Protective performance principle of thermal protection test

Thermal protective clothing is a safety protection for personnel working in high or ultra-high temperature conditions to avoid damage to the human body from heat sources.

The human skin is very sensitive to temperature. When the heat flow density of human skin reaches 2168J/cm, that is, when the skin temperature reaches 45℃, people will feel burning pain; when the heat flow density increases to 5102J/cm, that is, when the human skin temperature reaches 72℃, it will cause second degree burns on the skin. Therefore, people can wear thermal protective clothing to protect the skin; in a hot environment, wearing thermal protective clothing can reduce the rate of warming of human skin and provide time to dress to react and escape to avoid or reduce the damage of heat sources to the human body.

There are many forms of heat sources harmful to the human body, the nature of the different thermal protective clothing thermal protection performance requirements are also different. At present, the main forms of heat sources are flame (convective heat), contact heat, radiation heat, sparks and molten metal jets, high-temperature gases and hot steam, high heat from the electric arc, etc.

1.1 Heat convection.

Convective heat dissipation is the transfer of heat through the movement of fluids such as liquids (e.g. water) or gases (e.g. air). The heat transfer is called natural convection because of the fluid movement caused by the uneven fluid temperature. Natural convection of air within the boundary layer of the body and clothing surface. Fluid motion is caused by external causes and is called forced convection. The anti-heat convection performance of the fabric is mainly closely related to the weight, density and air properties of the fabric. Increasing the weight of the fabric can increase the time required for secondary skin burns. Also, multi-layer fabrics have better anti-heat convection effect.

1.2 Heat conduction.

Heat conduction is the transfer of heat along an object. It is transferred from a high temperature object to a low temperature object. This transfer is achieved mainly through successive collisions between adjacent molecules in a substance. Heat conduction refers to the heat to sparks, molten metal jets, etc. as a carrier, contact with clothing, through the clothing will transfer heat to the body, causing harm to the human body.

In general, the fabric's anti-molten metal heat transfer performance can be used to place the artificial skin made of standard PVC film behind the fabric and measure the harm to the artificial skin after the molten metal heat through the fabric. For the area and degree of injury, and can be divided into one to seven levels of fabric thermal protection performance. Level one is the best, no harm, level two and three are the primary burns with an injury area of less than 0.01 square meters, level four and five are the secondary burns with an injury area of less than 0.01 square meters, and level six and seven are the tertiary burns with an injury area of less than 0.01 square meters.

1.3 Thermal radiation.

Thermal radiation is the heat from the object along a straight line outward radiation. The essence of thermal radiation is the thermal radiation caused by the temperature of the object, proportional to the fourth power of the absolute temperature of the heat source. Unlike thermal conduction and thermal convection, thermal radiation is a non-contact heat transfer method that does not require any material as a medium, but rather in the form of electromagnetic waves to transfer heat. In the practical application of thermal protective clothing, heat radiation is one of the main forms of heat transfer for victim injury. Even when a flame burns, its energy includes up to 80% of thermal radiation.

In tests of the performance of thermal protective clothing against heat radiation, the fabric is often exposed vertically to a radiant heat source. At a defined distance, the heat source radiates heat to the fabric sample. By testing the time required for second degree burns on human skin behind the sample and the heat flow density, the thermal radiation protection performance of the sample can be evaluated.