Next Steps in Prototyping

2011 October 19

We have successfully tested the bean dynamics for coffee and cacao without any airflow added. The basic concept of the bowl with return vanes works. What has yet to be sorted out is how to evacuate the bowl quickly and completely and how airflow will affect the bean dynamics.

The roasted beans need to be evacuated from the roasting bowl in just a few seconds after reaching the optimal degree of roast to prevent over-roasting. We tested the output of each return vane to be able to calculate how much arc of the bowl edge would need to be opened up to allow the beans to be flung out of the bowl using centrifugal force. The test showed that each vane pitches 0.5 lbs. of coffee per second and there are 14 vanes pitching a total of 7 lbs. per second. With a maximum batch of 35 pounds of green coffee, which is 30 pounds of roasted coffee, our original idea of opening three vanes-worth of arc has an evacuation time of 20 seconds — too long. If we used the entire circumference of the bowl, the evacuation time would be 5 seconds, which is excellent.

This lead to the idea of lifting the entire ring of vanes allowing the beans to be channeled into annular chamber around the bowl and redirected downward into the cooling bowl below. I sketched a possible design for this idea.

We also wanted  to test the how bean dynamics might be improved by moving the return vanes to the the joint between the spinning bowl and the stationary mounting ring for the vanes. This would eliminate some drag before the beans hit the vanes.

We had the vane strip reworked  and then tested it with roasted coffee. There was some improvement in flow. We also noted that the smoothness of all transitions is critical in keeping bean flow uniform.

The next tests we plan on undertaking is painting a few of the beans and running them in unpainted beans while recording their movements with a digital movie camera. We want be sure that the mixing is very thorough and that there are no sticking points where overheating could occur.

Meanwhile, we have ordered the radial blade blower to begin building our airflow test jig. This particular blower has a 3/4 hp motor and can produce a maximum flow of 1600 cfm.

The jig will be moveable and adjustable for airflow (volume and velocity) and delivery height above the bowl. This should allow us to optimize our design with the addition of airflow.

2 Responses
  1. November 30, 2011

    Why divert the beans out and down from the roasting chamber? If you are using centripetal force to evacuate the bowl, the path of least resistance would be to allow the beans to flow out horizontally into your cooling apparatus. That would have the added benefit of keeping everything at an accessible height.

    • January 20, 2012

      Thanks for your suggestions regarding bowl evacuation. I may have to abandon my engineer’s bias towards symmetry. I have wanted to do a roasting bowl and a cooling bowl for that reason. Perhaps the cooling bowl will change. I appreciate your feedback.

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