

another_jim wrote:...there is contact. But it's slight....The system is designed to isolate the combustion gases from the beans.

Incorrect.

The system is NOT designed to isolate combustion gases from the beans. In a Probat-type roaster, heated air -- containing all the products of gas combustion -- is drawn through the drum by an exhaust fan and intimately contacts the beans.

This is clearly shown on the Probat drawing. Even Wikipedia states:

Wikipedia wrote:Indirect-fired roasters are roasters in which the burner flame does not contact the coffee beans, although the combustion gases from the burner do contact the beans. Direct-fired roasters contact the beans with the burner flame and the combustion gases.

Obviously Wikipedia is not always authoritative. For the benefit of the impressionable minds here on home-barista, I also emailed Marty Curtis. People here might not know Marty. As owner of Combustion Systems Sales & Service Inc, he makes his livelihood rebuilding coffee roasters. People also might not know that every time Marty steps up to the toilet he pisses out more knowledge about commercial coffee roasters than is possessed by everyone on this thread put together -- times 10. Here's what Marty said:

Marty Curtis wrote:The products of combustion will and do convey to the air that you will use for the convection proportion of roasting....Most of the coffee roasters that we artisan-type roast people play with use atmospheric or power burners. These burners burn gas and heat air. The hot air (containing products of combustion) then heats the drum and other surfaces as it moves thru the roaster combustion chamber. Then off to the inside of the drum where the beans are hanging out and tumbling. As the beans spin toward the top of drum they pick up heat from conduction. As the beans get to the point where they can no longer stay against the drum wall they fall back to the bottom of the drum. Here the convective heat transfer occurs (with the products of combustion moving thru).

Then there's another issue:

another_jim wrote:Mr Clean Coffee, Michael Sivetz, is famous for claiming that all combustion gases, from any fuel source whatsoever, spoils the roast.

This is obviously incorrect and Sivetz never said it. Anyone who's used one of his gas roasters knows that 100% of the combustion products from the burners pass directly through the roast chamber.

What Sivetz DID say is that CHAFF, tumbling and burning inside a roasting drum, transfers off-flavors to the coffee. That's at best. At worst, he suggests burnt chaff forms carcinogens. The purported advantage of his fluid bed design is that the chaff is swiftly blown out of the roast chamber before it can burn.

I don't think too many people take his burnt chaff claim seriously, even though the very first paragraph on his site humbly states, "Michael Sivetz is the world's foremost coffee scientist." :-0

The question still remains whether a wood-fired Probat-style roaster might flavor the beans with wood smoke. It is fact that wood smoke would intimately contact the roasting beans. However: the beans are more-or-less continuously outgassing H2O, CO2, CO, and various volatiles. This would limit the opportunity for smoke flavors to work their way in....but now I'm merely speculating.



...split from Wood roasted coffee by moderator...

D'oh. You're right about the Sivetz air roaster and his take on combustion gases versus recirculating coffee ones.

But with all due respect to Marty's piss; I don't see how combustion gases, or at least a lot of them, can go through the drum on a Probat design roaster. Here's the airflow on my Quest M3, which uses Probat style airflow:


Notice that air is either pulled through the drum, in which case it does not contact the flame, or it is pulled around the drum, in which case it does. The air flowing around the drum exits the roaster at the front. It can come into contact with the beans at that point, but the contact is probably not extensive. In order for the combustion gases to get into the drum, they would need to diffuse upstream, against the airflow. This may happen if the airflow is low enough, but again, how extensive can it be?

Not on the M3, but on the Probat L, is a set of dampers at the rear of the drum. Shutting these prevents air from flowing through the drum and creates an "aroma roast," where the beans baste in their own gases. I do not know how often craft roasters do this.

another_jim wrote:Here's the airflow on my Quest M3, which uses Probat style airflow

If you're gonna "prove" what a Probat commercial roaster does or doesn't do based on an amateur sketch of a little home roaster that some guy in Taiwan says "uses Probat-style airflow," well then, sorry Jim, I'm outta here.

Fine, if you don't like my amateur drawings of my amateur roaster, look at this certified professional one you cited earlier.


It shows exactly the same thing. The air enters the rear of the roaster and either flows throught the drum or around it through the flames.

another_jim wrote:Fine, if you don't like my amateur drawings of my amateur roaster, look at this certified professional one you cited earlier. .

I didn't realize that YOU had made that drawing. I apologize if I insulted you by calling it "amateur"; the drawing is certainly a lot better than I could do.

another_jim wrote:It shows exactly the same thing. The air enters the rear of the roaster and either flows through the drum or around it through the flames.

Still scratching my head why you'd say that. My read of the Probat drawing is that it doesn't show where the air enters the combustion chamber at all. But to me the artist clearly tried to show that combustion gas coming off the burners (appearing inside the yellow circle below) makes a u-turn at the back of the chamber and enters the perforated rear of the drum (red arrow).



