Document xz6vOdmxg4rdyNYjaYNMo7y7Q

710 CHAPTER 51 1960 Guide extent ean periods of heat exposure be offset by alternating periods of recovery? How is this expressed in terms of design of tiie 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 (Value* apply for a 154 lb bn, and do not include red passed Kind of Work Activity M Bto/hr Sleeping.......................................................... Sitting quietly.............................................. 250 400 Light Work Sitting, moderate arm and trunk movements.......................................................... (e.g., desk work, typing) Sitting, moderate arm and leg movements................................................ (e.g., playing organ, driving car 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................ 650-760 Standing, moderate work at machine or bench, some walking about........... 750-1000 Walking about, with moderate lifting or pushing....................................................... 1000-1400 Heavy Work Intermittent heavy lifting, pushing or pulling........................................................ 1500-2000 (e.g., pick and Bhovel 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 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 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 basis for fixing permissible limits of exposure. A start is made with the heat balance equation: where MRC=B + EaS (1) M ~ metabolic rate. R rate of heat exchange with environment by radiation. C TM rate of heat exchange with environment by convection. B = rate of heat loss from body in exhaled air. B = rate of heat loss from body by evaporation of sweat. A5 -- 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 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 tiie others and can be neglected. For a condition of heat balance, therefore, Equation 1 may be written: M R C = ^ (2) = 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 I. On both theoretical and experimental grounds, R and C can be calculated with reasonable accuracy by means of standard equations of heat exchange: R - KrAriU - I.) - 22(t. - I.) (3) where C - K, y/VA . - 1.) - 2 VV(t. - I.) (4) K, coefficient of radiant beat exchange, Btu per (hour) (square foot) (Fahrenheit degree temperature differ ence). K, varies in value; depending od the mean radiant temperature of the surroundings, . 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 = K, X A, for average size man with moderately high mean radiant temperature, U* . Ke n coefficient of convective beat exchange, Btu per (hour) (square foot) (unit velocity) (Fahrenheit degree tempera ture difference). A =* surface area of body, square feet. 2 = Kt X A, for average size man. V * effective velocity of air movement, feet per minute. lm = (black-body equivalent) mean radiant temperature of environment, Fahrenheit. U " ambient air temperature, Fahrenheit. t, " skin temperature, Fahrenheit. So long as there is no limitation on evaporation rate, the 1 Control of the Industrial Environment 711 CHARTS FOR DETERMINING HEAT STRESS RADIATION + CONVECTION-8TU/HR METABOLISM RADIATION CONVECTION -B T U /H R > r< 1. IOOX WETTED SKIN AT 95 F-BTU/HR Example: Determine Heal Sire** Index for worker doing light ana work white rtondmg a! o bends. Metabolism 400 Btufi EnWrocMPcnfaJ condrfioa*: Globe thermometer temperature Dry-bath temperature Wet-bulb temperature Air velocity NO deg 90 F 75 F 100 fpa SoMiort: Fellow the broken line* from the ghbe thermometer temperature and from dry-birth temperature to their tofenedton ao above diagram C to read a heat ftrea Index of 90. Fig. 1 .... Flow Charts for Determining Heat Stress Index Values