In a drum roaster, what is the approximate ratio of convective heat transfer to conductive heat transfer?

In a drum roaster, the roasting process primarily involves two types of heat transfer: convective and conductive. The approximate ratio of convective heat transfer to conductive heat transfer is typically around 70% convective and 30% conductive, making it key to achieving optimal roasting results.

Convective heat transfer arises from the circulation of hot air in the roasting chamber, which directly heats the surface of the coffee beans as they tumble. This method of heating is effective because the movement of hot air allows for even heat distribution, fostering a consistent and uniform roast.

On the other hand, conductive heat transfer occurs when the beans make contact with the metal surface of the drum. While this method contributes to heat transfer, it is less dominant than convection in this roasting setup. The higher proportion of convective heat is beneficial, as it allows for better control of the roast profile and minimizes the risk of scorching the beans, which can occur when relying too heavily on conduction.

Understanding this balance helps roasters fine-tune their equipment and techniques to achieve the desired flavor profiles and characteristics in the final coffee product.