OK, enough of the drawing micro-analysis.

Yesterday, as a favor, Tomas at Gimme Coffee opened up their Probat roaster to investigate the airflow path. He confirmed that combustion gas comes off the burners, goes to the rear and enters the back of the drum through a bunch of holes.

But with your Quest M3 drawing in mind, I asked him, "isn't there also a gap at the front of the drum where the combustion air could bypass the inside of the drum and go directly to exhaust?" He said, "Well, there's a very slight gap, but when you set up the roaster you adjust the clearance so that it's almost zero. If you do it right, there's almost no gap for gas to escape through."

So here's my Wiseguy Home-Barista theory: you are correct that the Quest M3 uses "Probat style airflow." BUT...it's the airflow of a Probat maintained by an incompetent mechanic.

Seriously: what's the point of having that gap? If, as you said, we were burning peat and didn't want that flavor to enter the drum, it makes sense. But the vast majority of modern Probats burn gas, right? And the M3 runs on odorless electric. If you're trying to design an efficient gas or electric drum roaster, you don't route cold ambient air through the same drum in which you're trying to roast coffee. And you don't dump heated air out the exhaust port until you've extracted all the energy you reasonably can.

I understand that in the little M3 a few watts of wasted power are no big deal. But on a commercial scale, the cost of wasted energy associated with that gap at the front of the drum could be huge.

Is your Quest roaster out of adjustment, and should the drum really be moved forward so that minimal air escapes at the front? Should the air intake to the heating chamber be at the bottom instead of at the rear? Am I missing something here?

The gap on the M3 is about 1/2 centimeter, smaller than a 10 screen size bean. It is required for the way this roaster works. The air intake is in the rear of the drum, as shown on my diagram. From there it divides, flowing either around the outside of the drum over the heaters or inside the drum through the beans. Without the gap, the air flowing on the outside would have nowhere to go, since the air exit is on the inside of the drum.

D'oh again. Sivetz gives an illustration of the earliest Probat roasters; and I have to say you're correct. The diagram shows that the air flows in, not from the rear, but from the floor and through the flames. Only then does it divide, either entering the drum at the rear and exiting at the bean entry; or flowing around it, and exiting from a second exhaust hole at the top, positioned outside the drum. The second exhaust hole means the gap in front is not required (except to prevent rubbing). It also allows the dampers at the back of the drum would be closed completely, blocking all airflow through the drum, whenever something smelly were being used for heat, or to achieve the "aroma roast."

(The M3 can do a version of the aroma roast by shutting down the fan and stopping all airflow, since the heaters are electric. This slows down roasting time, raises the drum temperature, and adds a lot of distillates to the taste. It also adds a lot of tarry crud to the drum. It's not to my taste, but I don't know how this compares to the Probat's closed damper roast. Completely sealed, airless drums were the norm for roasting until the 1930s.)

I got fooled, because in the commentary, Sivetz suggests that the Probat is a cleaner design (by his standards) then the perforated drums in early Burns roasters. But as far as I can see, the arrangement does nothing much different from a perforated design except allowing the drum to be sealed. Perhaps the maximum airflow is higher.

another_jim wrote:Without the gap, the air flowing on the outside would have nowhere to go, since the air exit is on the inside of the drum.

Understood, that's part of the reason why I said, "Should the air intake to the heating chamber be at the bottom instead of at the rear?" But of course even better would be to have that air enter the heating chamber at the front, and enclose the Quest with an outer shell to pull the intake air through.

I'm hoping now that our friend in Taiwan will make the "M4 roaster", see crummy diagram below:



The outer shell would use waste heat to preheat the incoming air. This would keep the roaster cooler to the touch, save energy, and possibly increase capacity.

Or maybe we could do this mod ourselves (I wuz thinking of getting an M3)????

No offense intended gentlemen...
But is there any chance that this thread will return to a discussion of coffee?

No offense intended Jon but these gentlemen are putting to bed (at last) a lot of the earlier issue of this thread.

They may have stretched it a bit into roaster design but it seems to me that it is agreeable now (for the people that were opposing the idea) that the combustion gas would indeed be in intimate contact with the beans.

This is not by any means a conclusion that wood roasting would taste differently, but it is relevant to the coffee discussion at hand. I think.

Beside I am learning a lot and at the very least I am getting an idea of how I could experiment wood roasting with my soon-to-arrive roaster.

So, if you do not mind let them go...

Sorry to post on this again, but it's just a redirect

AndyS wrote:I'm hoping now that our friend in Taiwan will make the "M4 roaster", see crummy diagram below:
... the outer shell would use waste heat to preheat the incoming air. Or maybe we could do this mod ourselves (I wuz thinking of getting an M3)????

Some existing M3 owners are getting there.. I'll also try enclosing the exhaust tube, to recover all the heat possible.