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.

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Local air temperature (°C)

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local air temperature (°c)

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Local air temperature (°Cj

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local air temperature (°cj

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Local air temperature (°Cl

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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

Category

Vertical air temperature difference (°C)

A

<2

B

•*

C

<4

TABLE 6.5 Range of Floor Temperature for Three Categories of Thermal Environment

Category

Range of surface temperature of the floor (°C)

A

19-29

B

19-29

C

17-31

TABLE 6.6 Radiant Temperature Asymmetry for the Three Categories of Thermal Environment

Category

Radiant temperature asymmetry (°C)

Warm ceiling

Cool wall

Cool ceiling

Warm wall

A

< 5

<10

<14

<23

B

<5

<10

<14

<23

C

< 7

<13

<18

<35

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)

Metabolic rate, M

Reference value of WBGT

Metabolic rate class

Related to Total (for a a unit skin surface mean skin area surface area (Wirr2) of 1.8 ma) (W)

Person acclimatized to heat (°C)

Person not acclimatized to heat (°C)

0 (resting)

M < 65

M < 117

33

32

J.

65 < M < 130

117 < M < 234

30

29

2

130 < M < 200

234 < M < 360

28

26

3

200 < M < 260

360 < M < 468

No sensible air

Movement

Sensible

Air

Movement

No sensible Sensible air air movement movement

25

26

23

4

M > 260

M > 468

23

25

20

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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