Posted by dBndbit on 09/04/2009 18:58
I like the idea of experiments and shoot-outs! Though we may run into problems trying to isolate the heat modes (contact, air, IR).
As I understand IR, all objects above the temperature of absolute zero are always radiating heat at some wavelength. So everything hot in the roaster is also radiating heat energy. It doesn't matter how things get hot, just that they are hot. The hotter they are the more they radiate, but the radiated energy also includes shorter and shorter wavelengths. At very high temperatures, objects start to radiate at wavelengths so short we can see it as a dull red glow. IR and visible light are the same thing, just different wavelength ranges.
Point being that isolating an 80+% any-one-mode of roaster may be difficult. I suspect a high-speed air spout roaster might be the only true high-percentage "convection" roaster, though convection is really the wrong word for this. And even so, the hot metal surfaces of the roast chamber get hot and become an IR source for the beans in addition to whatever they may catch from the air or conduct by contact.
Same would be true in any drum with or without perforations. Once heated to 450-500F, a solid drum becomes a source of IR for the beans. Even a hot air gun is radiating a huge amount of IR anywhere you can see the heat elements. (line-of-sight) That's one of the reasons their heating effect diminishes so quickly when you back off. IR effects drop at a strict inverse square of distance unless reflected or focused.
I guess what I'm saying is that I think IR can play a bigger role in pumping up the efficiency of most any roaster if it's given the chance by using reflective metals. And it might be a mistake to over-simplify our thinking about how heat energy is working at any given point in a roaster.
I really like Marshall's (endlesscycle) view of:
analyzing your roaster design in the shoes of a coffee bean with an engineering degree
Be the bean.