Document 8VxOn66vDn2eEJ97BD6Qqjwq5

HEATING VENTILATING AIR CONDITIONING GUIDE 1944 Table. 2. Highest Safe. Surface Temperatures for Heating Panel Type of Panel Plastered Ceiling (Pipes Embedded) Plastered Walls (Pipes Embedded) Floor, Anv Method Iron, Hot Water Medium* Iron, Steam Vapor3. ... Electrically Heated Panels*.,............................................ _.... Surface Temperature Dec F 115 120 ; 85 120 160 180 200 *Low surface temperature radiation is recommended regardless of the heating medium employed.. The difference between the desired and the actual total radiant emission represents the additional heating effect which must be supplied by the hot surfaces to be installed! The temperature of the proposed hot surface must then be selected from Table 2, and its emission per square foot at that temperature determined from Table 1. The difference between this emission and that of the unheated. surface replaced by the panel is divided into the total amount of additional heat needed, and the quotient will be the area of the required heating surfaces. These calculations depend on the accuracy of estimating the ultimate surface temperatures of the walls, windows, ceiling and floor surfaces under comfort conditions. Some unheated surfaces will absorb a large number of heat rays from the heated panels and thereby become warmer, giving off rays of longer wave length, while other surfaces will reflect a large percentage of rays and become simple reflectors of heat. Windows will be affected largely by curtains, shades or Venetian blinds and floors will be affected by rugs and carpets. . Example 1. The.surface areas and over-all heat transmission coefficient for a residence room having a volume of 5760 cu ft are given in Table 3. Determine the amount of radiating surface to maintain a room air temperature of 65 F and an MRT of 71 F, with an outside temperature of'zero, utilizing ceiling1 panels with circulating hot water at 130 F which will.maintain a surface temperature of approximately 115 F as given in Table 2. Solution. From Fig. 7 determine the estimated inside surface temperature for the various surfaces. In the case of the outside wall having a U = 0.25, it is found from the Table 3. Room. Data for Solving Example - :Surface .Area . Sq Ft Outside WaIl_,,L_L. Glass..................... Inside Wall._,,Ji__ Ceiling.__________ Floor___ ,________ 297 279 480 480 480 . Total:.-:_____:__ 2,016 Estimated U- Inside Surface . Temp Dec F 0.25 1.13 . 0.20 0.10 . 55.0 33.6 60.03 60.4 60.0 Emissivity e HeatEmis- r. sion Btu per Sq Ft per Hour' Total Heat , Emission * ;from Area Btu per .Hour 0.95 0.92 0.95" 0.95 0.93 115.1 94.3 119.5 119.8 117.0 . 34,200 26,300 57,300 57,500 . 56,200 Avg. 0:93 231,500 aNo heat loss through inside wall; assume wall surface temperature 60 F. ` 794 CHAPTER 45. RADIANT HEATING chart, that the intersection of this line with the zero outside temperature, that the surface temperature is 55 F. Since the glass temperature will depend on whether or not shades or curtains are provided, it may be assumed in offices and similar rooms that the whole glass surface will be exposed, whereas in residences, curtains may cover all or part of the window thus increasing the room MRT and reducing the human body heat loss. For this example it is assumed that the windows are partly covered with side curtains to the extent of about one-third. From Fig. 7 the surface temperature of the exposed glass corresponding to a U = 1.13, is 19 F. Assuming about a 2 F differential between the room air temperature Fig. 8. Heat Emission by Radiation from Panels when Surrounded by Surfaces of Various Temperatures Giving an Average MRT According to Curves and curtain surface temperature or 63 F. then one-third the difference between thisvalue. and the glass temperature results in a calculated average of 33.6 F. With a ceiling U = 0.20, the surface temperature for an unheated ceiling from Fig. 7 . is 57 F; multiplied bya factor 1.06 X 57 = 60.4F. The surface temperature of the floor witha U = 0.10 is from Fig: 7 a value of 62 F; multiplied by a factor 0.968 X 62 = 60 F. 'The emissivities are selected from Table 6, Chapter 3. .The glass emissivity of'0.92 . in Table 3 was determined by taking one-third of 0.95 (curtain) and two-thirds of 0.90 (glass). The heat emission in Btu per square foot per hour is taken from Table 1. The approximate natural mean radiant emission of the room from data, in Table 3 is 231,500 -s- 2016 = 114.8 Btu per square foot per hour which from Table 1 corresponds to an MRT of 57 F for an average emissivity of 0:93. With a ceiling surface temperature of 115 F and an emissivity of 0.95 from Table 1 the emission is 179.0 Btu per square foot per hour. The difference between 179 and-119.8 used in Table 3 is the additional heat emitted per square foot of warmed ceiling. - For an MRT of 71 F having a heat emission of 127 Btu corresponding to an average emissivity of 0.93, the total emission for all the room surfaces is 2016 X 127 = 256,000