Our triple pass drums are really drums with seven drying zones
which optimize the drying of hygroscopic products, using well established
principle of heat transfer. This is done by applying high air temperature
with high velocity to the wet product (inlet of drum) and low temperature
with low velocity to the dry product (outlet of drum).
For some products and where space allows it, we offer our single
pass drums which are of course of simpler construction than the
triple pass. To obtain the same high inlet velocity as with the
triple pass drum, we use an inlet cone which expands to the drum
diameter. In order to increase contact between product and drying
surfaces and to improve dispersion of the product in the hot air
streams, some of our single pass drums have a central displacement
tube, or “bullet”.
The single pass drums rotate slower and require more force on the
drum drive. Therefore, we have located the drive above the drum
on a bridge structure. As a result, the loading on the drum rollers
and tire is reduced increasing their useful life.
Both types of drums have bolted-on tires which can easily be replaced
and squared, with all fasteners accessible from the outside drum.
So called “quad pass” drums are really also three pass
drums with only four drying zones. The inlet air velocity is very
low. This reduces convective heat transfer resulting in higher product
temperatures reaching a level of boiling water. This type of “backward
flow” drum operates at higher outlet temperatures and lower
efficiency. The high drum outlet temperatures can cause product
break-up and increased dust.
Another special feature of Duske Dryers is its unique recirculation
system. Our dryers take hot exhaust gases and bring them back to
the burner housing. There are two major benefits here. First is
fuel efficiency. Since the gasses are already hot, less energy is
required from the burner to heat the air. Secondly, the amount of
exhaust going to the scrubber system is decreased. Typically, at
least a third of the airflow through the dryer can be recirculated.
The volume of recirculation is modulated with the burner firing
rate. With the higher exhaust humidity we have systems in operation
which make hot water through a heat exchanger.