Heat transfer is conventionally divided into three categories: convection, conduction, and radiation.
Conduction is the process of heat transfer between materials that are in direct contact. For example, when you place a saucepan on an electric hob, heat passes from the hob to the saucepan via conduction. Conduction then transfers the heat from the bottom of the saucepan to the inner surface, and from there to the rest of the pan and its contents.
Conduction transfers heat energy from the hotter material to the cooler one. In the coffee roaster, heat transfers to the beans by conduction when the beans touch any hot surfaces inside the drum, including the drum walls and paddles. Heat also transfers from bean to bean via conduction. Conduction is a relatively slow process that occurs only if the materials are in direct contact.
Convection is the process of heat transfer via the movement of a fluid. In the case of coffee roasting, the ‘fluid’ is air. Inside the saucepan, the water touching the bottom of the pan picks up heat energy by conduction from the surface of the saucepan. Hotter water is less dense, so it rises, carrying the heat energy with it. Meanwhile, cooler water flows in to replace it. This movement creates currents within the water that efficiently move the heat around.
Conduction (1) and convection (2) in a saucepan. By conduction, heat travels from the heating element to the hot plate, from the hot plate to the saucepan, and from the saucepan into the water. The hot water becomes less dense and rises, creating currents in the water. The movement of the hot water is called advection. Convection is the combination of conduction (the heat transfer into the water) and advection (the movement of the water).
These natural currents are not the only form of convection, however. If you stir the water, you make it circulate more quickly. The water still carries heat energy with it, so this is also a form of convection.