Document XR7JVdKyvjmakOMbeNymb5gRw

EXHAUST SYSTEMS, GENERAL b COMFORT VENTILATION In this case, 73 per cent (about threefourths) of the heat load under which the mao works comes from radiant sources, and can be reduced in practice only through proper shielding (or occasionally, through special protective clothing) Indexes of comfort. In comparing one work area with another, the effect!oe tem perature, already mentioned, is often used. However, if a significant part of the heat load is from radiant sources, a correction must be made How is an effective temperature reading interpreted7 The effective temperature most comfortable to the greatest number of persons in summer, with low sir velocities, is 71 F. According to Bedford, "Wherever practicable, the corrected effective tempera ture In spaces where men live or work should be kept below 80 F. It is especially desirable that this should not exceed 88 F . . . unless the temperature of the external air exceeds 100 F" Wartime studies at Fort Knox showed that effective tempera tures of 05 to 97 F were "impossible" Yaglou concludes, "An effective tempera ture of 95 F is the ceiling limit for short periods of work in heat" Any location where the wet bulb is 90 F approaches the upper limit for persons do ing light sedentary work. When the wet bulb approaches 95 F, sustained work be comes impossible, and heat prostration is frequent When humidity approaches satu ration, the wet bulb reading becomes allimportant Sometimes men have to expose them selves to temperatures as high as 300 F or more in kilns or m front of open furnaces-- but only for 2 or 3 minutes at a time. Since air cannot be breathed at temperatures above 240 F, the nose and other upper res piratory passages have to be protected Yaglou and Minardi developed a simple substitute for the corrected effectrVb tem perature called the wet bulb-globe tempera ture (WBCT) This requires no measure ment of air velocity nor use of complicated charts It is determined merely from the usual wet-bulb thermometer and the globe thermometer readings in this manner* WBGT = 07 (wet-bulb reading) -{-03 (globe thermometer reading). This gives directly a figure that corre lates well with the corrected effective tem perature, and is far more meaningful than the usual effective temperature when radi ated heat is present For example, with a wet bulb of 70 F and a globe reading of 100 F, die WBGT = 79 F, using the for mula )ust given. The heat stress index (HSI) devised by Belding and Hatchff is based on two cri teria of tolerable heat stress (a) thermal balance in the body must be maintained (buildup of heat m die body must be avoid ed), and (b) the maximum sweat rate over a long period of tune that a typical person must not exceed is approximately one liter per hour (equivalent to heat loss of 2,400 Btuh A liter has about 57 per cent more volume than a quart ) If the rate of body cooling through evapo ration of sweat, for example, is 1,200 Btuh to prevent gain of heat by the body, then half the available evaporative capacity of the man is being utilized This is defined as the heat stress index, (HSI) which in this instance is 50 HSI =%s!s-x 100 =^X 100 = 50 Emu 2400 Bedford, T. Environmental Warmth and Its Measurement Medical Research Coun cil War Memorandum No 17,1946 ""Yaglou, C. P. "Thermal Standards in Industry " American Journal of Public Health Year Booh, Part U, 40 131-142 (May 1950) tYaglou, C. P. and D. MinanL "Control of Heat Casualties at Military Training Centers." AM A. Arch Indust Health, 18 302,1957 See correction p. 485, lac. ctt HBelding, H. S. and T. F. Hatch. "In dex lor Evaluating Heat Stress in Terms of Resulting Physiological Strains" Heat ing, Piping and Air Conditioning, 27 129 (Aug 1955) See subsequent correction due to effect of clothing (amounting to about 30 per cent reduction in HSI) in "Labora tory Simulation of Hot Industrial Job To Find Effective Heat Stress and Resulting Physiologic Strain" (H S. Belding, et al) Amer lndust Hygiene Assoc. J, 21 25 (Feb 1980) 28-59