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Heat, smoke and chaff - fluid vs. drum
frankvw
In response to lots of reading here (and thanks for the great threads, BTW!) I've done some proof-of-concept tests with pan and drum roasting. The results are... interesting.

First off, the discussion about fluid bed vs. drum roasting (once you get past the fact that drum roasters are more energy efficient and can be scaled up to a tonne or so while FB roasters have inherent capacity limitations) reminds me of the semi-religious "Fuji vs. Kodak film" discussions in my photography days of decades back: it seems to be more a matter of personal preference (not to say prejudice) rather than based on hard comparative data. Some say there is a difference in taste, others say there isn't (at least not enough to pick it out in a double blind triangle test). So the long and the short of it is, probably, that it depends on many practical factors.

Offhand, I would say that if beans are exposed to heat at the same temperature (or temperature curve) for the same time, what happens inside the bean is completely independent on where the heat comes from. Granted, FB roasters heat the beans via hot air (convection) only while beans in a drum also receive heat via contact transfer, but if heating profiles are adjusted so that the insides of the beans follows the same temperature curves in both cases, there should (in theory) be no discernible difference.

That leaves two major other factors which are the main differences between FB and drum roasting: chaff extraction and smoke ventilation. In a FB roaster chaff is immediately blown off and does not burn. Smoke is blown off immediately as well. In a drum roaster, both chaff and smoke can hang around longer, which can have an effect on the outer surface of the bean and could account for the perceived difference in flavour.

Which brings me to the point of this post: some drum roasters have quite sophisticated smoke and chaff extraction facilities, and the beans come out clean and relatively "un-smoked". However, ventilating the drum removes heat, which reduces energy efficiency and affects the heating curve of the beans.

Simply put, I'm looking at two extremes here:
  1. No air flow through the drum at all coats the beans in burnt chaff and essentially smokes them;
  2. Blowing a lot of air through it means that the whole process begins to approximate FB roasting.

This suggests there is an optimum between the two somewhere.

But how to work that out? (I'm not good at thermal modeling; I'm looking more for an experience based general ballpark approach, if there is such a thing.)

Your opinions? coffee drink
Koffee Kosmo
The KKTO hybrid roaster is a combination of both

KK
I home roast and I like it. Designer of the KKTO
Roaster Build information
https://forum.hom...ad_id=1142

https://docs.goog...lide=id.i0
Blog - http://koffeekosm...gspot.com/

Bezzera Strega, Mazzer Robur Grinder, Pullman Tamper Convex,
(KKTO) Turbo Oven Home Roaster.
renatoa
The optimum between the above some of us found to be the so called turbo oven (lid).
Lots of convective heat, evenly distributed, actually this is the best heating method you can buy from the shelf as an existing appliance, and use as is.
Stronger airflow than in a drum, but lower than in FB, enough to exhaust smoke and chaff effectively.
Used in coffee roaster machines known as KKTO and SCTO, but there are at least other two usage scenarios I have imagined over time. One of them is derived from a Loring style drum machine.

LE: If you want a "Canon vs Nikon" Grin moment in this debate, please check the discussion for some opinions about "contact" roasting:
https://homeroast...post_71658
Edited by renatoa on 04/22/2021 7:11 AM
frankvw

Quote

Koffee Kosmo wrote:
The KKTO hybrid roaster is a combination of both
KK

Nice build. So where does the chaff go in that design? I assume the smoke is dealt with via the normal TO ventilation, is that right?
frankvw

Quote

renatoa wrote:
The optimum between the above some of us found to be the so called turbo oven (lid).

My main concern about going the turbo oven route (other than the increasingly unreliable electricity supply here which has interrupted many a roast, not to mention batches of beer being brewed) is that most things dumped on the South African market are of appallingly bad quality. Friends of mine bought one of these things and it didn't last for more than a week. And there's not much choice in these things here, either. One or two models is the best one can hope for.

Quote

renatoa wrote:
Lots of convective heat, evenly distributed, actually this is the best heating method you can buy from the shelf as an existing appliance, and use as is.
Stronger airflow than in a drum, but lower than in FB, enough to exhaust smoke and chaff effectively.
Used in coffee roaster machines known as KKTO and SCTO, but there are at least other two usage scenarios I have imagined over time. One of them is derived from a Loring style drum machine.

Where does the chaff go in a turbo oven? Whatever exhaust vents it seems to have don't look big enough, judging from the photos I've seen here.

Quote

renatoa wrote:
LE: If you want a "Canon vs Nikon" Grin moment in this debate,

No! No! No! Roflmao

Quote

renatoa wrote:
please check the discussion for some opinions about "contact" roasting:
https://homeroast...post_71658

Will do. Tnx!
renatoa
Sorry, didn't noticed you are the same guy from SA already advised about TA solution, and the specific local issues you have there.

Quote

Where does the chaff go in a turbo oven?


The lid is only upper half pf the story, more exactly the heating source.
The lower half is the oven itself, that has many incarnations, and each inventive mind created his own chaff exhaust solution. For me was a standard cyclone system, others approached it much simpler, as a variable aperture slot in the oven wall.

You are right to assume that smoke is already addressed by the lid itself.
Because that device is designed to cook food, it features by design a turbine and exhaust slots of the steam and smoke inherently to roasting process.
Edited by renatoa on 04/23/2021 1:07 AM
Koffee Kosmo

Quote

frankvw wrote:

Quote

Koffee Kosmo wrote:
The KKTO hybrid roaster is a combination of both
KK

Nice build. So where does the chaff go in that design? I assume the smoke is dealt with via the normal TO ventilation, is that right?



It’s primarily a two piece system
The beans roast in the perforated section and the chaff is collected in the outer section

The TO is not a closed system it draws in fresh air to heat and expels the access

Have a look at the blogspot link for explanation and photos
https://koffeekosmo.blogspot.com/


KK
I home roast and I like it. Designer of the KKTO
Roaster Build information
https://forum.hom...ad_id=1142

https://docs.goog...lide=id.i0
Blog - http://koffeekosm...gspot.com/

Bezzera Strega, Mazzer Robur Grinder, Pullman Tamper Convex,
(KKTO) Turbo Oven Home Roaster.
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