Electromagnetic energy of the radio frequency (RF) or microwave spectrum is a means by which energy may be converted to heat within the material to be dried. Water has a dipolar molecular structure. That means that in its natural state, a water molecule will demonstrate distinct characteristics of both negative and positive polarity, explains John Hokanson, vice president of Thermex-Thermatron, Louisville, Ky.
When subjected to an electric field, the molecule will rotate to align itself opposite to the polarity of the field. When field polarity is reversed, the molecule will rotate to realign itself accordingly. If the field is reversed 27 million times per second (RF) or 2,450 million times per second (microwave), the resultant molecular activation is manifest as frictional heat throughout the product. This completely changes the physical dynamic of drying.
In most cases, heat is generated and then transferred by airstream or radiant source to the product. Heat arrives at the product's surface. Drying throughout the product's cross-section depends on the transfer rates and dynamics of convection and conduction. In many cases, the material is simply a poor thermal conductor and the result is low throughput.
To improve speed, process temperatures are increased. This works until one of two effects results: either dry zones limit further increase in speed or the product's surface quality is adversely affected by the process temperature. Some products tolerate higher temperatures from a quality perspective, but begin to produce hazardous or undesirable effluents.
"The process at this point has reached its respective limits in throughput, quality, efficiency, and environmental impact and subsequent yields," Hokanson says.
The direct heat of RF or microwave redefines these process limits. Heat is now introduced at the location of every water molecule. Although the airstream continues to introduce some process heat and does prevent product heat loss, its basic function now is the removal of moisture from the product surface and drying chamber. Once the direct energy of RF/microwave is provided, it acts in push-pull unison with the airstream creating a drying synergy that is greater than the sum of its parts. Where substrate materials are dielectrically transparent, once the water is gone, heating stops.
Process temperatures now are at or below vaporization temperature. Product-surface damage is eliminated, along with any undesirable effluents. The direct application of energy to the product represents energy that is completely used in the process and not energy being discharged as hot flue gases. In addition, the chamber environment can now be redefined to prevent or control oxidation reactions.
The range of products and materials being dried today is considerable. RF/microwave provides the unique drying dynamic necessary to meet the challenge.
RF/Microwave Drying at a Glance
- The direct heat of RF or microwave redefines drying process limits compared to convection or conduction heating.
- RF or microwave's direct energy acts in push-pull unison with the airstream creating a drying synergy that is greater than the sum of its parts. Where substrate materials are dielectrically transparent, once the water is gone, heating stops.
- RF or microwave eliminates product-surface damage along with any undesirable effluents.
- The direct application of RF or microwave energy to the product represents energy that is completely used in the process and not energy being discharged as hot flue gases.
- RF or microwave use means that the chamber environment can be redefined to prevent or control oxidation reactions.