1. Types of veneer dryers
(1) For the common classification of veneer dryers, the heat transfer to veneer can be divided into the following types:
Air convection veneer dryer: It transfers heat to the veneer through hot air convection to dry the veneer, such as various mesh belt dryers.
Contact veneer dryer: It uses hot steel plates to contact with the veneer, and directly transfers heat to the veneer to dry the veneer. For example, the hot plate dryer that is being developed abroad has its advantages: the dried veneer is flat, the thermal efficiency is high, the drying speed is fast, and the energy saving is more than 36%.
Radiant veneer dryer: Use microwave, infrared (including far infrared) and other radiant energy as a heat source to dry the veneer.
Among them, microwave energy is a kind of electromagnetic radiation energy. The commonly used microwave frequencies for wood drying are 915 MHz and 2450 MHz, and the wavelengths are 0.328 m and 0.122 m, respectively. The microwave drying mechanism is usually explained by the microwave electric field causing the arrangement of dipolar water molecules in the veneer, which rapidly rotates with the electric field, causing friction to generate heat and evaporate water. Due to the limited technical conditions and the high cost of microwave drying, the single board drying is rarely used in microwave drying.
Infrared (including far-infrared) is also an electromagnetic wave. Their wavelength is 0.76 to 400 microns. They have the characteristics of ordinary light, and can be radiated, oriented, penetrated, and absorbed into heat. The characteristic of far infrared ray is that its wavelength is basically the same as the wavelength of the molecular movement of wood and water, both in the range of 3-16 microns, so far infrared ray can form a strong resonance with the molecular movement of wood and water. It absorbs heat radiation energy, so the far infrared ray drying board has higher thermal efficiency than ordinary infrared ray.
The use of electrical energy to generate infrared and far infrared rays consumes too much energy. Boron compounds, carbides, metal oxides, etc. are formulated into coatings according to a certain proportion, and coated on the surface of the heater to make infrared radiation elements, which can be heated by steam, direct combustion gas, etc. and this a more economical method. At present, infrared veneer drying is still in the test. Some people abroad believe that infrared dryers heated by combustion gas may be used in large quantities.
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