Some roasting machines, such as the Joper, heat the air outside the roasting drum and then duct the hot air into the roasting chamber. In an indirectly heated machine, the drum surface is cooler than it is in a classic drum roaster, thereby reducing the risk of bean-surface burning. This allows the machine operator to use hotter air temperatures during the roast. Hotter air temperatures allow for faster roasting, which is also time- and cost-efficient for roasting large batches in industrial machines.
The major disadvantage of indirectly heated drums — as with any roaster that uses hot air for heat transfer — is that the rate of heat transfer depends on airflow, and the operator’s options to adjust airflow settings are limited. The heat produced in the burner must provide the appropriate amount of heat energy to the beans during the roast. Because of the way some roasters are designed, this can be technically challenging to accomplish. Unless its design incorporates some method of recirculating hot air, an indirectly heated machine also uses more fuel than a classic drum roaster (Rosa 2019).
Recirculating Drum Roasters
Some indirectly heated drum roasters are designed to recirculate hot air as a means of improving energy efficiency. Compared with directly heated drum roasters, these machines can reduce energy consumption by 25% (Nogueira and Koziorowski 2020). However, when hot air passes through the roasting chamber, it picks up odours, smoke, and other pollutants, which, when recirculated, can taint the flavour of the beans.
Manufacturers of recirculating air roasters can cope with this problem by making the main heat source of the roaster also perform the function of an afterburner. Loring, for example, positions the machine’s burner at the base of the cyclone. The cyclone removes chaff and also performs the role of an afterburner. Afterburners operate at around 650°C (1200°F), so roasters of this style also need to include ductwork that allows for cool air to be mixed back in with the hot air before it is returned to the roasting chamber.