Document ga9J96oma0epEGZ8kqEeNYDge

7 1938Heating Ventilating Air Conditioning Guide expensive in construction, is effective and quite suitable for cathedrals and.large public buildings (Fig. 3). To provide a uniform floor temperature, special consideration should be given to the design of the air ducts so that equal distribution is.obtained:? 4. By attaching separate heated metal plates or panels to the interior surfaces. These1 plates or panels are placed either in an insulated recess so that the surface of the panel is flush with the surface of the walls or ceilings, or they may be secured to the face of the wall. They may be covered with wood veneers and decorated to harmonize with other parts of the room, or they can be cast into panels to imitate oak or other wood designs. With flat plate panels it is a common practice to use a frame of plaster, wood, metal or composition around the panels to allow for expansion. These plates may be heated with either hot water or steam and connected similarly to an ordinary radiator system. Fig. 3. Diagram of Air Ducts for Floor Heating 5. By electric heated metal plates or panels. These plates or panels are either placed in insulated recesses of walls or ceilings or fastened to the face as desirable. They should not have a surface temperature above 160 to 200 F. Some electric panels have a much higher surface temperature but a lower temperature gives a more comfortable condition and is more efficient. 6. By electrically heated tapestry mounted on screens and on the wall. For this purpose the screen is woven with an electric continuous conductor being the warp and wool or silk being the weft. Such screens are useful to plug in at any position for emergency local heating without taking care of a large room or office. Note. If all of the heating panel is installed at one end of a large room there may be a marked difference between the BET on the two sides of the body. It is usually desir able therefore that the heat be distributed at different parts in the room so that no uncomfortable effects will be felt from unequal heating. CALCULATION PRINCIPLES 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 758 41.Chapter Radiant Heating Table 1. Total Black Body Radiation to Surroundings at Absolute Zero2 Boot or 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 9t 1.00 0.95 0.90 0.S0 Bodt OR Mean Radiant Temper ature 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 t 1.00 9 0.9S 9 0.90 oio 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 72 137.4 130.5 123.6 109.9 40 107.6 102.4 96.8 86.1 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 96.9 150 237.1 225.2 213.5 189.7 56 122.1 116.0 109.9 97.7 160 251.1 238.8 226.0 201.0 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 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 XTM V 100.000.000 Q = total black body radiation, Btu per square foot per hour. e = emissivity. T = absolute temperature, degrees Fahrenheit. 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. 759