Modeling conductive heat transfer from rotary drum walls to a particle bed via discrete element method simulations, three time scales were determined: 1) the characteristic heating time of the bed, tau; 2) the particle thermal time constant, tau(p); and 3) the contact time between a particle and the wall, tau(c). Results fall onto a monotonic curve of tau/tau(c) vs. phi (tau(p)/tau(c)), with three heating regimes. At low phi, conduction dominates, and the system heats quickly as a solid body. At high phi, granular convection dominates, and the bed heats slowly at a nearly uniform temperature. At intermediate phi, the system heats as a cool core with warmer outer layers. The results of this work have important implications for improving the design and operation of rotary drums (e.g., energy-intensive calcination processes). By calculating tau(p) and tau(c) from material and operating parameters, the characteristic heating time, tau, can be predicted a priori. (C) 2016 Elsevier Ltd. All rights reserved.