For the indirect-mode, the grain bed is also considered to be a deep bed but is
described by four equations of mass balance, thin-layer drying, heat transfer rate and
energy balance inside the grain bed. The deep bed Eqs. (4)–(10) are used, but conduction
in the grain bed is assumed negligible due to the absence of direct radiation on
the top surface of the grain bed. Therefore, the first term on the right-hand side of
Eq. (9)