Document MG1EXB1qVZzMOXE1yrqxMBgaz

692 CHAPTER 51 1959 Guide extent can periods of beat exposure be offset by alternating periods of recovery? How is this expressed in terms of design of the control measures? These and other questions arise in the analysis of an industrial heat problem and must be con sidered by the design engineer and others before specifica tions are fixed. As there is no single set of thermal standards applicable to all work situations, failure to give proper recog nition to the many facets of the problem can result in inade quate control, measures. Biophysical Basis for Fixing Thermal Standards Under conditions of thermal comfort, the rate of internal heat production (metabolism) is just balanced by the rate of heat loss to the environment. This comfortable balance is Table 1.... Estimates of Energy Metabolism (M) of Various Types of Activity (Vetoes apply for a 154 lb men, and do sol intend* rust pomes) Kind of Work Adivffy M Btv/kr Sleeping.................................. .................... Sitting quietly........................ ................... 250 400 Light Work Sitting, moderate arm and trunk movements........................................................ fe.e.. desk work. tVDinz) Sitting, moderate arm and leg movements................................................ (c.g., playing organ, driving car in traffic) Standing, light work at machine or bench, mostly arms................................ 450-650 550-650 550-650 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 machine or bench, some walking about............ 750-1000 Walking about, with moderate liftingor poshing....................................................... 1000-1400 Heavy Work Intermittent heavy lifting, pushing or pulling........................................................ 1500-2000 (e.g., pick and shovel work) Hardest sustained work............................ 2000-2400 maintained without active sweating, with optimum skin and deep-body temperatures and without unusual load upon the heart. At the upper limits of physiological tolerance to sus tained heat exposure, thermal balance may also be estab lished, but this is accomplished only, with active sweating, elevated Ht> and body temperatures, and accelerated heart rate. The limit is fixed by tile maximum permissible degree to which these indexes of physiological strain--sweating, body temperature, %nd 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 stress. 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 baas for fixing permissible limits of exposure. A start is made with the heat balance equation: where MdcRdcC^B-\-E^s AjS (1) M = metabolic rate. R * rate of heat exchange with environment by radiation. C =* rate of heat exchange with environment by convection. B = rate of heat 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 beat load; those on the right represent the heat loss together with the heat storage or withdrawal from the body. R and C are positive when the environmental temperature is above skin 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 heat balance, therefore. Equation 1 may be written: M RC - E^ (2) TM. required rate of evaporation of sweat to maintain heat- balance. M 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: R = KMt* ~ t,) - 22(1. - i.). (3) where C - K. y/VA(t. - 1.) - 2 y/Vik - t.) (4) K, - coefficient of radiant heat exchange, Btu per (hour) (square foot) (Fahrenheit degree temperature differ ence). K, varies in value; depending on the mean radiant temperature of the surroundings, f. . A, = radiation area of body, square feet. At differs with body position and is around 0.8 of total body surface for a standing man. 22 Kr X At for average size man with moderately high 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 TM Ke X A, lor average size man. V -- effective velocity of air movement, feet per minute. tm " (black-body equivalent) mean radiant temperature of environment, Fahrenheit. 1 *=.;ambient air temperature, Fahrenheit. ! * skin temperature, Fahrenheit) So long as there is no limitation on evaporation rate, the Control of the Industrial Environment 693 CHARTS FOR DETERMINING HEAT STRESS Example: Determine Heat Stress index for worker dotes Gobi arm work while standing at a bench. Metabolism 600 Bfoh Globe thermometer temperatore Dry-bulb temperature Wet-bulb temperature Air velocity 170 dep 90 F 75 F 100 fpm Sofotfom Follow the broken finer ham the globe thermometer temperature and toom dry-bulb temperature to fheir Intonechon oa above diagram C to read a heal stress Index of 90. Fig. 1 .... Flow Charts for Determining Heat Stress index Values