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The wavelengths of various electromagnetic radiation sources range from the solar spectrum to the microwave range, and sunlight radiation can only penetrate the surface of the material. According to the wavelength of solar radiation, the material can only absorb part of the incoming radiation. Infrared radiation is often used in the drying of powder coating, sheet, film and other materials. Although most of the wet materials are poor conductors in currents of 50-60 Hz, the impedance drops sharply at high frequencies. This radiation can be used to heat the particles and reduce the resistance to internal heat transfer. The absorbed energy is selected by water molecules, and the drying process of the material achieves energy saving. However, due to the high operating cost of this drying method, only valuable materials can be used.
Radiative Heat Transfer Drying is the process by which the heat energy of the heating plate reaches the dry material. The key to this type of device is the radiant heat of the radiant heating plate and the wavelength that the selective material can easily absorb. For a long period of time, near-infrared wavelengths below 3μm have been used, but in recent years, far-infrared (greater than 4μm) ceramic heating plates with higher organic energy absorption have been used. Far infrared heating surface absorption is better, so as a coating film drying effect is better.
Experiments show that most of the far-infrared energy can be absorbed by materials within 1 mm from the surface, and even if it is thick, uniform heat transfer is difficult to achieve. Microwave drying has also been used in more and more applications in recent years. It uses microwaves as electromagnetic waves to directly enter the conductive body, causing polar molecules and polar groups in the material to vibrate and oscillate, and is internally generated by internal friction between molecules. Heat, to achieve the purpose of drying. This is a process in which electrical energy is converted into heat, and the thermal efficiency can reach 60% to 70%. The pure microwave drying operation is costly, but during deceleration drying, microwave drying can be used as an auxiliary means to increase the drying rate when the moisture is difficult to dry with hot air and conductive heat transfer is removed. In freeze drying, residual ice can also be removed by microwave drying. (1) Far-infrared dryer. The infrared rays generated by the electric energy will have the effect of absorbing penetration from the surface to the inside. Therefore, the drying rate is higher and the thermal efficiency is also higher. However, its nature is the same as that of light, so no shadow can be left when it is irradiated. (2) High-frequency dryer. When the material is placed in a high-frequency, high-voltage electric field, uniform frictional heat is generated within the material as the water molecules move. This method is suitable for thick plates and materials with poor thermal conductivity, mainly used in the wood processing industry. (3) Ultrasonic dryer. The sound wave hits the material with the appropriate frequency, and the cavitation effect is fully exerted with the intense molecular motion, which accelerates the drying rate and is suitable for the drying of heat-labile materials.
Where is the radiation dryer applied?
Radiation dryers include far-infrared dryers, high-frequency dryers, and ultrasonic dryers.