Document LO404L4DdQMO1366yr0adL13

HEATINC VENTILATING AIR CONDITIONING GUIDE 1944 Btu per hour. Or an additional (256,000 -- 231,500) = 24,500 Btu per hour will be required. The heated ceiling at 115 F and 0.95 emissivity releases 179 Btu or (179 -- 119.8) = 59.2 Btu per square foot per hour more heat than that allowed for an unheated ceiling." Therefore the surface required to be heated is approximately 24,500 4- 59.2 = 415 sq ft. Since the total ceiling area is'480 sq ft it is only necessary to utilize 415 sq ft to satisfy the necessary heating requirements. An alternative would be to heat the whole ceiling surface using a lower temperature circulating water. Also with the entire ceiling heated a slight margin of safety will be provided which is an advantage. Fig. 9. Heat Emission by Convection from Radiant Heat Panels with Still Air at Various Temperatures Example 2. Using the data in Example 1 calculate directly the required surface tem perature if the entire ceiling area is utilized and the design room conditions are identical. Solution. From Example t it was shown that 256,000 Btu per hour were required to maintain the desired MRT in the room having a surface area .of 2016 sq ft, andthat 24,500 Btu of additional heat was required above the natural heat emission of the room as shown in Table 3. Dividing 24,500 by 480 = 51.0 Btu per square foot per hour plus 119.8 Btu per square foot per hout which is the. heat emission of the unheated ceiling gives 170.8 Btu. From Table 1 and for an emissivity of 0.95 it is found that this amount of heat will be' emitted from a surface at approximately 108.5 F. ` This surface temperature may be obtained by using circulating water at about 125 F instead of 130 F, or the embedded pipes may be spaced wide apart and the water tem perature maintained at 130 F. Example 8. Determine the total heat emitted from the ceiling surface in Example i if it is maintained at a temperature to provide a room MRT of 71 F using the data in Figs. 8 and 9. 796 CHAPTER 45. RADIANT HEATING Table 4. Calculated Heat Losses for Example Surface Area Sq Ft U Heat Loss Calculation Total Heat Loss Btu per Hour Exposed Glass....... Covered Glass........ Inside Wall............ 297 186. 93 480 480 y 480/^ 0.25 1.13" 0.57b 0.16 297 X 0.25 X (65 - 0) 186 X 1.13 X (65 - 0) 93 X 0.57 X (65 - 0) No heat loss next to heated room Heated surface 480 X 0.10 X (65 - 0) 4,820 13,650 3,440 3,120 5760 cu ft X 1.25 air changes X (65 -- 0) X 0.018 8,430 Total 33,460 Two-thirds window area assumed to be fully exposed with a U = 1.13. bOne-third window area protected by side curtains with a reduction U = 0.57. Solution. The calculated heat losses of the room as outlined in Chapter 6 are given ih Table 4. The MRT for all unheated surfaces in the room may be determined from Table 3, by adding the total heat emission from walls, floors and windows and dividing by the total surface, or 174,000 4- 1536 = 113.5 Btu per square foot per hour. From Table 1 this emission from a surface having an emissivity of 0.93 corresponds to about 55 F. Utilizing the entire ceiling area with a heat emission corresponding to a surface tem perature of 108.5 F as determined in Example 2, and with a surrounding average M RT of the unheated surfaces of 55 F as previously calculated, it will be found from Fig. 8 that the ceiling surface will emit 50 Btu per square foot per hour by radiation. With an air temperature of 65 F this same surface will emit (42 X 0.48) = 20.2 Btu per square foot per hour by convection according to Fig. 9. Then the, Total by radiation = 480 X 50 = 24,000 Total by convection = 480 X 20.2 = 9,696 33,696 Btu per hour The difference between 33,696 and 33,460 Btu in Table 4 results in a safety factor of 236 Btu per hour. In case the ventilation rate of the room had been increased, more heat could be furnished by either adding wall panels or by introducing a positive source of ventilation air which could be externally heated to the correct temperature. MEASUREMENT OF RADIANT HEATING Radiant heating is intended to control the rate of radiant heat loss from the human body and should be measured by calorimetric methods. The apparatus for this purpose consists essentially of a cylinder, maintained at the accepted mean surface temperature of the human body, together with an accurate (usually electrical) measuring of the varying rate of heat supply required to maintain this exact temperature. This instrument, the eupatheoscope, is readily adapted to function like a thermostat so as to turn heat on or off, when the desired temperature of 80 F, or any other predetermined surface temperature of the cylinder, decreases or increases as a result of changes in the equivalent temperature. For testing work, the globe thermometer is a useful instrument. It consists of an ordinary mercury thermometer, with its bulb placed in the 797