DIAGRAM OF RF EQUIPMENT
SCHEMATIC
A basic schematic of a Radio Frequency dryer is shown below. The dryer receives standard power (i.e. 480V, 60 Hz) through
the Switchgear. In the Power Supply section, line voltage
is stepped up to high voltage AC through a transformer and
then changed to high voltage DC through rectifiers. In the
Oscillator section, high voltage DC is changed to high frequency,
high voltage RF energy and transmitted to the applicator or
electrodes where it is applied to the work. All of this is
controlled by a modern control system.
The key to effective application of
RF energy for drying is the right applicator, or electrode
design. Traditionally, heating was accomplished by creating
a uniform electric field between two parallel plates. This
approach is capable of heating thicker materials uniformly
because a high voltage gradient can be established in the
material. However, it does not work well for thin materials
such as webs. In order to establish a high voltage gradient
in a thin web material, the plates must be very close together
which can cause arcing between the plates.
For thin materials, the strayfield electrode design was developed.
This design creates an electric field between alternating
parallel rods that gives a higher voltage gradient in the
web for faster heating. A variation on this electrode design
for thicker webs is the staggered strayfield design. This
allows for more uniform heating of thicker webs. This has
also been used for thin beds of ceramic powders. As a general
rule, materials under 1/4” thick use the strayfield design,
materials 1/4 ” - 1/2” use the staggered strayfield design,
and materials over 1/2” use the parallel plate design.
In all of these electrode designs, the material can be either
self-supporting or can be transported on a conveyor.
Unlike conventional drying methods that heat most materials at similar
rates, RF has the ability to heat at very different rates.
Some of the unique properties of RF drying are:
- Water has a high loss factor compared to other materials,
so the amount of water determines the heating rate. Thus a
wet product heats quickly but as the water evaporates, the
heating rate decreases (i.e. self limiting process).
- The heating rate is different for different materials.
- The heating rate is proportional to the amount of water. This
means a wet area of a product heats faster than a dry area
of the same product and the result is even moisture content
throughout the product (i.e. moisture profiling).
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