Thermal protection principle of thermal protective clothing
2019-01-23 08:36 | writer: admin
Thermal protective clothing is a kind of protective clothing that protects people working under high temperature or high temperature conditions, so as to avoid damage to the human body caused by heat.
Human skin is very sensitive to temperature. When the heat flux of human skin reaches 2168 J/cm, that is, when the skin temperature reaches 45 °C, people will have a burning sensation; when the heat flux density increases to 5102 J/cm, that is, when the human skin temperature reaches 72 °C, it will cause Second degree burn of the skin.  Therefore, people wearing thermal protective clothing can protect the skin; in high temperature environment, by wearing thermal protective clothing, the heating rate of human skin can be reduced, and the wearer can react and escape time to avoid or reduce the heat source. Human body damage.
There are many forms of heat sources that cause harm to the human body. Their properties are different, so the thermal protection performance requirements for thermal protective clothing are also different. At present, the main forms of heat sources are flame (convection, heat), contact heat, radiant heat, sparks and molten metal sprays, high temperature gases and hot steam, and high heat generated by arcs.
1.1 Thermal convection
Convective heat transfer is the transfer of heat as a fluid such as water (such as water) or a gas (such as air) moves. The way in which the fluid moves due to uneven fluid temperature, thereby transferring heat is called natural convection. There is natural convection in the air in the boundary layer between the human body and the garment surface. Fluid transfer by external external causes for heat transfer is called forced convection.  The heat convection performance of fabrics is closely related to the weight, density and gas of fabrics. Increasing the weight of the fabric increases the time required to cause a second degree burn. At the same time, multi-layer fabrics also have a better heat convection effect.
1.2 Heat conduction
Heat conduction refers to the transfer of heat along an object. It passes from objects with high temperatures to objects with low temperatures. This transfer is mainly achieved by successive collisions between adjacent molecules in the material. In the application of thermal protective clothing, heat conduction refers to the use of heat, such as sparks, molten metal sprays, etc., to contact the garment and transfer heat to the human body through the garment, thereby causing harm to the human body.
Generally, the thermal conductivity of the fabric against molten metal can be placed on the back of the fabric by using an artificial skin made of a standard PVC film to measure the damage of the molten metal to the artificial skin after passing through the fabric. According to the area and extent of the damage, the thermal protection performance of the fabric can be divided into first to seventh grades. The first level is the best, no harm, the second level and the third level are the first degree burns with the injury area less than 0.01 square meters, and the fourth and fifth levels are the second degree burns with the injury area less than and greater than 0.01 square meters respectively. The grades are third degree burns with injury area less than and greater than 0.01 square meters .
1.3 Thermal radiation
Thermal radiation means that heat is radiated from an object in a straight line. The essence of thermal radiation is the heat radiation caused by the temperature of the object, and its magnitude is proportional to the fourth power of the absolute temperature of the heat source. Unlike heat conduction and heat convection, heat radiation is a non-contact heat transfer method that does not require any substance as a medium, but transfers heat in the form of electromagnetic waves. In the practical application of thermal protective clothing, heat radiation is one of the main forms of heat transfer for victims. Even with flame combustion, its energy includes up to 80% of heat and heat.
In the test of heat radiation protection performance of thermal protective clothing, the fabric is often exposed to the radiant heat source vertically. Within a prescribed distance, the heat source radiates heat to the fabric sample, and the second skin burn of the human skin behind the sample is required to be measured. The time and heat flux density were used to evaluate the heat radiation resistance of the sample .