Document 3Q5gobRwbY2pbB8YpQYyx03n

1104 CHAPTER 44 1958 Guide So long as' there is no limitation on evaporation rate; the sweat output is automatically limited by the body to that; amount which satisfies the required value of E. ' With restricted "evaporation, however, there is ex cessive sweating; the body becomes fully wetted and a maximum evapora tive cooling rate is established which does not increase with further sweat ing. This maximum evaporative cooling rate Emla has been established experimentally and can be calculated by the equation: where Bm,, = K.A7`(P. " P.) - 107-(P, - P.) (5) K, = coefficient of heat exchange by evaporation, Btu per (hour) (square foot) (unit velocity)(millimeter of mercury vapor pressure difference). 10 = K.X A for average size man, fully wetted. P, = vapor pressure of water at skin temperature, millimeters of mercury. P = partial pressure of water vapor, in ambient air, millimeters of mercury. A combination of Equations 4 and 5 yields Equation 6 which is a state ment of the maximum thermal conditions under which man .can maintain (M)Table 1. Estimates of Energy Metabolism of Various Types of Activity (Values apply for a 164-lb man, and do not include rest pauses) Kind op Work Sleeping........... Sitting quietly Activity. M Btu/hr. 260 400 Light Wore Sitting, moderate arm and trunk movements.................... (e.Q,, desk work, typing) Sitting, moderate arm andleg movements.......................... (e.g.i playing organ, driving car in traffic) Standing, light work at machine or bench, mostly arms. 460-550 550-650 550-650 Moderate Wore , Sitting, heavy arm and leg movements............................................................. Standing, light work at machine or bench, some walking about.............. Standing, moderate work at machine or bench, some walking about... Walking about, with moderate lifting or pushing.......................................... 650-800 650-750 750-1000 1000-1400 Heavy Work Intermittent heavy lifting, pushing or pulling. (e.Q., pick and shovel work) Hardest sustained work............................................ 1500-2000. 2000-2400 heat balance, giving proper recognition to internal heat production M, akin temperature t, and to all four of the thermal characteristics of the environment: Af + 22((w - i.) + 2 VV(t. - t.) = 107`(P. - P.) (6) Equation 6 provides a rational basis on which to construct a heat stress index and to establish thermal standards for different industrial work situations. . Two physiological criteria are important in fixing the limit of sustained heat exposure: 1. The increase in body heat content must notexceeda certainlevel. Practically, thia is reflected in the rise in skin temperature. This temperature may be limitea . to 95 F. 2. Thermal balance must be accomplished with a rate of sweating not greater than one liter per hour (equivalent to evaporative cooling of 2400 Btu per hr), ptutu on young acclimatized subjects have shown that this maximum rate of sweating be maintained over a work day without undue strain or physiological cost. The Heat Stress Index (HSI) proposed by Belding and Hatch' makes use of these criteria together with the previous biophysical equations. * so-called standard young man, in good physical condition and acclunataz to heat, can safely engage in simple physical work over a normal eignv Control of the Industrial Environment' ; 1 1105 horn1 day in a particular thermal environment so long as his skin tem perature does not exceed 95 F and he maintains thermal balance with a sweat rate not exceeding 1 liter per hr. By definition, this degree of stress has an (HSI) value of 100 and is given by any combination of M + R + C which is balanced by Fmax within their defined limits. For other situations where Freq is greater or less than Fmax (g2400 Btu per hr), the value of (HSI) is proportionately different from 100: (HSI) = 100 X B_______ req Em,* < 2400 (7) By using the globe thermometer temperature (see Chapter 52) of the environment to represent the combined effects of mean radiant tempera ture and air temperature, the Heat Stress Index chart Fig' 1 has been constructed, to permit quick determination of (HSI) when M, tg, <a, b and air velocity are given. -,The procedure for finding (HSI) will be obvious by following the dash line in Fig. 1. The Heat Stress Index was developed from research experience with young men in good physical condition and well acclimatized to heat. Table 2 assigns suggested physiological meaning to (HSI) values above and below 100, for older men as well as the standard young man and, also, for various kinds of jobs which differ in their demands upon mental and physical effort. From these it becomes possible to select (HSI) levels appropriate to each work situation. The Heat Stress Index (HSI) differs from the Effective Temperature (ET) scale in the rational rather than subjective basis of its construction. Practically, the commonly recommended ET = 80 F for heavy work at moderately high air velocity agrees very well with the (HSI) limit of 100 for acclimatized young men. In Fig. 2 are shown the combinations of dry and wet-bulb temperature (air and wall temperature alike) for three different air velocities and three different grades of work, all of which give (HSI) = 100. Points representing ET = 80 F with an air velocity of 300 fpm are also shown for comparison. CONTROL OF HEAT EXPOSURES Measures for control of heat exposures will be treated under the followmg headings: Control at Source, Local Exhaust Ventilation, Radiation Shielding, General Ventilation, Dilution Ventilation, Local Relief Ventila tion. Control at Source The magnitude of heat exposure can be reduced by insulating hot equip ment, locating such equipment most favorably (in zones of good general ventilation within buildings or even outdoors), covering steaming water tanks, and providing covered drains for direct removal of hot water, maintaining tight joints and valves where steam may escape. This method is an obvious one and requires no particular comment other than to emphasize the benefits to be derived from elimination of heat sources wherever possible. I-ocal Exhaust Ventilation The natural convection column of heated air rising from a hot process ay be captured by means of ventilated enclosures or exhaust hoods and amoved with a minimum of dilution by air from the surrounding space nen *cal exhaust ventilation can be used. Chapter 45 provides detailed