chang00 wrote:With faster drum speed, the bean has less conduction heat transfer and more convection. Faster drum speed=more fluid bed behavior and vice versa.

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;
2. Greater centripetal acceleration -> More contact time between bean and drum wall; however
3. Almost regardless of drum rpm (within a proscribed range), the beans spend almost equal time in the air, falling from the top to the bottom of the drum; and
4. Greater contact time -> Proportionally greater energy transference via contact conduction; while
5. Equal Air time -> Relatively equal energy transfer by convection conduction;
6. QED.

But I'm certainly willing to be wrong.

Rich



Topic split from Bella XJ-101 1kg Roaster by Moderator

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;
2. Greater centripetal acceleration -> More contact time between bean and drum wall; however
3. Almost regardless of drum rpm (within a proscribed range), the beans spend almost equal time in the air, falling from the top to the bottom of the drum; and
4. Greater contact time -> Proportionally greater energy transference via contact conduction; while
5. Equal Air time -> Relatively equal energy transfer by convection conduction;
6. QED.

But I'm certainly willing to be wrong.

Rich

I agree with Henry, when I was thinking of the following analogy. It's like grilling steak. If you flip it every few sec you cannot get nice searing grill marks, but the steak will get cooked more evenly through the center. Versus if you let it sit for couple min before flipping, you can get better searing, but you may risk not cooking the center to the desired doneness. So flipping often is like higher RPM.
Not sure if that makes sense...

But steak is soooo different from coffee

Surely there are mechanical engineers in the group. At least for me, this is how I arrived at the rotational speed of 65 rpm for my roaster.

Assuming the entire bean mass as one unit. I roast in 300g (0.3 kg) batches. The radius of my Mini 500 drum is about 0.2m (from recollection)

F=mass * acceleration (gravity) =mass * angular speed ^2 * radius = mass * (rpm)^2 * radius

When the angular speed is ~70 rpm, it exceeds 1 gravity therefore the bean mass will be "stuck" to the metal wall. So I set mine at 65, just below 1 gravity.

Of course the above assumption does not take into account the vanes in the drum and the bean mass is not fixed to one point. But you get the idea.

chang00 wrote:When the angular speed is ~70 rpm, it exceeds 1 gravity therefore the bean mass will be "stuck" to the metal wall. So I set mine at 65, just below 1 gravity.

The issue isn't whether the centripetal acceleration is greater than gravity's acceleration, but how much it alters the vector of force and the point (actually a set of points comprising an arc) of rotation at which the beans actually fall off the vanes. It's not an "all or nothing" but a "how much" proposition. We don't need mechanical engineers, just Physics 7A and 7B.

This in no way affects my opinion that you are one seriously smart guy.

Rich

fu11c17y wrote:So flipping often is like higher RPM. Not sure if that makes sense...

It makes sense, but it's mistaken to the degree of being bass ackwards.

Think of tying a rope to the handle of a bucket of beans and using the rope to swing it around. The faster the bucket goes, the harder the beans push against the bucket. Returning to your analogy, higher rpm is like pressing the steak to the grill with your spatula.

Rich

I have no clue what you guys are talking about

There's a statistical upper and lower control limit to roaster drum speed; too fast the beans are stuck to the drum, too slow they sit on the bottom. The mid point, mean, sweet spot is where hopefully your fixed speed roaster is set. Within the upper and lower practical drum speeds, one can set higher or lower for profiling as Chang00 stated.

So would you rather ride a one speed bike or a 10speed; I'll take the latter.

boar_d_laze wrote:It makes sense, but it's mistaken to the degree of being bass ackwards.

Think of tying a rope to the handle of a bucket of beans and using the rope to swing it around. The faster the bucket goes, the harder the beans push against the bucket. Returning to your analogy, higher rpm is like pressing the steak to the grill with your spatula.

Rich

Ok the steak grilling analogy may be too simplistic. You guys reminded me there is a lot more going on here...

How about another simple analogy (I like to dumb things down a bit ). Higher rpm is like shaking the drum faster, and basically the beans get more air contact the faster you shake the drum.

The analogy of tying a rope to the handle of a bucket of beans is true only if the angular momentum squared is larger than gravitation force, like Henry is saying. Beans will tumble around the bottom of the drum at this lower speed rather than get pressed to the grill with a spatula. The "all or nothing" point is true only if you somehow tape the beans against the drum and not let them fall down, or it's more true if the drum is rotating vertically, rather than horizontally like most drum roasters.

fu11c17y wrote:The analogy of tying a rope to the handle of a bucket of beans is true only if the angular momentum squared is larger than gravitation force.

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

Perhaps the bucket of beans analogy would be clearer if we specify that the person holding the ropes spins the bucket in a "horizontal" orbit as opposed to "vertical," by never rising his arms beyond horizontal to the level of the floor. That way we can, to some extent, isolate the effect of centripetal acceleration and not get confused by gravity. The faster the bucket spins, the greater the force of the beans pressing against its bottom.

Acceleration is acceleration. Gravity is acceleration towards the center of the earth. If another force, lesser than gravity, causes an object to accelerate away in any direction other than the center of the earth at the same time gravity acts upon the object, it will take more time to fall to earth than had gravity been the sole actor; the difference in time depending on the direction and amount of force. You're familiar with the idea that a football thrown by a strong-armed quarterback will take longer to fall to the earth than one dropped -- even though the force of the throw is not sufficient to overcome gravity.

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

hankua wrote:too fast the beans are stuck to the drum...

Chang00 argues the opposite.

Rich