Rather than rotating the drum to move the beans, a fixed-drum roaster uses mechanical paddles to stir the beans within the roasting chamber. This type of design harks back to the early days of coffee roasting, in which a person held the beans in a pan over a fire and stirred them with a spoon. Many early domestic roasting devices used a fixed pan with a mechanical stirrer that could be set over a fire.
A domestic roasting device using a fixed drum with a mechanical stirrer
When heated directly over the flame, a fixed drum has a high risk of causing bean-surface burning, as the surface closest to the flame can become very hot. With the development of hot-air roasting, however, fixed drums began to appear in commercial roasting machines.
The first commercial fixed-drum machine was the Gothot Rapido-Nova, described in a patent first filed in 1969 (Arndt and Rossi 1972). In this machine, rotating scoops stir and lift the beans as hot air is passed through them, and exhaust gases pass through an afterburner and are recirculated.
Detail from Gothot’s patent for a fixed-drum roaster. The beans are stirred in the drum by means of mechanical paddles (13) and heated by hot air blown into the drum (16).
Probat went on to buy Gothot seven years later, and the company uses the same basic design today in their Jupiter line of industrial coffee roasters (Global Coffee Report 2014). Probat refers to the design as ‘tangential’ because the hot air enters tangentially — in other words, parallel to the outer edge of the beans as they circulate within the drum. A similar principle is used in smaller contemporary roasting machines, such as those made by Loring and Bellwether.
Paddles lift the beans into the flow of hot air and the airflow is high enough to further agitate the beans,