ANS:B - Heat transfer

The controlling factor for a drum dryer is heat transfer. Explanation:

1. Heat Transfer:
   • In a drum dryer, wet material is spread onto the surface of a rotating drum.
   • The drum is heated from the inside, and as it rotates, the wet material comes into contact with the hot surface of the drum.
   • Heat is transferred from the drum to the wet material, causing evaporation of moisture from the material.
   • The rate of heat transfer from the drum to the wet material controls the drying process in a drum dryer. Factors such as the temperature of the drum, heat transfer coefficient, and thermal conductivity of the material being dried affect the rate of heat transfer and hence the drying rate.
2. Diffusion:
   • While diffusion may play a role in the drying process by facilitating the movement of moisture within the material, it is not typically the controlling factor in drum drying.
   • Diffusion refers to the movement of molecules from regions of high concentration to regions of low concentration. In the context of drying, diffusion may occur within the material as moisture migrates towards the surface to evaporate. However, the rate of heat transfer from the drum to the wet material is usually the dominant factor determining the overall drying rate in a drum dryer.