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Evaporative resistance evaluation
In order to evaluate the evaporative resistance of a fabric or multilayered fabric system under steady-state conditions (according to ASTM F 2370 standard), two circumstances are preferred: isothermal and nonisothermal . In isothermal circumstances, the temperature of the sweating thermal manikin’s skin surface and its ambient air are set at 35 ± 0.5°C (without fluctuating more than ±0.2°C during testing), and the relative humidity of the ambient air is set at 40 ± 5% by maintaining an air velocity in-between 0.4 ± 1 m/s at the surrounding of the manikin. As the manikin’s temperature is the same as the ambient air temperature, no dry heat exchange occurs between the manikin and the ambient air. In nonisothermal circumstances, various experimental parameters (the temperature of the manikin and ambient air, relative humidity of the ambient air, and the ambient air velocity) can be set as equivalent to the experimental parameters of the thermal resistance (Rt/Rcl) evaluation (discussed in the previous section). Because the ambient air temperature is lower than the manikin’s temperature, the dry heat loss can occur between the manikin and the ambient air simultaneously with evaporative heat loss. In nonisothermal circumstances, the ambient air temperature, relative humidity, and velocity are set as per researchers’ requirements. After setting up the isothermal or nonisothermal circumstances as per requirements, water of 35 ± 0.5°C is sprayed on the manikin until it is saturated;
Table 5.5 Differences between ASTM F 1291, ISO 15831, EN342, and ASTM F 1720 standards
Table 5.5 Continued
then, the water is continuously delivered to the manikin to keep it saturated for evaporation throughout the test period. Saturation is usually detected visually by a color change (surfaces that are wet are darker than those that are dry) but an IR camera can also be used to ensure that the surface is completely saturated. Next, the manikin is dressed up in the garment to be tested. The skin temperature of the dressed manikin is stabilized, and the clothed manikin system is allowed to reach steady-state (that is, the mean skin temperature and power input remain constant ±3%). After the system reaches steady-state, the manikin’s skin temperature and the ambient air temperature are measured every 1 min. The average of these measurements is taken over a period of 30 min to determine the evaporative resistance value of the clothing ensemble. The evaporative resistance (Ret) of the clothing ensemble with manikin’s surface (boundary) air layer can be determined by measuring the power consumption of the manikin (option 1) or by measuring the evaporation rate of the water through the tested garment (option 2). In option 1, the Ret can be calculated according to Eq. (5.33), where, Ret = total evaporative resistance provided by the clothing ensemble with surface air layer around the manikin (kPam2/W); A = area of the manikin’s sweating surface (m2); Ps = the water vapor pressure at the manikin’s sweating surface (kPa); Pa = the water vapor pressure of the air flowing over the clothing (kPa); He = power required for sweating area (W); Ts = temperature at the manikin’s surface (°C); Ta = temperature of the air flowing over the clothing (°C); and Rt = total thermal resistance of the clothing ensemble with manikin’s surface air layer measured by ASTM F 1291 (°C m2/W). In option 2, the Ret can be calculated according to Eq. (5.34), where, Ret = total evaporative resistance provided by the clothing ensemble and air layer around the manikin (kPam2/W); Ps = the water vapor pressure at the manikin’s sweating surface (kPa); Pa = the water vapor pressure of the ambient air flowing over the clothing (kPa); A = area of the manikin’s sweating surface; X = heat of vaporization of water at Ts (W); and dm/dt = evaporation rate of moisture leaving the manikin’s sweating surface (g/min). Here, the total mass loss due to evaporation from the clothed manikin is measured by two balances (one balance is used to measure the amount of water being fed to the manikin, while the other balance measures the weight change of the manikin) to give an accurate average over the period of the test. Furthermore, the water dripping from the manikin is captured (by a pan large enough to retain all water drippings) and measured at the end of the test with a calibrated balance having a resolution to the nearest gram, and the water loss from dripping is subtracted from the total mass loss to calculate the actual dm/dt. However, both options ignore the absorption of water vapor in the clothing, and several researchers have established that the water vapor from the manikin/human body would be absorbed or condensed within the clothing and influence the total evaporative resistance [311,440,441]. Additionally, the evaporative resistance in option 1 is calculated from the