Document 5kzE3mBpdp44NX0dZdLakQ2xD

American Society of Heating and Ventilating Engineers Guide, 1934 mined, 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, the type 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 Btu per square foot per hour, The total emission of radiation into the room from that surface would there fore bei 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--ait at, as, etc.,--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. The following example will illustrate the principles involved: Table 2. Surface Areas, Temperatures and Emissions for a Room of 5760 Cu Ft External Wall................... Glass................................... Inner Wall......................... Ceiling................................ Floor................................... Total.................... ..... A.REA Sq Ft 297 279 480 480 480 2016 Assumed Surface Temperature (Deo Fahr) 50 45 55 55 55 Heat Emission (BtuPePrerHrS)q Ft 110.6 106.5 115.1 115.1 115.1 Total Heai Emission from Area (Btu Per Hr) 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 -22qSn'3.g10 = 113.2 Btu per square foot 538 Chapter 37--Radiant Heating 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 would lose heat at the rate of 19.5 (142 -- 113.2) = 562 Btu per hour. 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) = 217 Btu per hour. In order to determine the amount of radiating surface necessary to maintain the MRT at 72 F, assume the surface temperature of the hot plates to be installed to be 200 F, which is approximately the temperature they would have if heated by steam. The 2016 sq ft total area of the surfaces of the room multiplied by 130.9, 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 263,890 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 263,890 Btu desired. The additional heat needed is the difference between these figures, or 35,580 Btu. Since, from Table 1, the emission per square foot at 200 F is 309 Btu, the required radiant heating surface needed is 3|'(^ = 115 square feet. 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. 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 converted 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 balance. In the solution of this particular example, the radiation loss from the human body was selected as 217 Btu per hour, which is that taking place under optimum comfort conditions, with a body surface temperature of 83 F in a uniform environment at 72 F. The mean radiant temperature necessarily was 72 F. If the optimum BET of 72 deg Fahr is desired, an air temperature of 72 F also must be maintained. If it is desired to maintain a lower air temperature than this, a mean radiant temperature greater than 72 F must be selected and the radiation loss from the in dividual must be recalculated from Equation 1. 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 60 F to 70 F. It would, therefore, 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, 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 539