Document 25vLdGenKJ14YJ1y2Y909D0r

240 CHAPTER 20 1962 Guide And Data Book Table 1.... Estimates of Energy Metabolism (M) of Various Types of Activity (Vaktet appfy for a 154 tb man, and do aol indodo wf pnumi) Kind of Wort Acfhflr M Btu/hr Sleeping.......................................................... Sitting quietly.............................................. 250 400 light Work Sitting, moderate arm and trunk movemeats.................................... ................... (e-g., desk work, typing) Sittiog, moderate arm and leg movements................................................ (e-g., playing organ, driving ear in traffic) Standing, light work at machine or bench, mostly arms................................ 450-550 550-650 550-650 Moderate Work Sitting, heavy arm and leg movements. 650-800 Standing, light work at machine or bench, some walking about................. 650-750 Standing, moderate work at mp*hine or bench, some walking about............ 750-1000 Walking about, with moderate liftingor pushing....................................................... 1000-1400 Heavy Work Intermittent heavy lifting, pushing or pulling........................................................ 1500-2000 (e-g., pick and shovel work) Hardest sustained work............................ 2000-2400 deep-body temperatures and without unusual load upon the heart. At the upper limits of physiological tolerance to sus tained beat exposure, thermal balance may- also be estab lished, but this is accomplished only with active sweating, elevated skin and body temperatures, and accelerated heart rate. The limit is fixed by the maximum permissible degree to which these indexes of physiological strain--sweating, body temperature, and heart rate--can be safely elevated. For practical work situations in industry, the permissible limits of these indexes must be set below the absolute physio logical maxima. The thermal relationship between man and his environ ment is dependent upon four independently variable thermal characteristics of the environment: air temperature, radiant temperature, moisture content of the air, and air velocity. These may combine in various ways, together with the rate of internal heat production, to create widely different degrees' of heat sires. The need is to provide a rational basis for combining them into a single index that will predict the mag nitude of the heat stress and will serve also as a basis for fixing-permissible limits of exposure. A start is made with the heat balance equation: AfRC-B + E& (1) where M > metabolic rate. R rate of heat exchange with environment by radiation. C = rate of heat exchange with environment by convection. B - rate of beat loss from body in exhaled air. E - rate of heat loss from body by evaporation of sweat. AS " rate of change in heat content of body. Terms on the left side of Equation 1 represent the heat load; those on the right represent the beat loss together with the heat storage or withdrawal from the body. R and C are positive when the environmental temperature is above skm temperature and negative when below. In practice, B and AS are of small magnitude compared with the others and can be neglected. For a condition of he*t balance, therefore, Equation 1 may be written: U R C " Bfo (2) * required rate of evaporation of sweat to maintain beat balance. Af can be measured by standard physiological techniques or estimated for a given work situation by reference to Table 1. On both theoretical and experimental grounds, R and C can be calculated with reasonable accuracy by means of standard equations of heat exchange: S - KrArit, - t.) = 22(t* - f.) (3) where C - K. SrAtI. - <J - 2 Svti. - i.) ) K, ** coefficient of radiant beat exchange, Btu per (hour) (square foot) (Fahrenheit degree temperature differ, enee). K, varies in value; depending on the mean radiant temperature of the surroundings, t, . Ar ** radiation area of body, square feet. A, differs with body position and is around 0.8 of total body surface for a standing man. 22 ^ Kr X At for average sise man with moderately high mean radiant temperature, . K, ** coefficient of convective heat exchange, Btu per (hour) (square foot) (unit velocity) (Fahrenheit degree tempera ture difference). A = surface area of body, square feet. 2 -K,Xi, for average sise man. V effective velocity of air movement, feet per minute. t* * (black-body equivalent) mean radiant temperature of environment, Fahrenheit. < * ambient air temperature, Fahrenheit. t, m skin temperature, Fahrenheit. 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 re stricted evaporation, however, there is excessive sweating; the body becomes fully wetted and a maximum evaporative cooling rate is established which does not increase with fur ther sweating. This maximum evaporative cooling rate ** has been established experimentally and can be calculated by the equation: Bm*x - KtAV* '{P. - />.) - lOF*-4(f> - Pa) (5) where K, * coefficient of heat exchange by evaporation, Btu per (hour) (square foot) (unit velocity) (millimeter of mer cury vapor-pressure difference). 10 -- K, X A for average sue man, fully wetted. P, -- vapor pressure of water at skin temperature, nulh* meters of mercury. P, *= partial pressure of water vapor in ambient air, miUi- meters of mercury. A combination of Equations 3, 4 nd 5 yields Equation 6 which is a statement of the maximum thermal conditions un der which man can maintain heat balance, giving propot recognition to internal heat production M, skin temperature Control of Industrial Environment 241 CHARTS TOR DETERMINING HEAT STRESS i~* tmmpfe Deferafa* Hoot Sfrat* Iwte* for wortar doing Ggb ora *ork whOo rfoodrnfl of a botxh. 600 Btub Globe flkrawmefar fawperafwre Drj-be&> fewperefora Wot-bulb fewperaftno Air ralocfly SqfafiV- follow tbo brota foe* from Itmghbo Oeraoaefer taBponrfere and from dry-bulb ttaporotm to I above diagram C to rood a hoot draw fade* ot 90. Pjg i____ Row Charts for Determining Heat Stress Index Values no deg 90 t 75 F 100 fpa