Document jykLx6Gpqk833m64Zyx5exboQ

HEATINC VENTILATING AIR CONDITIONING CUIDE 1944 correlated, a person is vaguely conscious of a strain in the thermostatic body mechanism. CONTROL OF HEAT LOSSES Heat is transferred from any warm dry surface to cooler surroundings principally by convection and by radiation; the total loss is substantially the sum of these two. Where the surface is moist, as with the human body, heat is also lost through evaporation from both the body surface and the respiratory tract. The rate of heat loss by convection depends upon the average tempera ture difference between the surface of the body and the surrounding air-, the shape and size of the body, and the rate of air motion over the body. The rate of heat loss by radiation depends upon the exposed surface area of the body, and upon the difference between the mean surface temperature of the body and the mean surface temperature of the sur rounding walls or other objects! This latter temperature is called the mean radiant temperature (MRT). Because these two types of heat loss supplement each other, a.required rate of total heat loss can result either from a relatively low air tem perature and a relatively high MRT, or vice versa. A heating installation should provide comfort for those individuals doing the least physical work, without causing undesirable changes either in the rate of heat generation, or in the body's heat regulating mechanism. Rate of Heat Production The normal rate of heat production in an average sized sedentary individual is about 400 Btu per hour.. The heat production for persons subjected to various rates of activity is given in Chapter 2. When con sidering radiant heating, one must study separately the evaporation, radiation and convection losses. The human body is of complicated shape, and radiation takes place freely only from the`exposed outer surfaces; there are considerable portions of the body such as the legs;* arms, lower part of head, etc., which radiate most of their heat to other* portions. It is necessary to determine the equivalent surface of the body from which heat is radiated and a similar value for convectioh. The total surface may be assumed as approximately 19.5 sq ft for convection .and 15.5 sq ft for radiation, in an average sized individual.. The loss by evaporation and respiration depends on the temperature and area of the moist surfaces (outside and respiratory) of the body, the air temperature, air movement and humidity. In air at a temperature of 70 F, this loss for a sedentary individual of average size'will be approxi mately 90 Btu per hour; and at 60 F, about 70 Btu per hour: These values are relative, because the total will vary materially with change of position, bodily activity, age, sex, race, etc. The balance of the heat generated in the average human body, approxi mately 300 to 320 Btu per hour at about 70 F room temperature', is the approximate amount of heat given off by radiation and convection. It is difficult to determine the exact proportions of these two; but it appears, that if the body losses are about 190 Btu per hour by radiation, or 12.25 786 CHAPTER 45. RADIANT HEATING Btu per hour per square foot of radiating body surface, the greatest com fort will result. This leaves about 120 Btu per hour to be lost by convec tion, or 6.01 Btu per hour per square foot of convecting body surface. The mean surface temperature of the human body, including the whole area not only of exposal skin but also of clothing and hair, has been estimated variously at from 75 F, particularly in England up to as high' as 83 F in America. It is, however, conceded that further research and experience will be needed to finally derive the most suitable value for the American climate. The final figures will vary, with sex, age, clothing, etc., but will probably come between these extremes. From installations already in use in America an average surface temperature of 80 F appears to be more nearly correct. The mean surface temperature of an inert body, which will cause given rates of heat loss by radiation and by convection in a uniform environ ment, having a given air temperature and a given mean wall temperature, may be calculated from fundamental equations1 for radiation and natural convection, with substitution of comparable cylinders for the irregular human body. ~ Sr-o.mo,[(ii)`-(|i)`] , ,,, where qt = heat loss by. radiation, Btu per square foot per hour. qc = heat loss by convection, Btu per square foot per hour. Ts = absolute temperature of the body surface, degrees Fahrenheit. rw = absolute temperature of the walls, degrees Fahrenheit. ra = absolute temperature of the air, degrees Fahrenheit. Ts + Fa im - 2 D = diameter of cylinder, inches, e = the ratio of actual emission to black body emission. If it is assumed that an average adult has a height of 5 ft 8 in. a body . surface of 19.5 sq ft for convection, and 15.5 sq ft for radiation, an equiva lent effect can be worked out for two cylinders, 5 ft 8 in. high by 13.15 in. diameter and 10.45 in. diameter, respectively. However, while the effects on a cylinder, of a particular size and shape may be used to estimate average similar effects on the human body, it should be remembered that the heat loss from the body varies greatly. Every movement alters not only its shape, but also the heat generated by the body and the velocity of the air passing over it and the surface exposed to radiation. This fact renders the results of any such computation only approximate. EQUIVALENT TEMPERATURE The equivalent temperature is the mean temperature of the entire environment which is effective in controlling the rate of sensible heat loss Surface Heat Transmission, by R. H. Heilman (A S.M.E. Truncations, Fuels and Steam Power Section, Vol. 51. No. 22. September-December, 1029). 787