Design Study – Centrifugal Bowl Roaster

2010 March 2
Share

One roaster geometry that has intrigued me is a bowl. While there are no shop scale roasters that use the bowl geometry, there is at least one large scale roaster that uses a modified bowl approach. The basic idea is to use a spinning bowl to throw the beans up the sides using centrifugal force. The beans then tumble back down to the center. In addition, the spinning action creates a swirl along the spin axis that further helps agitate the beans for uniform roasting.

I’ve been thinking about the use of a perforated bowl and a solid bowl. The perforated bowl allows for convective heat transfer via heated air blown through the holes and additional agitation of the beans by forcing them off of the surface of the bowl. The solid bowl is better for being able to independently heat the bowl for conductive heat transfer.

The first sketches I’ve been working on are some thumbnail sketches of possible airflow schemes. The airflow path in a roaster is of critical importance, since the majority of the heat transfer is done via convective heat transfer. While there are other variations, the sketch below shows five of the schemes that seem most promising to me.

The Sketches Analyzed

Sketch “A” is characterized by a central down vent. The heated air enters through the holes in the perforated bowl, helping to agitate the beans. It then flows down a central vent to evacuate chaff. When the roast is completed, the central duct is moved in such a way (by dropping or being lifted) as to suck down the beans into the cooling chamber. The bowl can be heated by a shielded burner to give adjustable conductive heat transfer. Advantages: for a sample roaster can have a glass top to watch the whole process, has some great cooling system possibilities. Disadvantages: complexity of the bearing system since the central vent is cylindrical and active, rather than a simple shaft.

Sketch “B” is characterized by bowl perforation supplied air and peripheral sidewall vents. The heated air enters through the holes in the perforated bowl, helping to agitate the beans. It then flows upward to the top of the bowl wall to evacuate chaff. When the roast is completed, bowl would either need to be tilted to dump out the beans or some sort of bottom door opened to allow the beans to fall into a cooling chamber. The bowl can be heated by a shielded burner to give adjustable conductive heat transfer. Advantages: a simple layout for the bearings and support system. Disadvantages: Cooling system is made more difficult to access.

Sketch “C” is characterized by bowl perforation supplied air and peripheral trough vent. The heated air enters through the holes in the perforated bowl, helping to agitate the beans. It then flows over the rim and into a vent trough to evacuate chaff. When the roast is completed, the bowl speed could be increased to spin the beans out into the trough and directed into the cooling chamber (this would require the bowl to be more shallow). The bowl can be heated by a shielded burner to give adjustable conductive heat transfer. Advantages: a simple layout for the bearings and support system, simplified cooling system. Disadvantages: Beans falling into the trough during the roasting process.

Sketch “D” is characterized by bowl perforation supplied air and an overhead vent. The heated air enters through the holes in the perforated bowl, helping to agitate the beans. It then flows upward to the top of the roasting chamber to evacuate chaff. When the roast is completed, bowl would either need to be tilted to dump out the beans or some sort of bottom door opened to allow the beans to fall into a cooling chamber. The bowl can be heated by a shielded burner to give adjustable conductive heat transfer. Advantages: higher capacity for the system, simplified ducting, simple layout for the bearings and support system. Disadvantages: Cooling system is made more difficult to access.

Sketch “E” is characterized by overhead duct supplied air and peripheral side vents. The heated air enters through an overhead duct in a relatively high velocity, which further agitates the beans. It then flows through vents in the wall above the bowl to evacuate chaff. When the roast is completed, bowl would either need to be tilted to dump out the beans or some sort of bottom door opened to allow the beans to fall into a cooling chamber. The bowl can be heated directly with a burner to give adjustable conductive heat transfer. Advantages: higher capacity for the system, simplified ducting, simple layout for the bearings and support system, potential for very high convective heat transfer. Disadvantages: Cooling system is made more difficult to access.

Share

Comments are closed.