APV Dryer Handbook - Umbc

20
material comes into
contact with the hot
gases and, as a result,
higher evaporative rates
can be achieved than
when using countercurrent
flow.
In addition, many
thermo-sensitive materials
can be dried
successfully by this
method. Such an
Figure 9. Fixed tube rotary dryer
arrangement lends itself
Photo courtesy of Mitchell Dryers Limited, Carlisle, England
to the handling of pasty
materials, since the rapid flashing off of moisture and consequent surface drying
limits the possibility of wall build-up or agglomeration within the dryer. On the other
hand, counter-current operation normally is used where a low terminal moisture
content is required. In this arrangement, the high temperature gases are brought into
contact with the product immediately prior to discharge, where the final traces of
moisture in the product must be driven off.
In both these processes, however, gas velocities can be sufficiently high to produce
product entrainment. Therefore, they would be unsuitable for low density or fine
particle materials such as carbon black. In such cases, the indirect-fired conduction
type dryer is more suitable, since the dryer shell is usually enclosed in a brick
housing or outer steel jacket into which the hot gases are introduced. Since heat
transfer is entirely by conduction, conventional flighting and cascading of the
material is not used. Rather, the inside of the shell is fitted with small lifters
designed to gently turn the product, while maintaining maximum contact with
the heated shell.
The steam tube unit is another type of indirectly heated dryer, which is particularly
useful for fine-particle or heat-sensitive materials. This dryer can be of either the fixed
tube variety – equipped with conventional lifting flights designed to cascade the
product through a nest of square section tubes; or alternately, a central rotating tube