The motions of the granular bed inside a rotary furnace are governed b
y complex dynamics consisting of two modes. The axial velocity of the
active layer on top of the bed represents the fast mode, while the ove
rall velocity in the bed follows a slower mode. The proposed nonlinear
model focuses on the dynamics of the slow mode. The difficulties of a
n analytic solution are avoided by a numerical method that employs sev
eral correlations. The model can reproduce the experimental transient
responses. It is concluded that rotary furnaces respond better to smal
l amplitude excitations than to large ones; consequently, closed-loop
control is well suited for this type of furnace.