RF drying - An Ideal drying system
Conventional mode of drying textiles after they have been dyed is a slow process. The heat transfer from the external heat source to the surface and its interior invariably result in the surface becoming heater than the interior. This can cause detrimental effects such as baking and dye migration in addition with the development of heat spots which is provided by the non-homogenous nature of textile materials. Textile industries can increase throughput and fill orders more prominently using Radio Frequency (RF) Dryers. RF drying offers high volume, high speed and high quality drying which is what exactly needed in textile industries. Because of the RF waves concentrate in the wettest, densest portion of the textile assembly, RF drying has a leveling effect that practically eliminates any problems of uneven shrinkage and over drying. But conventional methods of heating cannot produce such efficient dyeing.
RF Dryer Uses Dielectric Energy
-Occurs though rapid oscillation of molecular dipoles within an electrically non-conducting (dielectric) material.
-Caused by the application of alternating high frequency voltage. The work done by the dipole molecules in aligning themselves with the applied alternating field appears as heat generated within the dielectric material.
Dielectric heating encompasses the technologies of RF and MW heating. Although the heating effect is the same, the major differences between the two techniques lie in the frequencies used;
Those below 100 MHz are known as RF
while those above 100 MHz are MW. Commonly 13.56 and 27.12 MHz are used for RF.
Effect of Materials on RF Heating
The key measure of ‘heatability’ is the loss factor of the material which determines how well the material absorbs the RF energy. If the material has a high loss factor, it absorbs energy quickly and thus heats quickly and its vice versa. Water has relatively high loss factor while textile fibres generally possesses low loss factor which increase with moisture content. Thus in the process of RF drying, when a wet textile material is exposed to dielectric energy, the water molecules preferentially absorb power which increases the temperature of the water to its boiling point for its vaporization. The textile material, having only a low loss factor absorbs very little dielectric energy itself but becomes hot by means of heat transfer from the water. The rate of power absorption, and thus heating rate, decreases as the water content of the textile material falls during drying, thus the moisture content becomes uniform throughout the material so over drying will not occur.
