What is Infrared Radiation?

The atoms and molecules that make up a substance are in thermal motion and change their energy state from time to time. When the state of energy transitions from high to low, electromagnetic waves are radiated, taking away energy in the form of photons. In daily life, we encounter various types of radiation, such as solar rays, thermal radiation, radio waves, and x-rays. Although they have different forms, they are essentially the same in nature and are collectively referred to as electromagnetic radiation. All radiations follow the same laws of reflection, refraction, diffraction, and polarization, and they travel at the same speed, called the "speed of light." They differ only in frequency. There are several ways to generate infrared light. The most common is that it is generated by the thermal movement of material molecules. This is thermal radiation. ^[1]

Infrared is a type of electromagnetic wave with a wavelength range of 0.76 to 1000 m. Generally, infrared is divided into two parts with a wavelength of less than 5.6 m. Those nearer to the red light are called near-infrared; those with a wavelength greater than 5.6 m and far from the red light are called Far infrared. The division of near-infrared and far-infrared is relative. Some people also divide infrared into near-infrared, mid-infrared and far-infrared. A wavelength of 1-3 m is called near infrared; a wavelength of 2 to 40 m is called middle infrared; a wavelength of 40 to 1000 m is called far infrared.

The generation of infrared is closely related to the temperature. All objects in nature will radiate infrared when its temperature is above absolute zero (ie -273.15 ° C). The amount of radiant energy and its distribution by wavelength are determined by the surface temperature of the object. The radiant energy of the surface of an object is proportional to the fourth power of the surface temperature of the object; the wavelength range where the object's radiant energy is maximum (called the peak wavelength) moves toward a shorter wavelength as the temperature rises, and the peak wavelength is lower when the temperature is lower duration. That is, the higher the temperature of an object, the more it can radiate near-infrared with a shorter wavelength, and at a lower temperature, it can radiate with a longer wavelength.

Once the infrared is absorbed by the object, the infrared radiant energy is converted into thermal energy, which heats the object to increase its temperature. When the electromagnetic wave (ie, infrared) generated by the infrared radiator directly propagates to the surface of an object at the speed of light, and its emission frequency matches the natural frequency of the molecular motion of the object, it causes strong vibration of the molecule of the object, resulting in intense friction within the object. Heat. Therefore, infrared radiation is often called thermal radiation, and infrared radiation is called thermal radiation or temperature radiation. ^[2]

According to the aforementioned properties of infrared rays, when an infrared radiation coating is used, the coating layer can be heated to accelerate drying. When a beam of infrared light is irradiated on the coating surface, part is reflected by the coating surface, part enters the coating interior and is absorbed by the coating, which is converted into thermal energy to heat the coating from the inside of the coating, and part is transmitted through the coating to the substrate surface The coating is heated with the interior and converted from radiant energy to thermal energy from below the coating. Due to this self-heating effect, the coating can be heated quickly and effectively, and the curing of the coating is carried out from the inside out to the bottom up. The drying process is similar to preheat drying, and the drying effect is better. The schematic diagram of infrared irradiation, reflection, absorption and transmission is shown below.

Infrared radiation drying characteristics

1. Fast drying speed and high production efficiency. Compared with hot air drying, the drying time can be shortened by 3 to 5 times. It is especially suitable for heating and drying large surface areas.

2. Good drying quality. In the process of infrared radiation, part of the infrared is absorbed by the coating, and the other part passes through the coating to the surface of the substrate. Thermal exchange occurs between the substrate surface and the bottom of the coating, so that the direction of heat conduction is consistent with the direction of solvent evaporation. In this way, not only the heating speed is fast, but also defects such as pinholes, air bubbles, and "orange peel" during the drying process are avoided. In addition, infrared drying does not require a large amount of circulating air flow, so there is less flying dust, the coating surface is clean, and the drying quality is good.

3. Rapid heating and high thermal efficiency. Radiation drying does not require an intermediate medium, and can be directly transferred from the heat source to the coating, so the temperature rises rapidly. It has no heat consumption caused by the intermediate medium and reduces the heat taken away by some hot air, so it has high thermal efficiency.

