Metabolic heat and/or sweat-vapor transfer models
To date, a group of researchers have developed various analytical and numerical models on metabolic heat and sweat-vapor transfer through clothing to ambient environments; these models are discussed below for better understanding of clothing comfort [306-311,432,490,492,493]. Although these models do not accurately represent firefighters’ working scenarios, they provide fundamental knowledge to conduct research on firefighters’ metabolic heat and sweat-vapor transfer through their clothing.
Wu  developed a simplistic analytical model on wearers’ metabolic heat and sweat-vapor transfer through their clothing toward ambient environments (Fig. 6.7). Wu mentioned that the metabolic heat can easily transfer toward clothing through the microclimate region via conduction and/or radiation and/or convection . Then, this metabolic heat transmits within the clothing and reaches to its outer surface, depending upon the attributes (intrinsic thermal resistant, weight, thickness, etc.) of the fabrics used in the clothing. Finally, the metabolic heat transfers from the outer surface of the clothing toward wearers’ ambient environments occur by conduction and/or convection and/or radiation. This conduction and/or convection and/or radiation process depends upon the temperature difference between the clothed human body and the ambient environment. The conductive metabolic heat transfer takes place when the clothed human body is in contact with a relatively cold solid/liquid surface. The convective metabolic heat transfer occurs when relatively cold air flows over the clothed human body; this process is called forced convection. Even in the
Fig. 6.7 Analytical model on metabolic heat and sweat-vapor dissipation.
absence of air flow, the convective metabolic heat transfer can occur. In this case, the relatively cold air in the ambient environment of a clothed human body becomes warm due to conduction. This warm air, due to its low density, moves upward. In this condition, relatively high-density cold air comes to replace the vacuum over the clothed human body and again becomes warm, and this cycle continues. This process of metabolic heat transfer from the clothed human body is called natural convection. Additionally, the clothed human body also radiates heat to the ambient environment in the form of electromagnetic waves. Sweat-vapor primarily transfers from the human body to clothing, depending upon the attributes of the fabric (moisture absorbency, wicking, intrinsic evaporative resistance, etc.) used in the clothing. This sweat-vapor can easily reach to the outer surface of clothing if the clothing has low intrinsic vapor resistance . Here, the sweat-vapor transfer from the clothed human body to the ambient environment may occur by evaporation. This evaporation process depends upon the sweat-vapor pressure difference between the clothed human body and the ambient environment.
Although Wu’s study analyzed the metabolic heat and sweat-vapor transfer through clothing, this study did not elaborately highlight the impact of microclimate on the metabolic heat and sweat-vapor transfer process . Furthermore, this analysis was carried out for single-layered and thin regular clothing; so, it is expected that these findings can be partially applicable in the case of multilayered and thick firefighters’ protective clothing. To date, no/limited research has been carried out on firefighters’ metabolic heat and sweat-vapor transfer through their protective clothing. As the temperature of firefighters’ working environments is much higher than natural ambient temperatures, interesting findings are also expected from developing the analytical model on firefighters’ metabolic heat and sweat-vapor transfer through their clothing. Thus, it is suggested to develop analytical models on firefighters’ metabolic heat and sweat-vapor transfer through their clothing.