The heat transferred in directheat rotary dryers is expressed by the following equation: Q ¼ Uv aV (Dt)m (7:30) where Q is the rate of heat transfer, J/s, Uva is the volumetric heat transfer coefficient, J/(sm3K) or W/ (m3K), V is the dryer volume, m3, and (Dt)m is the true mean temperature difference between the hot gases and the material.