Document Jrr6Ejz4N1Ydb2bG0BRGBb7gB

American Society of Heating and Ventilating Engineers Guide 1937 it is possible to estimate these variables with a considerable degree of accuracy for any climate or construction. Assume that a mean radiant temperature of 65 F is desired. Table 1 shows that with all the walls at this temperature, and with an emissivitv of 0.95, the gross heat emission is 124 Btu per square foot per hour. The total emission of radiation into the room from that surface would there fore be A X 124, where A is the total inside area of the room. This is the desired emission. If the whole area be divided into a number of different parts which are each at a uniform temperature--au aas,--and each is multiplied by the value of the heat emission corresponding to that temperature, and if all these products are added together, their sum will represent the total actual emission of radiation into the room at these temperatures without the aid of any hot surface. The difference between the desired emission and the actual emission represents the additional heat which must be supplied by the hot surface. The temperature of the proposed hot surface must then be selected, and its emission per square foot at that temperature determined from Table 1. This emission is divided into the additional amount of heat needed, ad justed for the fact that the heating units will shield the walls behind them, and the quotient obtained will be the area of the required heating surface. It is evident that this method of calculation is approximate, and depends for its accuracy on a correct estimate of the ultimate surface temperatures attained by the actual wall surfaces. It is necessary also to calculate how much heat will be given off by the same surfaces by convection, and thereby to determine whether this amount of convected heat will warm entering ventilating air to the tem perature maintained. If it will not, additional convection surfaces must be introduced to make up the deficiency. MEASUREMENT OF RADIANT HEATING Convection heating, having as its object the raising of the air tempera ture to a specified degree, must be measured, by thermometric methods which indicate essentially the air temperature, and not the rate of heat loss from the human body. Radiant heating, having as its object the control of the rate of heat loss from the human body, can be measured only by methods which basically are calorimetric, that is, Which measure directly the rate of heat loss from an object maintained at the temperature of the body, irrespective of air temperature. The apparatus for this purpose consists essentially of a hollow sphere, or cylinder, containing a fluid which can be maintained accurately at 83 F (the accepted mean surface temperature of the human body), with an accurate means of measuring the rate of heat supply required to maintain the temperature at that exact point. The latter measurement can be made with sufficient accuracy by electrical methods. Although a BET of 72 F is desirable, the mean radiant and'air temperatures may both vary, provided the heat loss by radiation and convection from a surface at 83 F is maintained at the rate of 15.4 Btu per square foot per hour, . .Chapter 3,8-t-Radiant Heating which corresponds to ^ = 4.5 watts per square foot of exposed surface. . This instrument, the eupatheoscope, can readily be adapted as a thermo- t by electrical control to shut off or turn on heat when the critical temperature ^3 F in the vessel is increased or decreased. A modifi cation of the instrument is called the eupatheostat. Another instrument for maintaining comfort conditions is at present available only in a model adapted to British practice as it is designed for a temperature of 75 F. It consists of a blackened copper sphere of approxi-. mately 6 in. diameter in which is housed a cylindrical sump containing a volatile liquid. In operation, a small electric heating coil drawing about 5 watts creates in the sphere a vapor pressure which is constant as long as the heat losses from the sphere are standard. If the temperature of the air or die MRT becomes too high for comfort, a greater pressure is created, owing to a smaller loss of heat from the sphere. This increase of pressure acts on a diaphragm and shuts off the supply of heat to the room. For testing work, the globe thermometer is a very useful instrument. It consists of an ordinary mercury thermometer, with its bulb placed in the center of a sphere from 6 in. to 9 in. in diameter, usually made of thin copper and painted black. The temperature thus recorded is termed the radiation-convection temperature. EXAMPLE Example 1. The surface areas, temperatures, and emissions for a room having a volume of 5760 cu ft are given in Table 2. The figures for temperatures are fairly representative of American practice with well-built walls, and are based on an emissivity. of 0.95 which approximates that of most paints and building materials. Table 2. Surface Areas, Temperatures, and Emissions for a Room of 5760 Cu Ft External Wall---------------Glass. Inner Wall Ceiling..... ............... ....1.... Floor.____________ _____ Total................ ......... Area Sq Ft 297 279 480 480 480 2016 Assumed Surface Temperature (Deo Faer) . 50 45 55 55 55 Heat Emission S(Btu Per q Ft per Hour) 110.6 106.5 115.1 115.1 115.1 Total Heat Emission from Abba (Btu per Hour) 32,850 29,710 55,250 55,250 55,250 228,310 228 310 The mean radiant temperature of. the room is 2016 = sOuare foot per hour which, as seen from Table 1, corresponds to an MRT of 53 F for an average emissivity of 0.95. For an average individual having a body surface of 19.5 sq ft, under conditions of comfort with a body surface temperature of 83 F, the heat given off by radiation may be determined by means of Equation 1 as 217 Btu per hour, or 11.1 Btu per square foot per hour. This corresponds to an environmental emission of 142 -- 11.1 = 130.9 Btu. per. square foot per hour, and, according to Table 1, to an MRT of 72 F. If this body be placed in the room described, it will lose heat at the rate of 19.5 (142 - 113.2) = 562 Btu perliour. This loss is 345 Btu per hour, or 17.7 Btu per square foot per hour, more than the rate of heat loss for comfort, which is only 19.5 (142 -- 130.9) = 2i7 Btu per hour. 711