Document 6M96vkKLoZYojGrpnGV4Ea4R

of andAmerican Society Heating Ventilating Engineers Guide, 1935 area of the panel must be sufficient to supply the requisite quantity of heat at this low temperature. When carefully designed, this method produces comfortable 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^t 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 end of the room there may be a marked difference between the BET on the two sides of abody in the room. It is usually desirable therefore that the heat be distributed atdifferent 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 radiatio.i 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 660 38--Chapter Radiant Heating Table 1. Total Black Body Radiation to Surroundings at Absolute Zero2 * f'. . Bodt Radiation in Btn per square foot per hour H Boot Radiation in Btu per square foot per hour emitted `.OB emitted to surroundings with a tempera R OB to surroundings with a temperature of absolute `v.: Mean ture of absolute sero by bodies at various 1 Mean tero by bodies at various temperatures and Radiant temperatures and with emisBivity factor e IRadiant with emissivity factor 0 Teupeb- tTempeb- ATtTHB - Deg . Fabr e 1.00 0 0.95 0 0.90 0 0.80 1 ATUBE Deg | Fahr 1.00 0 0.95 00 0.90 0.80 30 99.3 94.3 89.4 79.4 71 136.5 129.6 122.9 109.3 . 35 . 103.5 --98.3 - 93.2- - 82.8 72 13774 130.5 123.6 109.9 .40 107.6 102.4 96.8 86.1 1 73 138.4 131.5 124.5 110.6 45 112.1 106.5 100.9 89.7 74 139.6 132.6 125.6 111.7 '' 46- 112.9 107.3 101.6 90.4 75 141.0 133.9 126.9 112.8 47 113.9 108.2 102.5 91.1 80 146.6 139.4 132.0 117.4 48 114.8 109.1 103.4 91.9 85 152.3 144.6 137.1 121.9 49 115.6 109.9 104.1 92.4 90 157.9 149.9 142.1 126.4 50 116.5 110.6 104.9 93.2 100 169.6 161.1 152.6 135.7 '- 51 '117.5 111.6 105.8 94.0 110 181.6 172.5 163.5 145.4 52 118.4 112.5 106.5 94.7 120 194.8 185.0 175.4 155.9 53 119.4 113.4 107.4 95.5 130 210.1 199.6 189.1 168.1 54 120.2 114.2 108.2 96.2 140 223.2 212.1 201.0 178.5 * 55 121.1 115.1 109.0 96.9 150 56 122.1 116.0 109.9 97.7 160 237.1 251.1 225.2 238.8 213.5 226.0 189.7 201.0 57 123.1 117.0 110.9 98.5 170 270.5 257.0 243.5 216.4 58 124.0 117.8 111.6 99.2 180 288.0 273.8 259.1 230.4 59 124.9 118.6 112.4 99.9 190 306.5 291.0 275.8 245.1 60 125.8 119.5 113.4 100.7 200 325.2 309.0 292.8 260.3 1 61 126.6 120.3 114.0 101.4 210 348.0 330.6 313.1 278.4 62 127.7 121.4 114.9 102.2 220 371.5 353.0 334.4 297.1 63 128.6 122.2 115.8 102.9 250 437.8 415.9 394.0 350.2 64 129.6 123.1 116.7 103.7 300- 575.0 546.1 517.5 460.0 -. * 65 130.5 124.0 117.5 104.4 350 740.0 703.0 666.0 592.0 66 131.6 125.0 118.4 105.4 400 942.1 895.0 847.5 753.5 67 132.5 125.9 119.3 106.0 450 1176.0 1117.0 1059.0 941.0 68 133.5 126.8 120.1 106.8 500 1464.0 1390.0 1318.0 1171.0 69 134.5 127.8 121.1 107.6 550 1791.0 1701.0 1613.0 1434.0 .70 135.5 128.8 121.9 108.4 600 2405.0 2284.0 2165.0 1925.0 These factors are calculated from the formula where Q ( 0.1723 XT*\ V 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 temjperature 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, 661