within mainly by conduction. The evaporation of moisture takes place through the product layer into the surrounding atmosphere.
The output dried product is then scraped off the drum at some point during the revolution of the drum (about ¾ of the revolution
from the point of feeding) and collected in a bin or conveyor for further processing. In a double-drum design, the liquid material is
fed into the space between two drums, rotating in opposite directions. The gap between the two drums controls the thickness (0.5–
2 mm) of the feed layer applied to the drum surfaces. Drying takes place on both drums as the reservoir formed between the drums
allows a thin film of the material to coat on the surface of both drums. The dried product is again scraped off at the appropriate
point and collected for further processing. In a twin-drum dryer, the drums rotate away from one another, and the feed is applied
by the feeder of each drum. This type of drum dryer is used for materials with solids that are dusty when dry (Okos et al., 1992).
The drying process in a drum dryer can take place quite rapidly, since both heat transfer through the metal drum and mass
transfer of water out of the thin film are typically quite good (Heldman and Hartel, 1997). The residence time of the material on
the drum normally ranges between 15 and 60 s, regulated by the revolution speed of the drum (Heldman and Hartel, 1997). The
material properties that affect drum adherence include viscosity, surface tension, and wetting power. It should also be emphasized
that care must be taken for heat-sensitive materials as the dried products are typically exposed to high temperatures (above
100 C). For materials sensitive to heat damage, a vacuum drum dryer may alternatively be used to reduce the drying temperature.
This system is similar to the other drum dryers except that the drums are enclosed in a vacuum chamber. However, the
equipment and operation of vacuum drum dryers are relatively expensive, which limits this technique to only high-value products
(Tang et al., 2003).