- The first commercial coffee roasters were directly heated drums, and they are still the most common design today.
- Controlled airflow allows for more efficient heat transfer via convection and faster roasting.
- Thicker or double-walled drums reduce the amount of conduction and increase the amount of convection.
- Using more convection reduces the risk of bean-surface burning, which permits higher roasting temperatures and faster roast times.
- Gas burners can be either atmospheric or forced-air. Forced-air burners control the gas-to-air ratio in order to burn more efficiently at all power settings.
- Unlike directly heated drum roasters, hot-air roasters heat the coffee entirely by convection, which further reduces the risk of bean-surface burning and allows for higher roasting temperatures than those in directly heated drums.
- Hot-air roasters include indirectly heated drums, fluid-bed roasters, and fixed drum roasters that use paddles to stir the beans.
- Hot-air roasters became more commercially viable after the development of recirculation systems, which save heat and greatly reduce the amount of gas needed to power them.
- Some recirculation systems incorporate an afterburner to remove smoke and other compounds from the roasting gases.
- Commercial roasters include continuous roasting machines ideal for roasting coffee in bulk, but these have become less popular as customers seek more varied types of coffees.
- Individuals prefer different roasting machine designs for various practical reasons. Little evidence shows that one kind of roaster is associated with a specific flavour, except for the particular characteristics associated with fast roasting.
Afterburner A combustion chamber that heats the exhaust gases from a roaster to very high temperatures in order to completely combust any remaining smoke, particulates, and organic compounds. Afterburners are required in some jurisdictions to reduce air pollution.
Cyclone A device for separating chaff and larger particulates from exhaust gases in a roaster. The cyclone blows the exhaust gases in a spiral motion, and the chaff is separated out via centrifugal forces.