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Heating Ventilating Air Conditioning Guide 1939 If the area in square feet of each part of the space is multiplied by the emission value corresponding to its actual temperature, and these products are added together, the gross amount of radiant heat discharged into the room by the wall surface per hour is obtained. This quantity, divided by the total interior surface, gives the average amount of heat coming into the room from the surface of the walls per square foot of surface per hour. Interpolating in Table 1, the total radiation from a surface at 83 F for an emissivity of 0.95 is 142 Btu per square foot per hour. The difference between 142 Btu and the average amount of heat coming into the room is the amount which will be lost per square foot per hour by radiation from a body at 83 F. If a rate at which it is desired that heat be lost from the body by radiation and convection be assumed, the mean radiant emission from the walls required to give the desired result can be determined from Table 1, as can also the required air temperature for the corresponding convective effect. The determination of the amount of radiant heating surface needed in a room requires knowledge of the climate, the type of structure, theltype of heating, and the surface. temperature of the walls. This problem can be solved only on an empirical basis. After some experience, however, 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 emissivity of 0.95, the gross heat emission is 124 Btii per. square foot per hour. The total emission of radiation into the .room from that surface would there fore be4 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 Oj, Os,--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. ite 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 tern-., perature'maintained. If it will not, additional convection surfaces must I be introduced to make up the deficiency. 776'.' Chapter 41. Radiant Heating Table 2. Subface Areas, Temperatures and Emissions for a Room of 5760 Cu Ft External Wall.................... Glass-......................... ......... Inner Wall..........._...... ...... Ceiling................................. Floor.. ................................ Total--......................... Arica Sq Ft 297 279 480 480 480 2016 Assumed Subta.ce Temperature (Deo Fahb) 50 45 55 55 55 Heat Emission (Btu Per Sq Ft per Hour) Total Heat Emission from Area (Btu per Hour) 110.6 106.5 115.1 115.1 115.1 32,850 29,710 55,250 55,250 55,250 228,310 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. The mean radiant temperature of the room is 228,310/2016 = 113.2 Btu per square 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 area of 15.5 sq ft under conditions of comfort with a body surface temperature of 83 F, the heat given off by radiation when calculated by means of Equation 1 is 217 Btu per hour, or 14 Btu per. square foot per hour. This corresponds to an environmental emission of 142 -- 14 = 128 Btu per square foot per hour, aind, according to Table 1, to an MRT of 69.2 F. If this body be placed in the room described, it will lose heat at the rate of 15.5 X (142 -- 113.2) = 446 Btu per hour. This loss is 229 Btu per hour more than the 217 Btu per hour calculated, or 14.77 Btu per square foot per hour, more than the rate of heat loss for comfort: In order to determine the amount of radiating surface necessary to maintain the MRT at 69.2. F, assume the surface temperature of the hot plates to be installed to be 160 F, which is approximately the temperature they would have if heated by hot water. The 2016 sq ft total area of the surfaces d! the room multiplied by 128 which is the emission in Btu per square foot per hour necessary to maintain a body surface tempera ture of 83 F, gives a total desired emission of 258,048 Btu per hour. It is necessary to supply enough radiant heating surface to increase the total actual mean radiant heat emission by the room from 228,310 as shown in Table 2, to the .258,048 Btu desired. The additional heat needed is the difference between these figures, of 29,738 Btu. Since, from Table 1, the emission per square foot at 160 F is 238.8 Btu, the required radiant heating surface needed is 29,738/238.8 = 124 sq ft. The effect of this surface suitably placed would be to raise immediately the mean radiant temperature to the required degree and to maintain it. at that value as long as the surfaces remained at the values assumed. .. The calculation may be simplified by preparing tables showing, at the usual temperatures, the area of hot surface required to bring each square foot of actual wall surface at various temperatures up to a general standard of from 60 F to 70 F. It would then be necessary only to . multiply the respective areas by the appropriate factors, and to add the results, to obtain the required total. 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, arid 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.-