What is fan speed in coffee roasting?

Fan speed controls the airflow through the roasting drum - how much air is being drawn through the bean mass and out of the roaster. On drum roasters with adjustable fans, speed is typically expressed as a numbered setting. On the Aillio Bullet it runs from 1 to 9; on other machines it may be expressed as a percentage or airflow rate.

Fan speed has several interconnected effects on the roast. Higher fan speeds increase airflow, which draws more heat away from the bean mass (convective cooling effect) and removes smoke and chaff more aggressively. Lower fan speeds reduce airflow, allowing more heat to build in the drum but also allowing smoke to linger longer - which can contribute to smoky off-flavours at very low settings. Fan speed also affects the movement of chaff: sufficient airflow is needed to carry detached silverskin out of the drum and into the chaff collector.

Many roasters vary fan speed during the roast rather than holding it constant - starting lower in the drying phase to preserve heat, then increasing in development to manage smoke and temperature. For home roasters on machines like the Bullet, learning how fan adjustments affect the rate of rise and overall heat trajectory is one of the more nuanced aspects of developing a refined roast profile.