Rich, I think you are not reading my post clearly.

As an example, when the rotation is ~> 70, mathematically the beans will get "stuck", as the centrifugal force is greater than gravity. When it is slower than ~70, the beans tumble.

As Hank mentioned, this is one area I can adjust the profile.

boar_d_laze wrote:Does that mean we expect the moon to fall into the earth? Or to fly away from it?

If the centripetal acceleration imparted by rotation of the drum were greater than the acceleration imparted by gravity the beans would never leave the inner surface of the drum. The question isn't whether or not they tumble -- or course they do -- it's the amount of time they remain in contact with the interior surface of drum at one rpm insufficient to overcome gravity versus the amount of time they remain in contact with the interior surface of the drum at another rpm, also, but differently, insufficient.

Rich

The moon would fall into the earth if we attach a rope to it and force it to revolve around the earth just a bit too closely. We have a fixed drum radius here, and the gravity is not pulling toward the axis of the drum. OR maybe I missed your point here... Let me think about that one a bit...

Meanwhile... back to the drum roaster...

So let's start with 0 rpm on a drum roaster, you get max bean-to-drum contact because you have zero agitation. Let's assume uniform distribution of angular momentum, so all beans are in contact with the drum at this point. Let's also assume all beans rotate at the same speed with the drum for simplicity, since that's been our assumption. Then now, bump up the rpm faster than 0, but slow enough that gravity still brings the beans down to the bottom of the drum. The agitation causes the bean tumbling. You and I agree to this.

Where we depart, is that for me, the higher the rpm, the more often the beans tumble from the bottom of the drum up the wall and back down to the bottom. This agitation effect is what I was focusing on.

What you focus on is how long the beans stay in contact with the drum. Let's explore that. At a very high rpm, the beans stay in contact with the drum due to its overcoming gravitational force, which both of us agree on, and we can call this infinite drum contact, zero convection. I believe your line of thinking is that the delta effect over the delta rpm should be monotonically increasing or decreasing. Put differently, basically I believe your conjecture here is we have high rpm giving us infinite drum contact, so low rpm must give us low or zero drum contact. Well this is in fact true in free space. But the act of agitation renders this continuum model inaccurate.

You neglected to consider the effects of air flow in the drum unless the discussion is purely academic. Airflow and drum rotation work together to modulate heat transfer. If Henry wants to increase conduction, he can slow down the drum speed and lower air flow. For more convection; increase drum speed with higher air flow.

Granted it's near impossible to quantify the degree of conduction vs convection by changing drum rotation; except maybe with careful cupping. Henry's well versed in coffee roasting theory so I'll take his word on this one.

hankua wrote:You neglected to consider the effects of air flow in the drum unless the discussion is purely academic....

If this is directed at me, my entry into this thread was entirely limited to correcting a mistake Henry made regarding whether faster rpm leads to greater or lesser bean contact with the drum. Period.

If you want to discuss general roasting technique and theory we can. But let's start a different thread.

Granted it's near impossible to quantify the degree of conduction vs convection by changing drum rotation; except maybe with careful cupping.

For purposes of quantification, take the appropriate measurements and calculate the Rayleigh number. When it comes to quantification, iunlike a working familiarity with fluid dynamics, "careful cupping" doesn't add a great deal to the determination[/i].

Henry's well versed in coffee roasting theory so I'll take his word on this one.

• A. Not the point; and
  B. A logical fallacy.

Rich

fu11c17y wrote:... So let's start with 0 rpm on a drum roaster...

Andy,

I'll PM you my phone number and we can have the discussion without confusing and boring everyone else.

Rich.

chang00 wrote:Rich, I think you are not reading my post clearly.

Perhaps I misunderstood what you meant, but this is what you said, which I quoted, and to which I responded:

With faster drum speed, the bean has less conduction heat transfer and more convection. Faster drum speed=more fluid bed behavior and vice versa.

I'd love to discuss the role various proportions of contact conduction vs convection conduction and all that stuff, but that wasn't the substance of my earlier response.

Rich

Rich, please do read the entire posts and not quote out of context.

Generally speaking, in a solid drum one pass roaster, convection is responsible for about 85% of heat transfer, and conduction from the drum to beans in perhaps about 5%.

If you have access, search Dr Henry Schwartzberg's writing regarding this subject and calculation.

After I finished reading this thread and pondering the how high the beans travel up the side depends on RPM ideas it occurred to me that most drums have fins on the inside to stir up the beans and it's possible that in that case, the height the beans travel might not be significantly related to RPM until the RPM has passed a speed near what drums normally turn at.

Of course that might be completely incorrect.

Ira

Most commercial solid drum roasters two sets of spiral fins, which compose of two inner and outer fins, with several flights or steps. The outer fins generally move the beans from back to front, and the inner fins the opposite direction. The smaller roasters have fewer steps, and I believe the larger commercial ones have more, like six. The steps move the beans side ways, in addition to the back and forth of fins.

Because of these fins and steps, the movements of beans do not follow the simplified "arch" or calculation for centrifugal force. It is also the reason smaller roasters will receive slightly more conductive heat from the drum to bean, although this can change if a different drum material is used; ie cast iron vs mild steel, and double layered drums.

Even at the point where the centrifugal force is greater than gravity, due to the shape of the coffee bean, only a small percentage of total surface is in direct contact to the drum. The convection heat on all other surfaces of the bean is still responsible for the majority of heat transfer.

boar_d_laze wrote:Are you sure about this? I imagined it would be the other away around on the bases that:
1. Faster drum rotation -> greater centripetal acceleration of the beans
... But I'm certainly willing to be wrong.

Andy worked out the basic physics of this long ago. There is a maximum rotation speed at which the beans stick like motorcycle riders on at state fair's cylinder. But at any slower speed, well designed turning vanes increase the bean agitation. Home drums like the Behmor, Hottop and Quest, small sample roasters like the open Probat drums or the old Burn design have relatively lazy drums. The bigger shop roasters since the Probat and Gothot designs of the 1930s have faster moving drums and the beans swarm like riled up wasps.