Far Infrared Heating Technology

Far infrared heating technology emerged in the early 1970s. It utilizes radiation heat transfer, transmitting energy through electromagnetic waves, and is a key energy-saving technology promoted for widespread adoption. When the wavelength of far infrared rays matches the absorption wavelength of the heated object, the object absorbs a large amount of far infrared rays, causing the molecules and atoms inside the object to undergo "resonance" and generate intense vibration and rotation, thereby raising the object's temperature and achieving the purpose of heating. This technology can improve heating efficiency, but attention must be paid to enhancing the absorption capacity of the heated material for radiation, so that its molecular vibration wavelength matches the far infrared spectrum wavelength. Due to its significant energy-saving advantages, far infrared heating technology is widely used in the heating, melting, drying, and curing processes of various products or materials such as paint, plastic, food, medicine, wood, leather, textiles, tea, and tobacco.

Principle of Far Infrared Heating

In any heating device, the heat source transfers thermal energy to the heated object through three forms: convection, radiation, and conduction. Conduction refers to the phenomenon of heat transfer relying on the thermal motion of microscopic particles such as molecules, atoms, and free electrons within an object without relative displacement of its parts or when different objects are in direct contact.
Three forms of heat conduction
Convection is the phenomenon of transferring heat from one place to another through fluid motion. Both conduction and convection require direct contact between hot and cold objects or rely on conventional substances as a medium to transfer heat. However, the mechanism of thermal radiation is completely different; it transfers heat by emitting visible and invisible rays from the object's surface. Radiation heating has a fast transfer speed and does not require any medium, thus greatly reducing losses during heat transfer and improving thermal energy utilization.
In far infrared heating technology, radiation heating is emphasized as the primary method. When far infrared rays irradiate the heated object, part of the rays are reflected back, and part penetrate through. When the wavelength of the emitted far infrared rays matches the absorption wavelength of the heated object, the object absorbs a large amount of far infrared rays, causing the molecules and atoms inside the object to undergo "resonance" — generating intense vibration and rotation, which raises the object's temperature, thereby achieving the purpose of heating. To improve heating efficiency using this technology, it is important to pay attention to matching radiation.
Conduction process of far infrared rays within an object
So-called matching radiation refers to when the frequency of infrared rays irradiating an object matches the molecular vibration frequency of the material composing the object; the molecules will resonantly absorb the infrared radiation energy. Through energy transfer between molecules, the internal energy (vibrational and rotational energy) of the molecules increases, meaning the average kinetic energy of the molecules increases, manifested as a rise in the object's temperature. The main meaning of matching absorption is that the frequency of selective radiation emitted by the infrared heater is consistent with the vibration frequency of the molecules of the heated material itself; the resulting resonant absorption is called matching absorption [2
Matching absorption is significant for thin-layer heating, such as paint baking, plastic processing, and dehydration drying of certain salts. However, it is of little significance for heating thick materials because far infrared rays have low penetration ability for general substances, typically only penetrating a few micrometers to a few millimeters. Even if the energy is not absorbed by surface molecules, it will be absorbed within the object. In this case, the main approach is to reduce reflectivity to increase absorptivity. For some heated materials, due to baking quality or processing requirements, it is desired that the object be heated both internally and externally simultaneously, such as drying wood to prevent cracking, or melting plastic to avoid excessive external temperature leading to aging. Therefore, for heating such materials, non-matching absorption should be considered, allowing some far infrared rays to penetrate into the interior to achieve uniform heating.

Judgment Criteria and Design Principles for Far Infrared Heating Technology

The evaluation of a far infrared heating system mainly depends on whether radiation heating is the primary method during the entire heating process, with conduction and convection heating as supplements. The larger the proportion of radiation heating, the better the system's performance [3]
Through extensive measurement and theoretical research on the thermal energy conversion efficiency of far infrared drying tunnels and ovens by far infrared workers, two methods for judging radiation drying tunnels were proposed in the mid-1980s: First, measure the spatial temperature inside the drying tunnel and the surface temperature of the elements. Only when T_space < T_element by 20–40°C can the drying tunnel be called a radiation drying tunnel. When the elements meet the far infrared energy-saving conditions, it is further called a far infrared drying tunnel. Second, change the continuous operation to static baking. If no uneven paint film phenomenon (color difference) occurs, it can be called a radiation drying tunnel; otherwise, it cannot be called far infrared heating.>Design Principles for Far Infrared Heating Technology
After years of research and practical exploration, scientists have summarized the following three design principles for far infrared drying tunnels and ovens [3
(1) Uniform radiation field design: Uniform radiation field design aims to ensure that regardless of the workpiece's position during movement, the radiation energy received on its surface is uniform. Uniform radiation field design uses radiation optics principles to calculate a set of data, namely: element arrangement pattern, element spacing, reflector type, and distance between elements and workpiece. This breaks away from the qualitative design of element spacing of 100–350 mm and element-to-workpiece distance of 50–400 mm.
(2) Uniform temperature field design: To prevent the destruction of the uniform temperature field inside the drying tunnel, the temperature difference between the top, bottom, left, and right inside the drying tunnel should be maintained at ±5–10°C.
(3) Uniform temperature control technology design: On-off temperature control leads to large local temperature fluctuations. Thyristor voltage regulation reduces element radiation and increases convection. Using power regulators for temperature control brings far infrared technology to a state of perfection.