Idea Sketches: How to Move Roasted Beans to the Cooling Bowl

2011 August 30
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Once coffee has been roasted to the correct degree, the coffee must then be cooled quickly. In virtually all roasters, this means emptying the roasted beans from the roasting vessel (usually a drum) into a cooling section. The beans are then cooled rapidly as the roasting vessel is refilled with green coffee to start the next batch.

In the bowl roaster, I have decided to use stacked bowls: one for roasting and one for cooling. In the roasting bowl, hot air is circulated through the beans, in the cooling bowl cool air is circulated through the hot beans. Ideally, the bowls are not spaced very far apart because there are two human factors to be considered. First, I would like for the roasting bowl to be viewed without the need for steps, a riser, or a ladder. Second the cooling bowl needs to be able to empty into a container not more than 2 feet high that sits on the floor.

There are three ways I have considered for moving the roasted beans from the roasting bowl to the cooling bowl below. The first way is to pivot the bowl and dump the beans into the bowl below. I’ve eliminated this method because it increases the required bowl spacing and makes the drive mechanism more complicated. The second way is to use gravity by lifting the diverter cone at the end of the roast cycle and letting the beans drop into the bowl below.

The third way is to use the centrifugal force of the spinning bowl (already the means for mixing and bean movement) to spin the beans out of the bowl through a duct and down to the cooling bowl. This would involve retracting a section of return vanes to create the opening for the duct.

The duct would have vanes that help direct the flow of the beans in a relatively gentle manner.

Each method has its merits and problems. Dropping coffee through a hole in the bottom of the bowl is simple. The problems are 1) the bowl must be slowed down which may require braking, 2) the cone actuator mechanism goes through the area where the bowl support bearings and drive gear are, which complicates things, 3) the bowl shape for best airflow may not be the best shape to allow quick flow of the beans down the hole, and 4) a scraper blade may be required for speedy flow and that will require an actuator, which is a complication. The centrifugal duct method allows the emptying to happen while the bowl is spinning (no requirement to slow down). The problems are 1) it is a more “violent” process which could be hard on cacao, 2) it requires a mechanism to retract a ┬ásection of the return vane ring, which is a complication, and 3) it may not empty as fast as may be desirable.

We plan on trying to modify the the prototype to be able to test the centrifugal duct method. Due to the way the prototype is built, it is impossible to test the drop method, but we can observe bean dynamics that will help us project how it might work. It is possible that we will choose one method to empty beans into the cooling bowl and the other method to remove the beans from the cooling bowl.

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4 Responses
  1. keith potter permalink
    August 30, 2011

    How interesting. I had initially thought “stop the bowl, spin it backwards and keep the same vanes, just catch them on the back side.” This would be fast, but touches on all the negatives of “stopping the bowl,” and, likely, burning. I can’t remember if it has a reversible drive, anyhow.
    If the vanes were interchangeable, though, the centifugal duct could be dropped in place of the vanes, or could be mounted on the back side before then removing 3-5 vanes, so as to keep the bowl moving at the same/roast speed. For some reason, I always picture a jet engine concept for the vanes whenever I see the prototype. See here: http://bit.ly/pScvVB and here: http://bit.ly/nxmBxJ

    Just my random ramblings, keep up the good work and the blogging, I love reading about the progress…
    ~Keith

    • September 9, 2011

      It does look like a jet engine, sort of. We will be testing various ways to divert the coffee into the duct. Thanks for you ideas!

  2. September 8, 2011

    Joe- I would propose another method, for which all of the mechanics may already be present:

    Use a vacuum hose.

    Presumably, the exhaust fan/turbine can generate enough vacuum pressure to drive a “shop vac” like suction hose. Hopefully this hose would have enough movement for the operator so they can be sure to fully vacate the roasting chamber of all coffee, quickly and easily

    That’s the problem I see with the centrifugal design. It seems likely to me that it will take a long time to get all of the beans out of the chamber, resulting in an uneven roast.

    The gravity/hole method seems like it would have the fastest evacuation time, which would is why I would lean toward that for the final solution.

    • September 9, 2011

      Steve, we are going to test the evacuation time using the centrifugal method. The beans are traveling at 20 ft/sec, which is pretty fast. I am hopeful that it can evacuate faster than typical drum roasters, but only testing will say. I will have to noodle on the vacuum idea. Thanks for the thoughts!

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