Document peopM6M5zny1Jd6V6oNQQ0LJw

of and 1936American Society Heating Ventilating Engineers Guide, increase can be compensated for by raising the MRT. Similarly, with a higher air temperature the same total heat loss will be maintained by a correspondingly lower MRT. The loss by evaporation depends on the air temperature, air movement, and humidity; it is increased if the humidity is reduced. For the usual conditions of heating by radiators or convectors, where the air tempera ture ranges from 70 F to 73 F, approximately 75 per cent of the total heat loss of 400 Btu per hour occurs by radiation and convection, and the balance, or 100 Btu per hour, occurs by evaporation. In the case of radiant heating, if the air temperature is reduced to 60 F, 84 per cent of the 400 Btu per hour, or 336 Btu per hour, is lost by radiation and con vection, and 64 Btu per hour are lost by evaporation. The mean normal surface temperature of the human body, taken over the whole' area, including not only the exposed skin surface but also sur faces of the clothes and the hair, has been very extensively used as 75 F, particularly in British literature. However, results obtained by Aldrich* in rooms in which the air and wall surface temperatures were approxi mately 72 F gave mean values nearer to 83 F than to 75 F. The mean body surface temperature which will maintain the optimum heat loss by radiation and convection in a uniform environment of 72 F may be calculated from fundamental equations for radiation and natural convection by substituting a comparable cylinder for the body. Heilman* gives the following equations: <>> where Hr = heat loss by radiation, Btu per square foot'per hour. Hc = heat loss by convection, Btu per square foot per hour. r,, = absolute temperature of the body surface, degrees Fahrenheit. 7w = absolute temperature of the walls, degrees Fahrenheit. Ta = absolute temperature of the air, degrees Fahrenheit. v. Ta + Ta ---------- 2 D = diameter of cylinder, inches. e = the ratio of actual emission to black body emission. If it be assumed that a normal adult has an average height of 5 ft 8 in. and an average body surface area of 19.5 sq ft, the surface of his body will have the same area as that of a cylinder 5 ft 8 in. long with a diameter of 13.15 in. The value of e for skin and clothing is practically 0.95. Ta and Tv are each taken as 72 F, or 532 Absolute. The sum of Hr and Hz is taken to be 15.4 Btu per square foot per hour, which is derived as the normal rate of heat loss due to convection and radiation from a sedentary individual by dividing his total sensible heat loss by his area. Solution of >A study of Body Radiation, by L. B, Aldrich (Smithsonian Miscellaneous Collections, Vo!. 81. No. 6, Dec'Semurbfaecr,e1H92e8a).t Transmission, by R. H. Heilman {Trans. AJi.M.E., Fuels and. Steam Power Section, Vot. 51. No. 22, September-December, 1929). 690 38--Chapter Radiant Heating Equations 1 and 2, using average figures as outlined, gives a value of approximately 83 F for the normal temperature of the body surface. This agrees more closely with the values obtained by Aldrich than with the 75 F used by British investigators. British Equivalent Temperature The British Equivalent Temperature BET is the temperature of ah environment which is effective in controlling the rate of sensible heat loss from a sizable black body in still air when the body has a maintained surface temperature of 83 F. The BET is, therefore, a function of both the air temperature and the mean radiant temperature. Its numerical value in a uniform environment (walls and air at the same temperature) is equal to the temperature of the walls and air. In a non-uniform environ ment (walls and air at different temperatures) the BET is equivalent to that of a uniform environment in which an 83 F surface loses sensible heat at the same rate as it does in the non-uniform environment. As originally defined, the BET was based on a body surface temperature of 75 F, but 83 F has been accepted as giving results more nearly conforming with American practice4. The higher the BET the less the heat loss from the body, the rate of loss in still air being approximately proportional to the difference between the BET and the mean body surface temperature. If the BET were 83 F, there could be no sensible heat loss from a surface at that temperature, so the temperature of a normal body surface would have to rise to a point where the heat generated in the tissues could be dissipated. When convected heat is used, the temperatures of the air and walls are nearly the same, and the optimum value of the BET from the physio logical point of view is 72 F. Under these conditions the mean surface temperature of a normal body would have the optimum value of 83 F because the rate of heat loss by radiation and convection would be 15.4 Btu per square foot per hour and that by evaporation 5.1 Btu per square foot per hour, which would just balance the rate of heat production of 20.5 Btu per square foot per hour. This BET of 72 F in a uniform environment is .exactly equivalent to the effective temperature of 66 F as defined by the American Society of Heating and Ventilating Engineers (see Chapter 3), because, in a uniform environment, a drybulb temperature of 72 F in still air with a relative humidity of 30 per cent gives an effective temperature of 66 F, which has been determined to be the optimum. METHODS OF APPLICATION There are two general methods of applying radiant heating, as follow: 1.- By warming the interior surfaces of the building. Pipe coils are embedded in the concrete or plaster of the walls, ceiling or floors, the heating medium being hot water or, in some cases, steam. This has the effect of warming the entire concrete Or plaster surface in which the pipes are embedded. Since the temperature of the heating medium should not exceed about 120 F on account of the possibility of cracking the plaster, the ; ^Application of the Eupatheoscope for Measuring the Performance of Direct Radiators and Convectors in Terms of Equivalent Temperatures, by A. C. Willard, A. P. Kratz, and M. K. Fahnestock (A.S.H.V.E. Journal, llrating, Piping and Air Conditioning. July, 1933). , 691