Flame Cap Kilns come in many different shapes and sizes. They all seem to work quite well, but there are some fine points of design that will help users determine the most appropriate configuration. Parameters to consider include feedstock type, available materials and cost. Keep in mind that large round cone shapes can be more expensive to fabricate in small welding shops than pyramid or trench shapes. On the other hand small cone shapes may be easier in some cases. Tubes can often be found ready-made. Look for other ready-made containers like small dumpsters or old tanks. You also need to consider how you will quench and unload your char. More on that later.
This Flame Cap Kiln Design Guide is a work in progress, but I want to share it now because I am getting busy with other projects and I don't know when I'll be able to get back to this to make it more complete. You can download the Flame Cap Kiln design guide, or just take a look here: