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Hi all - first up, thanks for the welcome! Great first impression!

I have some rather esoteric questions to ask the brains trust (ie. you) about heat transfer ratios in different roaster types. I admit the theoretical nature of these questions. I am looking to use this info to build a better roaster, but I recognise that a theoretical perfection won't necessarily correlate to cup quality. There's my disclaimer!

My background: a mate and I roast with a Corretto (heat gun, bread maker) and have been doing so for nearly 18 months now. We've roasted a lot of coffee in that time (nearly 200kg/450# - we have thirsty friends!) and have experimented constantly with different ways of utilising the heat gun's output. Our system works best with 500-800g (1.1-1.75#). Any less and the mass of beans is too small to maintain its heat evenly; any more and the HG needs to be turned up too high to reach FC by 10-12mins. Still I sometimes wonder if it's the best it can be. Koffee Kosmo has generously shared his Turbo Oven roaster ideas here and elsewhere and that method has me intrigued. As is true for many of us, the idea of a 2kg/4-5# roaster is very appealing for future ventures...

So I started looking into various designs for roasters of this size... Unwisely, I decided to have a look at some of the first principles of roasting. What heat transfer methods do they use? What kind/rate of mixing? How much airflow? How do these (and other factors) affect the structure of the bean post-roast, moisture loss percentage, rate of staling, etc.

Pretty soon I found this article by Terry Davis (Ambex Roasters) where he asserts that modern drum roasters are 80% convective in terms of heat transfer. I later found this thread on this very forum where someone was reporting back having spoken to Karl Schmidt from Probat. Karl had given him a similar story for their (drum) roasters; 80% convective, 16-18% conductive, 2% radiated. That thread also gives some information on fluid bed roasters (96% convection).

There are several other articles/threads I've come across - you probably all know what it's like looking for this kind of information; especially on forums, the really useful info is just so disparate!


I want to build some kind of roaster that will roast close to 1kg/2#. In this thread, I just want to throw some ideas around about how knowledge of heat transfer methods and other thermodynamics will help me (and others) to build better roasters.
Here are some starters:
- Drum: 80% convection, 16-18% conduction, 2% radiation. Low airflow but excellent physical agitation. Large heat mass (all that metal!) so can afford a small temperature differential (am I right there?).
- Fluid bed: close to 100% convection. Very high airflow with good (though not even?) physical agitation. Large temperature differential, mitigated by high air flow.
- Corretto (heat gun/bread maker): I'd guess ~90% convection. High airflow, though the air doesn't go "through" the beans... Concentrated stream of very hot air can be a problem. Very inefficient system (ie. no lid) such that heat input is required to be very high.
- Turbo oven (think KK's design): I'd guess that the TO roaster would have a higher radiated heat transfer component, but still >85% convective. Lower air flow, though it does pass through the beans. Seems to be quite efficient (eg. 1400W can handle ~700g/1.5# roasts) with available heat meaning that heat input can be more gentle.

My perfect roaster, based on the above, would have:
- good airflow
- a significant component of heating that is not convective
- efficient use of available BTUs
- excellent physical agitation
>> all of which are meant to encourage gentle heating.

What think you?
Cheers
Stuart.