4. The equipment is compact and flexible to use. Due to the short drying time of infrared radiation, the equipment has a short length and a small footprint. The structure is simpler and more compact than hot air drying equipment, which is convenient for construction and installation. It is flexible to use and easy to operate. It is very convenient to adjust the temperature with a transformer.

5. There are certain requirements for the shape of the workpiece. Because the infrared rays travel straight, it is difficult to dry the coating in some places where it is not exposed. The arrangement of the radiators should be considered, especially the design of the reflecting plate, and the illumination efficiency must be considered as much as possible. For workpieces with complex geometric shapes, the irradiation shadow is more serious, and it is difficult to control the irradiation distance to be approximately equal, which may cause the paint film on the surface of the workpiece with a short irradiation distance to change color, but the distance or the shadow part is not completely dry. It is difficult to guarantee the drying quality of complex workpieces.

6. Due to the rapid temperature rise of the coating, the coating solidifies in a short time (20 ~ 30min), and sometimes the solvent does not have time to evaporate, which also affects the film formation quality, which should be controlled.

7. If the temperature is too high, the paint film may become discolored and brittle, and the light red paint film tends to change color more easily. ^[2]

When infrared drying coating is used in production, a certain number of infrared radiators are often assembled into a through-type drying chamber. The painted parts or products are carried by a conveying device and passed in a drying room to cure the coating.

Far-infrared radiation drying is an earlier application of radiation drying. It is superior to hot air drying in many ways, but it has disadvantages. Therefore, there is a far-infrared radiant hot air drying chamber which combines the two together.

At present, there is no fixed drying room in China, and it must be designed according to the specific conditions of production. When designing the drying room, select the appropriate radiator and arrange it properly. Determine the optimal radiation temperature and distance, determine the size and structure of the drying room, and take into account the thermal insulation and ventilation of the drying room.

Schematic diagram of infrared radiation drying room

The far-infrared radiation drying room is mainly composed of a room body, a radiant heater, a ventilation system, and a temperature control system.

For the type and structure requirements of the far-infrared drying room, refer to the general hot air drying room. But its size is small and there are few brick structures.

As the main body of the radiation drying chamber, its function is to maintain a certain temperature in the drying chamber, reduce heat loss, and improve the drying effect. The design of the cross-section size and shape of the chamber body and the configuration of the radiators are determined according to the nature, shape and size of the workpiece to be heated, and the type, temperature and irradiation distance of the selected radiator. The length and volume of the chamber body are determined according to the size of the workpiece, heating time, transportation speed and output.

Far-infrared heating and drying uses radiant heating, but it is not possible to simply use radiant heating. When the far-infrared radiator works, it also heats the indoor air to a certain extent, so the hot air heating also plays a certain role. Therefore, the drying chamber should have appropriate insulation measures, and the insulation material should be covered in the chamber to reduce heat loss and improve operating conditions.

Radiant heater, also called radiant element, refers to the element that can emit far infrared rays. The radiant heater is composed of a far-infrared coating, a heating element, a substrate and accessories.

Common radiation coatings are oxides, carbides, nitrides, sulfides, borides, etc. of most elements located in the periodic table of the chemical element 2, 3, 4, 5 and so on. They will radiate to varying degrees at a certain temperature. Out of different wavelengths of infrared. One or more materials can be selected and mixed according to requirements, and applied to the surface of the radiator by different processes. When selecting a far-infrared component, a far-infrared coating with a wavelength matching the coating should be selected according to the requirements of different coatings.

The role of the heat source is to provide enough heat to the radiation coating to radiate far infrared rays. Theoretical research shows that the energy of the far infrared radiated by the radiation coating is proportional to the fourth power of the absolute temperature of the radiator surface. Therefore, increasing the temperature can increase the amount of far-infrared radiation. Electricity, coal, steam, etc. are usually used as heat sources. The most practical application is resistance wire heating, that is, electric far-infrared. ^[3]