thermal resistance (Rt), where, Rt is calculated from the dry manikin test. However, the manikin is in a wet condition during the evaporative resistance test; and this wet manikin can change the Rt. It has been found that water vapor condensation may occur within the clothing ensembles during the wet manikin test, which can lower the thermal resistance of clothing during the evaporative resistance test; this phenomenon is more prominent in thicker clothing than the thinner clothing . Thus it can be inferred that the evaporative resistance obtained from option 1 can be lower than the actual one. Because the water vapor absorption or accumulation within the clothing releases heat and changes the thermal properties of clothing ensembles, the measurement of both thermal and evaporative resistance needs to be taken after the stabilization of water vapor accumulation within the clothing in options 1 and 2. Similar to intrinsic thermal resistance (Rcl), intrinsic evaporative resistance (Recl) of clothing ensembles is also determined by subtracting the evaporative resistance of the air layer on the surface of the nude manikin’s sweating surface (Rea) from the Ret [Eq. 5.35, where, Recl = intrinsic evaporative resistance of the clothing ensemble (kPam2/W); Ret = total evaporative resistance of the clothing ensemble with surface air layer (kPa m2/W); Rea = the evaporative resistance of the air layer on the surface of the nude manikin’s sweating surface (kPam2/W); fcl = clothing area factor (dimensionless) estimated using the ISO 9920 standard, or a photographic method described by McCullough, Jones, and Huck ]. Note: The total evaporative resistance (Ret) of the garment under nonisothermal circumstances can also be called apparent total evaporative resistance (ARet). Here, the “apparent” term is used as a modifier to the total evaporative resistance (Ret) to reflect the fact that condensation may occur within the garment, and the ARet values of the garment can only be compared to those of other garments measured under the same nonisothermal conditions.
The ASTM F 2370 standard is widely used to quantify and compare evaporative resistances provided by clothing ensembles having different designs, fabrics, garment layers, closures, and fits; the evaporative resistance values of clothing ensembles under isothermal circumstances can also be used in models to predict the physiological responses of people in different environmental conditions. However, the ASTM F 2370 standard does have several limitations . For example, this is a static test that provides a baseline clothing measurement on a standing manikin, and the effects of body positions and movement are not addressed; the obtained evaporative resistance values applies only to the particular ensembles evaluated and for the specified environmental conditions of each test, particularly with respect to ambient air velocity and sweating simulations; the measurement of evaporative resistance provided by clothing ensembles is a complex process and dependent on the apparatus and technique used; and technical knowledge concerning the theory of heat transfer, moisture transfer, temperature, air motion measurement, and testing practices is essential for an operator, who evaluates evaporative resistance. Furthermore, the ASTM F 2370 standard has been developed based on the premise that the vapor pressure at the manikin’s skin surface is 100% and the sweating rate should be high enough to saturate the skin. However, this premise is not true in unstaged scenarios, in which optimal sweating rates of human beings vary due to many factors, such as their ambient environmental conditions, activities performed, and type of clothing worn. It has been observed that if the sweat rate is low and the clothing is thin, all of the water vapor dissipates through the clothing towards the ambient environment. In this situation, 100% water vapor pressure at the skin surface cannot be attained. It seems that the ASTM standard should measure the exact water vapor pressure near the skin surface, although this is very difficult.
At this time, ASTM F 2370 and ISO 9920 are the only standards to evaluate the evaporative resistance of clothing. The testing protocols of these standards are almost the same; however, ISO 9920 standards also provide a method for estimating the evaporative resistance of permeable clothing ensembles [Eq. 5.36, where, Ret = total evaporative resistance of clothing with manikin surface air layer (m2kPa/W); Rt = total thermal resistance of the clothing with manikin surface air layer (clo); im = moisture permeability index (dimensionless), and LR = Lewis Relation (16.5°C/kPa)]. Although Eq. (5.36) is for permeable clothing ensembles, this equation is only applicable to normal indoor clothing, it cannot be applied to protective clothing, which is usually made of materials with low moisture permeability.
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