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Moisture accumulation properties

A fabric may absorb the moisture generated internally from the wearers’ bodies and this moisture may accumulate in the fabric’s structure. At a high ambient temperature or during strenuous activity, wearers (especially firefighters) perspire profusely (perspiration rate can be 2000 g/h); thus, fabrics used in clothing next to their skin become saturated with moisture. Additionally, clothing becomes moistened due to external conditions, such as sprayed water from a hose and/or water on the floor (when a firefighter crawls to avoid smoke and high radiant heat). Studies show that the presence of internal and external moisture have a complex effect on the thermal insulation characteristics of a fabric, depending upon the types of fabrics, amount of moisture, and types and intensity of thermal exposures [24,81,83,92,97,305,349,481].

In general, it has been observed that the thickness of nonwoven fabrics used in firefighters’ thermal protective clothing is higher than the thickness of used woven fabrics. Therefore, moistened or wetted nonwoven fabrics could provide higher thermal protective performance than moistened or wetted woven fabrics under a specific thermal exposure. In this context, it is necessary to remember that firefighters’ thermal protective clothing consists of a multilayered (shell fabric, moisture barrier, thermal liner) fabric structure. Additionally, firefighters wear firestation uniforms and underwear below their protective clothing. Due to this multilayered fabric structure along with uniform and underwear, moisture could be distributed within different layers and the amount of moisture within each layer could be different. Contextually, Keiser et al. [81] found that the moisture content of a particular fabric layer is not only dependent upon the material properties of that particular layer, it is in fact, mainly dependent on the material properties of the neighboring fabric layers or even whole combinations of fabric layers. They also observed that the sweat generated moisture mainly accumulates near the underwear, uniform, and/or innermost layer of thermal protective clothing. This moisture can be completely absorbed by underwear if it is made from the hygroscopic fibers (eg, cotton). However, the hydrophobic underwear (eg, aramid) cannot absorb the moisture and they usually transport the moisture to adjoining layers. In this context, it is possible that the combinations of hydrophilic and hydrophobic fabrics could transport the moisture toward the outermost fabric layer of the thermal protective clothing. This transported internal moisture in the outer layer fabric may evaporate in the presence of thermal exposures and this water vapor could transmit toward firefighters’ skins. This situation can lower the thermal protective performance of firefighters’ clothing by generating steam burns on firefighters. Similarly, if the external moisture (eg, sprayed or extingushing water) accumulates on the outer layer fabric of thermal protective clothing, it could evaporate and generate steam burns on firefighters. Altogether, it is expected that the firefighters’ thermal protective clothing should be designed in such a way that it could avoid the accumulation of moisture in the outer layer fabric and can enhance the overall thermal protective performance of the clothing.

Barker et al. [92] also mentioned that the amount of moisture in the fabrics of thermal protective clothing could significantly affect its thermal protective performance under the radiant heat exposure of 6.3 kW/m2. They mentioned that the thermal conductivity of moisture is much higher than the thermal conductivity of air or fibers (25 times as great as that of air); as a consequence, the accumulated moisture increases the thermal conductivity of the fabric and causes a rapid transmission of thermal energy toward firefighters. In contrast, they also concluded that the heat capacity of moisture is much higher than the heat capacity of air or fibers, which can help to store the thermal energy inside the fabric and reduce the transmission of thermal energy toward firefighters. Here, Barker et al. [92] found that the thermal conductivity of the fabric plays a key role under radiant heat exposure if its moisture content is lower than 15%. On the other hand, the specific heat of the fabric plays a key role if its moisture content reaches over 15%. As a result, a fabric with <15% moisture content shows lower performance; however, the performance becomes high if the moisture content of the fabric becomes more than 15%. It is also notable that if a fabric absorbs a significantly high amount of moisture (more than 15% of its weight), this ultimately provides a cooling effect for firefighters by reducing the thermal energy transfer. Furthermore, Keiser et al. [83] found that a minimally moistened fabric provides lower protection in steam exposure, too. Here, although the moisture in the fabric sample does not contribute to the transfer of the steam toward firefighters, the high thermal conductivity of moistened fabric generates quick burns on firefighters’ bodies.

Moreover, Lee and Barker [167] found that the types of thermal exposures have significant effect on the thermal protective performance of fabrics. They evaluated the thermal protective performance of fabrics under two different types of thermal exposures of the same intensity (ie, 84 kW/m2)—(1) 100% radiant heat, and (2) mixed (50:50) flame and radiant heat. Here, it has been found that thermal protective performance of fabrics is lower under the radiant heat exposure in comparison to the mixed flame and radiant heat exposures. This is because the flame impinged on the fabric generates a convective thermal energy flow parallel to the fabric. This convective flow takes away the moisture vapor from the fabric due to the ablative effect. As a result, the transfer of thermal energy toward the back of the fabric (or wearers’ skins) becomes less, and the thermal protective performance of the fabric increases. As the ablative effect is missing in the radiant heat exposure, the thermal protective performance of fabrics is less in this exposure.

Lee and Barker [167] further investigated the impact of intensity of thermal exposures on thermal protective performance of fabric. For this, they evaluated the thermal protective performance of 60-80% wetted single layer Kevlar/PBI fabrics under the 84 kW/m2 and 20 kW/m2 radiant heat exposures. Through this evaluation, it has been found that the thermal protective performance of the fabric is low at the intensity of 84 kW/m2 in comparison to the intensity of 20 kW/m2, even though the heat capacity of the wetted fabric was initially the same under both the intensities. This is because the vaporization of water takes place at 84 kW/m2 radiant heat exposure. This vaporization process causes rapid transfer of thermal energy toward firefighters’ bodies and that results in steam burns on firefighters. As observed, the heat capacity of the fabric increases and water vaporization does not occur under the 20 kW/m2 radiant heat exposure. This situation results in higher thermal protective performance of fabrics under the 20 kW/m2 radiant heat exposures. External moisture and internal moisture also behave differently depending upon the intensity of the exposure [349]. Under the high heat flux conditions, external and internal moisture tend to decrease and increase the transmission of thermal energy toward firefighters, respectively; however, the external and internal moisture behave in opposite ways under low heat flux.

 
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