Thermal radiation drying is the use of heat source, through infrared radiation, directly transfer energy to the heated object, it is essentially different from conduction and convection heating, energy transfer does not require intermediate medium, infrared wavelength range of 0.75 ~ 1000 μm, of which the wavelength of 0.75 ~ 2.5 / μm is near-infrared, radiator temperature 2000 ~ 2200 °C, radiation energy is very high; The wavelength is 2.5~4μm, and the radiator temperature is about 800~900°C; The wavelength is greater than 4 μm for the far infrared, the radiator temperature is 400 ~ 600 °C, the radiation energy is low, although the radiation energy of the far infrared is low, but the molecular vibration wavelength range of organic matter, water molecules and metal oxides are above 4 μm, that is, in the far infrared wavelength region, these substances have a strong absorption peak, under the irradiation of the far infrared rays, the molecular vibration is intensified, the energy is effectively absorbed, and the coating film is quickly cured, therefore, the thermal radiation drying is actually the far infrared radiation curing, It has the following advantages: (1) high thermal efficiency; (2) Fast heating and short curing time, which can shorten the length of the equipment and reduce the floor space; (3) The surface of the substrate and the coating film are heated at the same time, so that the heat transfer direction is consistent with the solvent diffusion escape direction, and the defects such as bubbles and pinholes on the surface of the coating film are avoided; (4) The structure of the equipment is simple, the investment is small, the solvent vapor is naturally discharged by the principle of hot air rising, a large amount of circulating air is not required, the amount of dust in the room is greatly reduced, the appearance quality is high, although the thermal radiation drying equipment has many advantages, it is not suitable for the workpiece of complex shape, so as not to be cured in the shadow part, despite this, radiation and convection can be used in combination, and the characteristics of fast heating of thermal radiation and good appearance quality of the coating film are used as heating in the heating section. The holding section uses convection hot air to heat uniformly, so that the coating film cures completely consistently.
Different coating materials have different blackness (i.e., absorption capacity), and materials with high blackness have strong absorption capacity and high thermal efficiency. The blackness of the paint is mostly between 0.8~0.9.
In the far infrared region, the radiation wavelength and absorption wavelength can be basically matched, the drying efficiency is high, and the curing is also fast. Near-infrared rays only vibrate electrons, absorbing them all with a thin layer of 1 μm on the metal surface and a thin layer of 0.1 μm coating. Therefore, radiation curing allows the surface of the metal and the entire coating film to absorb and convert the radiant energy into heat energy at the same time, so that the coating film is effectively cured and the metal is not heated as a whole. A newly developed high-infrared radiation element makes the far-infrared, mid-infrared and near-infrared energy reasonably distributed, and uses the near-infrared to make the surface of the workpiece heat up rapidly, so that the heat transfer direction is consistent with the solvent release direction, and the heating and curing speed is faster.
The moisture and solvent vapor in the drying chamber will absorb the radiant energy and attenuate the radiation flux, which is not conducive to the drying of the coating film and should be eliminated in time.
In order for the coating film to effectively obtain radiation energy, the radiation distance should not be too far. Generally, the flat part is 100mm, and the more complex workpiece is 250~300mm.
The radiant energy is directly proportional to the fourth power of the absolute temperature of the surface and inversely proportional to the wavelength. Therefore, for far-infrared rays, a radiator with a surface temperature of 400~600°C is used to make it have a higher radiant energy, and the wavelength is between 4~15μm. For coatings with molecular vibration wavelengths beyond this range, a special infrared absorption synergist can be added (different coatings, different synergists) can be added to match the coating absorption with the radiation wavelength, reduce the drying temperature, and shorten the curing time.
Due to the high surface temperature of the radiator, the high temperature of the upper part of the chamber cannot be ignored due to the natural convection of hot air. Thus, in the altitude direction, the number of emitters decreases from the bottom up.
The thermal radiation drying equipment is composed of a chamber, an infrared radiator, an air curtain, a ventilation system and a temperature control system. The chamber body and air curtain are the same as those of the convection drying room. There are two types of ventilation systems: natural exhaust and forced ventilation. For water-based paints, low-solvent content coatings, and moisture drying, natural exhaust can be adopted, and air inlet holes can be set every 1.5~2.5m at the bottom of the chamber, and the air intake can be purified through the filter screen and the air intake volume is adjusted with the ram, so that the solvent vapor is discharged from the exhaust chimney in a uniform flow. Coatings with high solvent content need to be forced ventilation, and the density should be higher in the solvent volatilization section and exhaust port.
There are two types of radiators: gas type and electric heating type. From the perspective of appearance, there are several types of electric heating type, such as tubular type, plate type and bulb type, and there are many applications of tube type and plate type. The tubular emitter is a quartz tube internal resistance wire, coated with far-infrared radiation coating, and backed by an aluminum reflector; If metal pipes are used, the pipes need to be filled with magnesium oxide powder and thermally conductive insulating materials. The plate emitter is a resistance wire set in the silicon carbide plate, the surface is coated with far-infrared radiation coating, and the back side is a thermal insulation material, so the heat utilization rate is low. In addition, there is a resistive band type direct heating electric heating far-infrared emitter, which is a far-infrared radiation coating directly sintered on the surface of the resistance wire, and the radiator itself has low heat consumption and fast heating, which is very suitable for intermittent heating and drying room.
The power of the electric heating far-infrared emitter is generally 3~5w/cm2. The new high infrared radiation element consists of tungsten filament (2200 °C), quartz tube (800 °C) and reflective screen (600 °C) to form full infrared band radiation, with a power of up to 15~25w/cm2, with high radiation intensity, high radiation density, fast heating (about 1min), and drying time below 5min.