I haven't been able to figure out exactly why a fan is necessary in drum roasters or what exactly it does and why one can't do whatever it does by controlling the heat at the source.

If anyone can explain it to me, I'd appreciate it.

Regards,

dan

Drum roasters without any fan would heat the beans extremely unevenly, since they'd rely on only conductive heat ( from an extremely hot drum surface) and a bit of radiant heat to roast. You'd be left with severely tipped scortched beans, that are likey still grassy and underroasted in the centers without the convective heat from air being heated and drawn thru the drum.

It would also be an extremely inefficient way to roast the coffee.

I roast with an RK drum in a Weber gas grill and control only the heat. And I can get very good tasting roasts that are not tipped or uneven. I guess I get some air flow through the drum as you need air for combustion. The grill has vents to facilitate air flow - side vents at the bottom and a rear vent and I open the doors in front to further facilitate air flow. I used to work in a boiler house where we had both forced draft and induced draft boilers - I think of this as an induced draft operation. I think I could improve my roasts with more air control, I've even considered reconstructing the grill cover with a chimney that would further the induction process but I don't have those skills. There are lots of folks using these RK drums and some of them sell there coffee commercially so I think I'm not alone in thinking you can do a pretty good roast without air control. Interestingly, some of the folks who use these grills even restrict the rear vents on their grills to retain the heat better.

There is a slight operational paradox here:

Convection oven roast faster than still ovens. Coffee will roast faster in a faster airflow at the same temperature than in still air.

But if you turn up the air in a drum roaster during the roast, and leave the heat the same, the first thing that happens is that the air temperature drops, so that you will slow the roast down.

If you keep both these effects in mind, you will have no problem regulating air flow and heat to get the profile you want.

As Tom mentions, every roaster design will have a minumum airflow required for every given drum temperature to prevent charring.
-- Some roasters this minimum is built into the base design. For instance, Probat L roasters have enough agitation to keep the beans moving fast enough in the drum to sop charring. Old style Burns perforated drums relied on convection currents to move air (these do have an upper drum temperature limit).
-- Other types, you have to make sure the amount of airflow is enough for the current stage of the roast and drum temperature (Probat sample roasters and the Quest M3 are like this -- they allow you to get the drum very hot early in the roast, when it can't hurt the beans; but you'll do in the roast if you ever get too hot with low airflow late in the roast).

It's helpful to understand these dynamics at the outset, when you are learning how to use a new roaster. But in the end, you have to get so used to running your roaster that handling its dynamics is as automatic as walking or driving a car.

I'm asking the question because I'm thinking of making a more traditional roaster than my "milk can" roaster to handle instead of about 3 or so pound to handle 6 or so pounds.

With my current design, which I've used for thousands of roasts, I have never gotten tipping or charring and I don't have a fan. So, I'm trying to get my head around why the fan is necessary. Obviously, I guess it is, since the 'real roasters' all have them.

So, what I'm understanding so far is:

1. You can use them to regulate the temperature by cooling the bean mass. I can see why one would want this in an electric roaster, but not so sure about a gas roaster since you can drop the temperature pretty fast by lowering the flame.

2. Moving hot air through the beans results in a more even roast. I can see that, certainly, but wonder if one can avoid it by simply making the beans move faster?

(On a side note an unrelated, I've modified a normal weber BBQ by introducing air through the bottom via a 12 volt computer fan. Using real charcoal I can raise the temperature from 200 to 800 + in just a minute or two. So, air flow certainly is important and in the case of the BBQ it doesn't require a very hefty fan. My son is programming an Arduino to control the fan, take measurements with a thermocouple and report them back to my iPhone.)

All modern commercial roasters, without exception, use airflow. The reason, again, if you missed it in my first post is convection, that is using airflow to transfer heat to the beans at reduced environmental temperatures. This both reduces cost and improves quality.

Also, you reasoning about maneuverability is fallacious. Turning down the heat, whether gas or electric, will reduce the heat going in; but cannot speed up the heat going out. If you want to maneuver a roaster with a lot of thermal mass, i.e. a heavy drum and case, you need air to shed heat. Alternatively, an inherently maneuverable roaster with a light drum and case still needs air to stop the beans from charring on the drum's hot spots.

If you want to hand build a museum piece roaster, just do it; but don't kid yourself that an unventilated design can ever be as good as a ventilated one.

Not to mention pristine chaff collection. Those who have roasters with large fans and cyclones know this in their zero-chaff results.

I don't see a specific notation of moisture management. With out controlled extraction of the offgassing water the process would trend toward baking vice roast.

I think those last points are right on. Despite the fact that my RK drum throws off a fair amount of chaff in the grill - and I get off a lot more in my 2 step cooling process - first with a 6 h.p. shop vac and then with a colander and a leaf blower - I still have more chaff than I'd like in the finished product. It certainly isn't pristine. And secondly, my finished weights are higher than it seems to me they'd be if I had better air flow.

MerleApAmber wrote:I don't see a specific notation of moisture management. With out controlled extraction of the offgassing water the process would trend toward baking vice roast.

There is very little unbound H20 present in the beans after first crack finishes. Preventing "baking" would be more an issue of not roasting too long, at too low a temperature, or any other fault in the roasting process that destroys the aromatics.

To your point about "moisture management", it would entail a proper drying phase for each coffee, tailored to its density,hardness, and charge weight so that the beans are evenly dry throughout, without actually having any roasting occurring at this stage. If you're in the caramelization phase of a roast, moisture management shouldn't even be on the radar. Not blowing your fine aromatics out the back of your roaster should be, however.