There are three basic modes of heat transfer: conduction, convection and radiation. Conduction means that when two objects with temperature difference contact each other, heat is transferred from the high-temperature object to the low-temperature object; A typical example is hot plate heating. Convection refers to the mutual diffusion and fusion of fluids at different temperatures to achieve heat transfer; The most typical example is the mixing heating of hot water and cold water. Our most common hot blast stove is actually a mixture of convection and conduction. The hot air is brought to the furnace body by the fan to make the temperature of the furnace body reach the set temperature. This process is convection; Then the hot air transfers heat by contacting the surface of the heated object. This process is called conduction. The common point of conduction and convection is that the object needs to be contacted, or the heat source directly contacts the object to be heated, or the energy of the heat source is transferred to the object to be heated through air and other media. The speed of energy transmission is slow and the energy density is limited.

The heat transfer mode of radiation is completely different. It transfers energy through infrared light. The heat energy transferred from the sun to the earth is realized through infrared ray. It has the following two basic characteristics:

1. Infrared radiation heat transfer does not require media. That is, non-contact heat transfer. Because light is an electromagnetic wave, the electromagnetic wave does not need a medium to propagate, and it can also propagate in vacuum. The infrared light emitted by the heat source shines on the surface of the object to be heated and is absorbed by the molecules on the surface, which causes the increase of the molecular vibration energy level and temperature.

2. The energy density of infrared heating is very high. The energy carried by the infrared ray is related to its wavelength and the power per unit area of the infrared light source. The maximum energy can reach 1000kw/m2. In practical sense, the infrared power density can reach 200kw/m2, and it is easy to design the general infrared heating furnace at 10-40kw/m2. The energy of 600 ℃ hot air at the speed of 10m/s is only 40kw/m2.