Document YZQNz771825mejEJ5J5wGEny

HEATING VENTILATING AIR CONDITIONING CUIDE 1943 range, the blood capillaries near the surface become dilated, allowing more blood and heat to flow into the skin, and thus increase its temperature and consequently its heat loss. If this method of cooling is not in itself sufficient, the stimulus is extended to the sweat glands which allow water to pass through the surface of the skin. This method of cooling is the most effective of all, as long as the vapor pressure and dew-point temperature of the air are sufficiently low to allow for evaporation. In high humidities, where the difference between the dew-point temperature of the air and body temperature is not sufficient to allow rapid evaporation, increase in heat loss may be had by increasing air movement. The body, under hot conditions, is in the zone of evaporative regulation, and for moderately extreme conditions perfect balance between heat production and heat loss may be attained, although at the cost of considerable discomfort. In a cold environment, where environmental conditions are such as to remove heat too rapidly, the organism adapts in some degree by con stricting the blood vessels leading to the surface, thereby reducing the blood flow and heat available for dissipation to the environmental sur roundings. This adaptation is, however, partial and incomplete, and in an environment too cold for the clothing worn the temperature of the body tissues may fall, with accompanying discomfort and ultimate danger of serious chill. The process may go on for hours. The individual may move about and increase metabolism through muscular activity and thus balance the excessive heat demand of the environment, or he may reduce the loss by greater insulation of his body in the form of clothing. Some of these phenomena which are important are shown graphically in Fig. I. The dotted curves, from a study at the John B. Pierce Labora tory of Hygieneu, are for subjects lightly clothed in a semi-reclining position and give the relation between the dry-bulb temperature of the environment (with about 45 per cent relative humidity) and the metabolic rate, the rate of heat dissipation by radiation and convection combined, and the latent heat loss due to evaporation of,' perspiration and moisture from the respiratory tract. The smooth line curves, from the work of the A.S.H.V.E. Research Laboratory14,, give the same relationships for healthy, male subjects (18 to 24 years of age), seated at rest and normally clothed for winter-heated and air conditioned occupancy. The data for the semi-reclining subject also include the rate of heat storage (either positive or negative) due to a rise or fall in body temperature. For the normally clothed subjects a curve gives the total heat loss (that is, the sum of the radiation, convection and evaporative losses). Here, storage is given by the difference between the metabolism and total heat loss. The small difference between the metabolic or heat production rates for the two types of subjects may be accounted for by the difference in activity. Heat exchange between the body and the environment by radiation and convection is greater for the lightly clothed subject, both for cool con ditions where there is Considerable heat loss, and for very warm conditions where there is a sensible transfer from the atmosphere to the body. The two curves for evaporative loss serve to show how physiological control l,A.S.H.V.E. Research Report No. 1107--Recent Advances in Physiological Knowledge and Their Bearing on Ventilation Practice, by C.-E. A. Winslow, T. Bedford, E. F. DuBois. R. W. Keeton, A. Missenard, R. R. Sayers and C. Tasker. (A.S.H.V.E. Transactions, Vol. 45, 1939, p. 111). ,4Loc. Cit. Note 11. 48 CHAPTER 2. PHYSIOLOGICAL PRINCIPLES uses evaporation of perspiration to.maintain equilibrium, particularly at high temperatures. Below about 75 F for the normally clothed' subject, and below about 85 F for the lightly clothed subject, evaporation loss is minimal and probably due to uncontrolled evaporation from the relatively dry skin and from the respiratory tract. Above these temperatures con trol is had by availability of perspiration for evaporation. The difference in the curves above 75 F is probably largely determined by the difference in clothing and activity. Above temperatures from 95 to 100 F (probably that of the average outside surface of the clothed body) radiation and convection combined changes from positive to negative, and slightly above this temperature even the greatly increased latent heat loss ceases to suffice to take care of the rate of heat production and the negative Fig. 1. Relation Between Metabolism, Storage, Evaporation, Radiation Plus Convection, and Operative Temperature for the Clothed Subject radiation and convection loss, and storage or a rise in body temperature is the consequence. Above this range, even though there is inability to dissipate heat rapidly enough, metabolism actually increases, which may be accounted for by the predominance of the purely chemical laws of increased chemical reaction with rise in temperature, over physiological control, and indicates the point where a breakdown in thermal equilibrium begins. For higher temperatures life can only survive to the point where these accelerated processes will result in a rise in body temperature to the limiting level of from 106 to 108 F. Air movement is an important factor in increasing heat loss by either convection or evaporation. The result is accomplished through removal of hot humid air from near the body surface and replacing it with cooler and relatively drier air. This is an important factor in maintaining thermal equilibrium either for persons at rest or at work in hot, humid conditions. For conditions in the comfort zone and below, excessive 49