Hot Environments
The ISO philosophy for the assessment of hot environments is to use a simple, “fast” method of monitoring the environment, based on the wet bulb globe temperature (WBGT) index (ISO 7243). If the WBGT value exceeds the provided “reference” value, or if a more detailed analysis is required, then ISO 7933 provides an analytical method of assessment.
18 20 22 24 26 Local air temperature (°C) |
1.8 20 11 24 26 Local air temperature (°Cj |
18 20 22 24 26 Local air temperature (°Cl |
FIGURE 6.6 Acceptable mean air velocity as a function of local air temperature and turbulence intensity for the three categories of thermal environment.
TABLE 6.4 Vertical Air Temperature Difference between Head and Ankles (I. I and 0.1 m, Respectively, above The Floor) for the Three categories of Thermal Environment
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TABLE 6.6 Radiant Temperature Asymmetry for the Three Categories of Thermal Environment
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The WBGT heat stress index is calculated inside buildings and outside buildings without solar load as
WBGT = 0.7tnv/ + 0.3tg (6.4)
And outside buildings with solar load as
WBGT = 0.7 fnw + 0.2 + 0.1 ta, (6.5)
Where
Tnw is the natural wet bulb temperature, °C is the temperature at the center of a 150 mm-diameter black globe thermometer, °C ta is the air temperature, °C
The WBGT value for the hot environment is compared against a WGBT reference value, which is included in an informative annex (Table 6.7). The reference values have been established allowing for a maximum rectal temperature of 38 °C for the persons concerned. This corresponds to levels to which
Almost all individuals can be ordinarily exposed without any harmful effect,
Provided there are no preexisting pathological conditions.
TABLE 6.7 Reference Values for WBGT (ISO 7243)
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Note: The values given have been established for a maximum rectal temperature of 38°C for the person concerned. |
Ft the WBGT of the hot environment exceeds the WBGT reference value, then the heat stress at the workplace needs to be reduced or a more detailed analysis made (i. e., using ISO 7933). The standard also includes a method to plan a work/rest schedule that will provide a tolerable environment.
The value used in ISO 7933, required sweat rate, SWreq is based on the heat balance equation (6.1). Assuming the heat storage is equal to 0, the necessary evaporation from the skin, Ereq, to ensure a heat balance is calculated as follows:
Јrecj = M-W-C-R — Eres — Cres. (6.6)
The maximum evaporation, Emax, that can be absorbed by the environment is estimated from the equation
^max — {P$ks~~ Pa)/R-er) (6-<‘/
Where
Psj, = saturated water vapor pressure at the skin
Pa = water vapor pressure in the environment
R. = total evaporative resistance of clothing and boundary layer
TOC o "1-5" h z Based on the required evaporation and the maximum evaporation it is
Then possible to estimate the following factors:
• Required skin wettedness,
^req ^req/^max (6.8)
• Sweating efficiency,
. n. -6.6(1a
R = l-0.5e (6.9)
• Required sweat rate,
SWreq = EKq/r (6.10)
These parameters are used to evaluate how stressful a given hot working environment is. Depending on the physiological limitations for factors such as sweat rate, total sweat loss, heat storage, and skin wettedness, which are listed in Table 6,8, it is possible to evaluate whether a given environment is acceptable for continuous work. The method also allows calculation of an acceptable working time. Detailed equations for the calculations can be found in the standard (ISO 7933). The relation between the operative temperature and SWreq for different combinations of activity and clothing is shown in Table 6.9.
A computer program is provided for ease of calculation and efficient use of the standard. This rational method of assessing hot environments allows identification of the relative importance of different components of the thermal environment, and hence can be used in environmental design. The WBGT index is an empirical index, and it cannot be used to analyze the influence of the individual parameters. The required sweat rate (SWreq) has this capability, but lack of data may make it difficult to estimate the benefits of protective clothing.
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