Slide 1Slide 2Environmental Stress (Ch. 13)BasicsBasic TemperaturesSlide 6Heat TransferThermal Adjustments - 1Thermal Adjustments - 2Slide 10Slide 112) Wet-Bulb Globe TemperatureLimits for Sedentary Job3) NIOSH Heat Stress IndexSlide 15Control of Heat Stress Engineering RedesignsSlide 17Slide 18Control of Heat Stress Administrative FactorsCold Stress - MeasurementCold Stress – ControlSlide 22IE 5531IE 553 Engineering of Human WorkDr. Andris FreivaldsClass #30IE 5532IE 553 Engineering of Human WorkDr. Andris FreivaldsClass #30IE 5533Environmental Stress (Ch. 13)•Human modeled as cylinder•Heat exchange process •Need to maintain equilibrium•Net gain or loss of heat = 0•S = M + R + C + Cd - E – W•Where: S = storage, M = metabolismR = radiation, C = convection, W = work Cd = conduction, E = evaporation,IE 5534Basics•Core = 37°C (or 98.6°F ) •Shell/extremities ~32°C (90°F, sacrifice?)•Physiological gradient = TC - TSk –Controlled physiologically•Physical gradient = TSk - TA –Controlled manually by clothingIE 5535Basic Temperatures•Dry bulb temperature (TDB) = “normal” temperature•Globe temperature (TG) = measure of radiation•Wet bulb temperature (TWB) = measure of evaporative capacity, depends on humidity•Dew point = temperature at which air is 100% saturatedIE 5536Psychometric Tables(Interrelationship of DB, WB and Vapor Pressure)IE 5537Heat Transfer•Radiation - heat gain/loss, absolute temperatureR (kcal/hr) = 4.375 (TW - TSk)TW = [(TG + 460)4 + 1.03×108V0.5(TG - TDB)]0.25–460•Convection - heat gain/loss, air movement (ft/min)C (kcal/hr) = 0.189 V0.6 (TDB - TSk)•Evaporation - heat loss (only), evaporation of water (1 l = 580 kcal)Emax (kcal/hr) = 2.575V0.6 (PSk - PA)Psk = 42 mm Hg at 37°CIE 5538Thermal Adjustments - 1 •Neutral to cold–TSk decreases, –Vasoconstriction (blood in, maintain core)–TC decreases–Shivering, goose flesh?–Hypothermia•Neutral to hot –TSk increases, –Vasodilation (blood routed to surface)–TC increases–Sweating (evaporation!!)IE 5539Thermal Adjustments - 2•Acclimation - short term, laboratory•Acclimatization - long term physiological adjustments to extremes–For heat = TC rise decreases over days–Corresponding increase in sweat loss –Rather quick (2 weeks, as opposed to reverse change)IE 55310Heat StressIE 55311Heat Stress Indices1) Effective Temperature• Psychological index = combo temp & humidity• Or Corrected ET (use GT for DB)IE 553122) Wet-Bulb Globe TemperatureWBGTout = 0.7 WB + 0.1 DB + 0.2 GTWBGTin = 0.7 WB + 0.3 GTIE 55313Limits for Sedentary JobIE 553143) NIOSH Heat Stress Index•S = M + R + C + Cd - E – W •If S = 0, then Ereq = M + R + C •HSI = Ereq/Emax •If HSI < 1 then OK, if >1 then heat stress•S (kcal) = (Ereq - Emax) x Time •S (kcal) = BW (kg) × 0.83 × ↑TC •Allowable ↑ in TC =1˚CIE 55315Evaporation - (1 l = 580 kcal)Emax (kcal/hr) = 2.575V0.6 (PSk - PA)Psk = 42 mm Hg at 37°CIE 55316Control of Heat Stress Engineering Redesigns•Modify environment:–↑convection (ventilation)–↓radiation (shielding)–↓humidity (AC??)•Modify work:–↓metabolism–W/R schedulesIE 55317Condition fpmAC space 50General 75-200Spot-light 1,500Spot-med 2,500Spot-high 3,500Ventilation (Heat stress)• Drops off as 1/d• At d = 30 x dia, ventilation = 0• Combine general and localIE 55318Ventilation (Fresh air) 300 ft3 (8.5m3) per person/hrIE 55319Control of Heat StressAdministrative Factors •Select personnel–Acclimatize before start–Rotate through jobs•Hydration•Cooling vestsIE 55320Cold Stress - Measurement•Wind chill index = rate of cooling as a function of ambient temperature and wind velocity given as equivalent temperature• Frostbite and hand dexterityIE 55321Cold Stress – Control• Tradeoff between heat production and insulation• Clo = insulation to maintain comfort at 70˚F (21˚)IE 55322Insulation (Tradeoff with exposure
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