Indirectly Heated Drum

Machines with indirectly heated drums send hot air from a combustion chamber through the roasting drum. This design protects the drum from direct flame contact, allowing the machine operator to use higher roasting temperatures with less risk of bean-surface burning. Like classic drum roasters, indirectly heated drum roasters mix the beans in the drum for even roasting and dump the beans into a separate cooling bin for efficient cooling at the end of a roast.

Advantages: Indirectly heated drums provide a clean roasting environment and permit faster roasting at higher temperatures, with less risk of bean-surface burning, than most drum-roaster designs.

Disadvantage: This design is a little less fuel efficient than the classic drum roaster.

Indirectly Heated Drum Roaster

This Jasper has an indirectly heated drum.

Fluid-Bed

Fluid-bed roasters rely on high airflow to keep the beans aloft and rotating in the roasting chamber. Because beans lose density as they roast, to maintain proper bean rotation these machines require very high airflow early in a roast and declining amounts of airflow as a batch progresses.¹⁶

Most fluid-bed roasters do not include a separate cooling bin; instead, room temperature air is passed through the roasting chamber at the end of a batch to cool the beans. This system is not ideal because the chamber’s surfaces are hot, which inhibits the cooling process. Many users of fluid-bed roasters buy and use separate cooling bins.

Advantages: Fluid-bed roasters are affordable and reliable, have a small footprint, and pose little risk of bean-surface burning.

Disadvantages: Excessive airflow damages flavor and decreases fuel efficiency; the machine operator must compromise between the gas and airflow settings desired for optimal flavor and those required for proper bean rotation.

A fluid-bed roaster transfers heat to beans almost exclusively by convection. Air heated in the burner box (l) passes through the roasting chamber (2) and leaves the roaster through the chimney, while the cyclone (3) traps chaff. The beans enter the roaster through the funnel (green), circulate on a bed of hot air in the roasting chamber, and exit the roasting chamber through a door (not shown).

Recirculation

In contrast to the single-pass roasters listed above, recirculation roasters send a portion of a roast’s exhaust air back through the burner chamber to recapture its heat, thereby decreasing the fuel requirements of the roasting process. These machines have recently become popular by virtue of being very fuel efficient, limiting bean-surface burning, and providing a very stable, moist, and repeatable roasting environment. The stable environment offers the side benefit of improving the ability of automated roasting software to track a programmed roast profile. The one significant drawback to these machines is a higher risk of smoky flavors in the coffee due to the beans dwelling in smokier air during roasting.

Advantages: Recirculation roasters offer fuel efficiency and fast roasting, with limited risk of bean-surface burning. They facilitate performance of automated profiling software, if any is used.

Disadvantage: Roasters sometimes report development of smoky flavors.

This recirculation roaster sends most of the exhaust air back through the drum and transfers heat almost exclusively by convection. In this design the burner also serves as the afterburner, incinerating particulate in the exhaust air before sending a portion of it up the chimney. The white arrows represent airflow. As in the drum-roaster illustration, beans enter the roaster through the green funnel, roast in the roasting drum (orange), and cool in the cooling bin (blue).

The Lilla (left) is on early attempt at a recirculating-air design, while the Loring is the best-in-class design.