Document LpbnOXn0pJQvr3X2nZm4wMLYg

HEATING VENTILATING AIR CONDITIONING GUIDE 1941 It will be noted that the proportionate heat loss by the three processes involved varies widelyj as indicated in Table 6. At air temperatures above the temperatures of the body surfaces, the body will, of course, be gaining heat by convection and losing heat only by evaporation. Increased air movement will favor both these processes and its net effect will depend on the relative humidity of the atmosphere. The phenomena involved are illustrated in Fig. 11 which shows the influence of air movement (at varying air temperatures and humidities) upon the upper limit of the zone of evaporative regulation. It will be noted that (for the nude subject in a semi-reclining posture) increase in air movement consistently increases evaporative cooling, and therefore heat tolerance, when relative humidity is high and air temperature low. When Fig. 11. Contour Chart Indicating Upper Limits (Wetted Area = 100 per cent) of the Zone of Evaporative Regulation for Various Air Velocities for Unclothed Subjects relative humidity is low and air temperature is high, however, an increase in air velocity from 17 to 30 or 100 fpm actually decreases heat tolerance. Under these conditions air temperature is above skin temperature and the increased demand for evaporative cooling exceeds the actual increase of evaporation due to the higher air movement. When an air velocity of 500 fpm is reached, the limits of the zone are broadened throughout its range. For very hot and dry environments, still air is more desirable than a slightly greater air movement but a high air movement is still' better. At low air temperatures the effect of increased air movement upon convection loss is a simple and direct one; but the exact quantitative influence of air movement upon the rate of cooling of a hot body has/until recently, been unknown. Studies at the John B. Pierce Laboratory of Hygiene have indicated that the cooling effect of air actually increases as 66 CHAPTER 2. PHYSIOLOGICAL PRINCIPLES k Percentage of Total Heat Loss Effected by the Three Chief Table 6. terc routes of Thermal Interchange Air Temperature- Deg F 60 70 80 90 Air Movement--- fpm 15 100 Evaporation--------- per cent 18 16 250 15 15 100 22 19 250 18 15 100 29 27 250 25 15 100 73 67 250 55 Radiation----------- per cent 43 26 19 40 26 17 37 24 17 14 10 13 Convection.--------- per cent 39 58 66 38 55 65 34 49 58 13 23 32 the square root of its velocity1. Under the conditions of the experiments in question (semi-reclining lightly clothed subjects) the effect is repre sented by the formula: / /n\ wherCe = Convection loss in kilogram-calories per square meter of body surface per hour. \T -- Surface temperature of the body (clothing and exposed skin) minus air temperature, degrees Fahrenheit. V -- Velocity of air, feet per minute. The extent of the effect may be indicated by the computed data in TaAbsle e7m. phasized in an earlier paragraph the problem of local drafts causing differential cooling of special areas of the body is one that must always be kept in mind. Experience and recent field studies by the A.S.H.V.E. Research Laboratory52 place the desirable air movement --" rK Qr,rl 2ft fnm under ordinary room temperatures during the Table 7. Corresponding Equ^ent Air Tempers Producin^^ Heat Observed Ambient Air Deg F 95 90 85 80 . 75 70 65 60 55 50 Equivalent temperature at standard air MOVE MBNT OF 15 FPM WHEN OBSERVED AIR MOVBMB NT IS 15 fpm 95 90 85 80 75 70 65 60 55 50 100 fpm 95.0 88.5 81.7 75.2 68.7 62.2 55.5 48.9 42.3 35.7 250 fpm 95.7 87.7 79.5 71.6 63.5 55.5 47.5 39.5 31.8 23.5 500 fpm 96.0 87.0 78.0 69.0 60.3 51.2 42.5 33.6 24.7 15.8 "" "The Influence of Air Movement on Heat Losses for the Clothed .Human Body, by C.-E. A. Winslow, A. P. Gagge and L. P. Herrington (American Journal of Physiology, October, 1939, Vol. 127, p. 505). "A.S.H.V.E. Research "Report No. 1016--Classroom Drafts in Relation to Entering Air Stream Temperature, by F. C. Houghten. H. H. Trimble. Carl Gutberlet, and M. F. Lichtenfels (A.S.H.V.E. 67