Indirect (drum drying) is more economical
Efficiency
In contrast with indirect drying on a drum dryer, a large quantity of hot air is
necessary for "direct drying" in order to achieve evaporation. In indirect drying, heat
is transferred by, for instance, steam to the (metal) wall, which in turn transfers it to
the product on the other side of the wall. All the heat transferred by the wall is also
used to dry the product and does not leave the machine or chimney unused. This
makes indirect drying a much more efficient process. An extra advantage is that
large dust recovery systems (such as filters) are no longer necessary due to the
absence of drying gas