At present, China has a total of about 120,000 industrial furnaces, the annual total energy consumption of 250 million tons of standard coal, accounting for about a quarter of the total energy consumption of the country, accounting for 60 % of the total energy consumption of industry. Therefore, the energy conservation of industrial furnaces has great potential. The energy-saving technology of heat insulation materials developed rapidly in recent ten years has attracted much attention because of its remarkable energy-saving benefits and attractive development prospect. The data of many countries show that the new high-efficiency heat insulation materials can save 15 % ~ 30 % energy in industrial furnaces instead of the traditional heat insulation refractory bricks. For the heat storage and heat dissipation of industrial furnace lining, generally accounts for 20 % ~ 45 % of the total energy consumption of the furnace. the selection of high quality, energy-saving refractory material can reduce the heat storage and heat dissipation loss of the furnace body, improve the thermal efficiency, so as to achieve the purpose of energy saving.
1 composite nano heat insulation board characteristics
Composite nano-thermal insulation panels have very low thermal conductivity, which is different from any conventional fiber or ceramic thermal insulation materials based on gas phase SiO _ 2 ultrafine particles, metal oxides and reinforcing fibers. These particles and fibers form a nanoscale microporous structure that limits air convection, heat conduction, and radiation, thereby allowing the material to ensure ultra-low heat transfer performance over all application temperature ranges.
The advantages of the composite nano heat insulation board are obvious. Firstly, the material can increase the volume of the container and reduce the cost by reducing the thickness of the lining. Secondly, that material can minimize the fatigue degree of the furnace wall, eliminate hot spot at the same time and reduce the operating temperature of the shell; Again, materials can reduce operating costs, reduce critical operating temperatures, reduce heat dissipation, and increase customer operational flexibility.
The application of composite nano-thermal insulation board has been paid more and more attention with the increasingly stringent requirements of various industries on the thermal insulation of process equipment. In foreign countries, the development and application of this material has been relatively mature, in heavy industry, civil, military, aerospace, construction application level has been gradually improved. The research and development of microporous thermal insulation materials in China started late, but the momentum is rapid.
Heat insulation principle of composite nano heat insulation board
2.1 material composition structure
The composite nano heat insulation board is mainly composed of ultrafine SiO _ 2 particles, opacifying agent and peripheral wrapping material. Generally speaking, the pore size of microporous materials is less than 100 nm, which belongs to the research field of nano - materials. At the same time, the proper volume density of the material can keep the thermal conductivity at a low value; The microporous thermal insulation material is composed of ultrafine SiO _ 2 particles, which have an ultra-low thermal conductivity in the range of 5 ~ 25 nm. its unique amorphous structure makes it a core part of the microporous thermal insulation material. Long chains of particles are formed between the particles due to chemical bonding, and these chains are finally mixed to form part of the insulating material. In addition to the special structure of SiO _ 2 itself, other structures of the insulation material also play a key role in the insulation process ( see fig. 1 ). The addition of opacifying agent can reduce the radiation heat transfer of the material. Typical shading particles include carbon black, TiO _ 2 and so on. this uniform distribution of shading particles in the material basically prevents the path of infrared radiation, and can be mixed with ultrafine SiO _ 2 particles to achieve the purpose of blocking radiation. According to Boltzmann's law, the amount of infrared radiation is directly proportional to the fourth-order temperature difference between the hot and cold surfaces of the material, and the effect caused by infrared radiation is particularly prominent when the temperature difference is greater than 100 c, so the opacifying agent is an indispensable key factor for the heat insulation material.

由于开云游戏尺寸的孔隙度范围很小，这就决定了气体分子会失去自由流动或者相互碰撞的能力，很大程度上控制了气体对流，使空气分子间的对流换热基本停止。

根据分子运动及碰撞理论，气体热量的传递主要通过高温侧的高速运动分子向低温侧的低速运动分子相互碰撞完成。由于微孔隔热材料的孔隙度都在开云游戏级别，这些气孔大部分都小于气体分子的平均自由程，这样，气体分子将只能与气孔壁发生弹性碰撞而无法参与热传递。同时，对于固体热传导来说，这种开云游戏级的空隙只能允许热流在分子接触点方向上进行传导。然而，SiO2超低的导热系数和由大量气孔造成的分子接触点的排布程度决定了这种导热方式基本被阻断。

2.2隔热参数与性能

复合开云游戏隔热板的导热率见表1。

复合开云游戏隔热板的导热系数在各类传统隔热材料中优势明显。如图2所示，这种材料的导热性能在各温度区间都有上佳表现。而且，使用温度越高这种优势越明显。

Due to the small porosity range of nano - size, gas molecules will lose the ability to flow freely or collide with each other, which largely controls the convection of gas and stops the convection heat transfer between air molecules.
According to the theory of molecular motion and collision, the heat transfer of gas is mainly accomplished by the collision between the high-speed moving molecules on the high-temperature side and the low-speed moving molecules on the low-temperature side. Because the porosities of the microporous thermal insulation materials are in the nanometer level, most of the pores are smaller than the average free path of gas molecules, so that the gas molecules can only elastically collide with the pore walls and cannot participate in heat transfer. At the same time, for solid heat conduction, such nanoscale voids only allow heat flow to be conducted in the direction of the molecular contact point. However, the ultra-low thermal conductivity of SiO _ 2 and the arrangement of molecular contact points caused by a large number of pores determine that the thermal conduction mode is basically blocked.
2.2 insulation parameters and performance
The thermal conductivity of the composite nano heat shield is shown in table 1.
The thermal conductivity of composite nano-thermal insulation board has obvious advantages in all kinds of traditional thermal insulation materials. As shown in fig. 2, the thermal conductivity of this material is excellent in each temperature range. Moreover, the higher the use temperature, the more obvious this advantage is.

It should be noted that due to the particularity of the structure of the microporous heat insulation material, aerosol particles are sensitive to liquid, therefore, when the material is exposed to water, oil and other liquids, the structure of the microporous material will be partially decomposed. Therefore, should try to keep isolated from liquid during operation.
Taking an aluminum smelting furnace of a company as an example, the benefit analysis was carried out after the application of the product: 25.4 mm composite nano heat insulation board was replaced in the original lining, and other structures were unchanged. The effect of the improvement is obvious: the gas consumption is reduced by 13 %, the cost can be saved by RMB 54,500 yuan per year, the return can be obtained in 9 months, and the total income is about RMB 218,000 yuan. In a word, adding this product to other products can get more profit than before.
4 Conclusion
In this paper, the thermal insulation principle, material properties, energy saving effect and application prospect of composite nano thermal insulation board are analyzed comprehensively, especially in industrial furnaces. Therefore, the composite nano-thermal insulation board is worth in-depth study and promotion of thermal insulation materials, has a broad market prospect and application value.