The inductive electric heater was used to correct the unevenness in moisture content
on the left side. It is a ‘low-frequency’ (50 Hz) inductor, with a ma netic circuit,
producing up to 3.4 kW over a 20 cm width (heated surface 0.26 # m ), for a wide
range of gaps (3-10 mm), without impedance adaptation. Figure 7 shows the inductor installed near the outside surface of the drum dryer
(3 mm gap). When the inductor is energized, an alternating electromagnetic field
intersects the portion of the drier shell adjacent to it. Since the wall dryer is
electrically conductive, an eddy current is induced in the wall, and heat is generated
from the resistance to this eddy current.
Three zones are possible for installing the inductor (Fig. 7). However, this
technique brings heat to the wall, which must be heated before increasing the local
heat flux (left side), thus requiring the time of several rotations. Thus, the inductor
location is rather a question of practicability, washability, etc. The environment in
zone A is very humid because of the steam produced by the drying of product. That
is why the actuator was placed under the drum in zone C, between the feed zone
and the knife.
Figure 8 shows the process response when the local actuator is switched on. In the
first part of the test, the system is used without the inductor. The initial unevenness
between left side and middle is obvious, the left side being 15% wetter. When the
inductor is started, the left moisture content decreases rapidly, changing from
Xf=4-2% to Xf=2*8%. The response time is about 100 s, with an applied power of
3 kW.
The inductor efficiency is about 75%, i.e. 2-25 kW is transferred to the wall. The
additional evaporating flux does not consume 2.25 kW (on 20 cm width). In fact,
most of electrical power input also results in a reduction of the energy coming from
the heating steam condensation; only a small part (10%) is net energy to increase
the local mass flux. We must choose an inductive power adapted to the unevenness
to control and to the size of the drum.