Document nmeJYzLLrpdGpwDEMBbkbOY4R

1936American Society of Heating and Ventilating Engineers Guide, area of the panel must be sufficient to supply the requisite quantity of heat at this low temperature. When carefully designed, this method produces coihfortable and eco nomical results. 2. By attaching separate heated plates or panels to the interior surfaces of the structure. These plates or panels are placed either in an insulated recess flush with the surface of the walls or ceiling or bolted on its face. They may be decorated as desired. As it is difficult to make an invisible joint between the edge of such a plate and the plaster, it is common to use a frame of plaster, wood, metal or composition around the panel. These plates may be placed either on the ceiling or the wall, or in some cases as a margin around the edge of the floor. If floor heating is required the temperature over the whole area.should not exceed 70 F: If the entire warm surface is installed at one etid of the room there may be a marked difference between the BET on the two sides of a body in the room. It is usually desirable therefore that the heat be distributed at different points in the room so that no uncomfortable effects will be felt from unequal heating. PRINCIPLES OF CALCULATION The calculations for radiant heating are entirely different from those for convective heating. The purpose of the latter is to determine the rate of heat loss from the room by conduction, convection, and radiation when maintained in the desired condition; radiant heating involves,the regu lation of the rate of heat loss per square foot from the human body. The first step in the calculations for radiant heating is to ascertain the necessary mean radiant temperature (MRT); next, the size, temperature, and disposition of the heating surfaces required in the room to produce this MRT are estimated; and after this the determination of the convec tive heat is made. Mean Radiant Temperature If the whole of the interior surface of a room were at the same tempera ture, this temperature would represent the MRT. Such a condition seldom exists, however, since the actual surface temperature in any heated space having surfaces exposed to the outer air varies greatly for different sides of the enclosure. It is therefore necessary to ascertain by calculation the mean of these interior surface temperatures. The mean temperature in this sense is not the arithmetic average of the actual thermometric temperatures of the surfaces, but the temperature corresponding to the average rate of heat emission per square foot of surface. The temperature corresponding to this mean emission can be taken from Table 1. Conversely, the emission at different temperatures and also the emissivity factors can be obtained from this table. For instance, 1 sq ft of surface at 50 F will emit 104.9 Btu per square foot per hour to surroundings at absolute zero if the emissivity of the surface is 0.9. 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 692 38--Chapter Radiant Heating - Table 1. Total Black Body Radiation to Surroundings at Absolute Zero3 Boot ' OB Mean Radiant Temper ature Deg Fahr Radiation in Btu per square foot per hour emitted to surroundings with a tempera ture of absolute zero by bodies at various temperatures and with emissivity factor e ee 1.00 0.95 0.90 0.80 Boot or ` Mean Radiant Temper- aturb 1 Deg Fahr Radiation in Btu per square foot per hour emitted to surroundings with a temperature of absolute zero by bodies at various temperatures and with emissivity factor e 1.00 e 0.95 - e. 0.90 .e 0.80 30 35 40 45 46 47 48 49 50 51 52 53 . 54 ' 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 99.3 103.5 107.6 112.1 112.9 113.9 114.8 115.6 116.5 117.5 118.4 119.4 120.2 121.1 122.1 123.1 124.0 124.9 125.8 126.6 127.7 128.6 129.6 130.5 131.6 132.5 133.5 134.5 135.5 94.3 98.3 102.4 106.5 107.3 108.2 109.1 109.9 110.6 111.6 112.5 113.4 114.2 115.1 116.0 117.0 117.8 118.6 119.5 120.3 121.4 122.2 123.1 124.0 125.0 125.9 126.8 127.8 128.8 89.4 93.2 96.8 100.9 101.6 102.5 103.4 104.1 104.9 105.8 106.5 107.4 108.2 109.0 109.9 110.9 111.6 112.4 113.4 114.0 114.9 115.8 116.7 117.5 118.4 119.3 120.1 121.1 121.9 79.4 82.8 86.1 89.7 90.4 91.1 91.9 92.4 93.2 94.0 94.7 95.5 96.2 96.9 97.7 98.5 99.2 99.9 100.7 101.4 102.2 102.9 103.7 104.4 105.4 106.0 106.8 107.6 108.4 71 72 73 74 75 80 85 90 100 110 120 130 140 150 160 170 180 190 200 210 220 250 300 350 400 450 500 550 600 -136:5 137.4 138.4 139.6 141.0 146.6 152.3 157.9 169.6 181.6 194.8 210.1 223.2 237.1 251.1 270.5 288.0 306.5 325.2 348.0 371.5 437.8 575.0 740.0 942.1 1176.0 1464.0 1791:0 2405.0 129.6 130.5 131.5 132.6 133.9 139.4 144.6 149.9 161.1 172.5 185.0 199.6 212.1 225.2 238.8 257.0 : 273.8 291.0 309.0 330.6 353.0 415.9 546.1 703.0 895.0 1117.0 1390.0 1701.0 2284.0 122.9 123.6 124.5 125.6 126.9 132.0 137.1 142.1 152.6 163.5 175.4 189.1 201.0 213.5 226.0 243.5 259.1 275.8 292.8 313.1 334.4 394.0 . . 517.5 666.0 847.5 1059.0 1318.0 1613.0 2165.0 109.3 109.9 110.6 111.7 112.8 117.4 121.9 126.4 135.7 145.4 155.9 : 168.1 178.5 189.7 201.0 216.4 -230.4 245.1 260.3 278.4 297.1 350.2 460.0 592.0 753.5 941.0 1171.0 1434.0 1925.0 These factors are calculated from the formula / 0.1723 XT*\ )l, 100,000.000 0 = total black body .radiation, Btu per square foot per hour. e -- emissivity. T = absolute temperature, degrees Fahrenheit. 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, 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, 693 ' %