Document byg2LDMeLK222agyYX4KD6yo0

796 T. T. HATCH GLOBE TEMPERATURE (*F) METABOLISM 4RADIAT10N 4 CONVECTION (B T U /H R ) -1----- 1-- e?lOOO 0 1000 2000 5000 RADIATION 4 CONVECTION (BTU/HR) B /// 5000 - 1V/////// v ///2500 A/// 2000 /f A// -/A/1500 AA/ //// // / /V metabolism (BTU/HR) ISOO ~ - HEAT STRESS (INDEX) 40 $l| ;sb 20 100 :l; when icm 2400 v 80 uif lnUrctpt||c Of Ertt/wlllW S thUlInfM5^!; /60 MAXIMUM EVAPORATIVE CAPACITY 1Em0IQ 500 1000 1500 2000 2500 3000?$ BO 90 100 UO 120 ISO 140 500 1000 1500 2000 2500 3000 DRV BULB TEMPERATURE (*F) HEAT OF VAPORIZATION FROM '*![* 100% WETTEO SKIN AT 95'F (BTUv/HRh^ Figure 3. Flow charts for determining heat stress with excaample showing method ''W&mm of use^^V Belding and Hatch.1*) (Explanation on /actnjffcoft ' asTM HEAT CONTROL IN THE HOT INDUSTRIES 797 ``assumption of direct relationships of sufficient simplicity to cover practical situafttions adequately. It is therefore of interest to examine the proposed limit lines %that purport to combine the several factors into a single index to determine the ".physical basis, if any, in their apparent physiological equivalence, jb Haldane's original proposal to use the wet-bulb temperature as a single ^thermo-physiological index was based more upon physical reasoning than physio logical consideration. The completely wetted surface of man, according to this (reasoning, should assume a temperature directly related to the wej.-bulb tempera ture of the air. The latter bears a direct relation to the dry-bulb and dew-point ftemperatures of the air such that the rate of heat flow into the thermometer bulb l!?y convection and radiation just equals the rate of heat loss by evaporation irffom the wet bulb. For the ordinary thermometer with a 'A-inch bulb, the con vection coefficient is given by: Ka = 0.5 s/V. The radiation coefficient (independ ent of bulb size) has a value of Kr = 1.0 for temperatures around body tem1 perature. Since the psychrometer is whirled at considerable velocity, K0 is many (times greater than Kr, which can therefore be neglected. Consequently, the slope of the wet-bulb temperature line on the psychrometric chart is: AVPo = - y = - 0.27 At Hg/ F. The convection coefficient of a cylindrical body varies (approximately) in versely with the square root of the diameter. For man, therefore, Ka is much smaller than lor.:the_thermom&teE-apd-for-a-nude-man-has-been -shown experi mentally to be: Ka = 0.11\/K For V = 1000 f.p.m., Kc = 3.5, which is not much 'greater than Kr -- 1.0 and, hence, the latter cannot be neglected in considering 'the balance between heat input, C + R, and heat loss, Es^ith lower velocities K,, is even closer to Kr, and in still air it is actually smaller; In practical terms, this means that the line of thermal equivalence in man will have a steeper slope on the psychrometric chart than does the wet-bulb temperatt|^;line. Using the coefficients, of heat exchange given abqyg.!,and assuming an air velocity of 400 f.p.m., the iine of thermal balance has a slope.of --0.66 compared !with --0.27 for the wet-bulb line. Eichna's lines of physiological equivalence have "slope of --0.44, which fallB'b'etwehn'the slopes of wet-bulb line and calculated cedureTTIn7er.vfvlTroppmgfvi/erfrom intercept of globe temperature with air speed; this gives combined heat-loads of radiation and convection. Extend vertical line to 'enter B. At intercept with metabolism, drew horizontal line; obtain total heat load in terms nf evaporation required for heat: balance. CE,,,.).. Extend horizontal line-to eitterVQi ('Enter X. Taw horizontal line from intercept of .dry- and wet-bulb temperature; obtain vapqr.pressure V intercept with air speed draw vertical line; obtain maximum evaporation,from, wet skin, at 5 F. (Em,.). Extend vertical line to enter Z. Proceed to intercept with hqrizo.nt.al' line.frpm. . (If Em,,. exceeds 2400, enter Z at 2400). Read heat stress index value. Example: Globe 110, dry-bulb 90, wet-bulb 75, air speed 100, metabolism 600 (light arm'' ork standing at bench). For solution follow broken lines: Heat Btress = 90. ......