Document LorZdMR1qpe7D76xporMLD7Gq

HEATING VENTILATING AIR CONDITIONING GUIDE 1942 Table 1. Approximate Unit Thermal Conductivities op Miscellaneous Materials3 Material * Conductivity, k Btu per Hour per Sq Ft per Dbg F for Onb Inch Thickness Ain................................................................... . Aluminum.- - .......................................................................... Cast Iron......... '.......................................................................... Glass............................................................................................ Soil...................................... ......... :............................... 0.168 1416.0 720.0 336.0 2640.0 3.6--7.32 240.0 330.0 2.4--12.0 312.0 4.08 ^Thermal conductivities depend to some extent on temperature. The above magnitudes are approxi mate only. Refer to Heat Transmission, by W. H. McAdams (McGraw-Hiil Co.. 1937) for additional values: Thermal Convection Equation ~ he (k -- k) (2). This rate equation states that the thermal convection current per unit transfer area (dq)/(dA), Btu per hour per square foot, is proportional to the temperature difference, {ts -- U) which is the temperature of the surface less that of the fluid3. The proportionality factor is termed the unit convection conductance (sometimes called the film coefficient), hc, Btu per hour per square foot per degree Fahrenheit. These convection conditions are illustrated in Fig. 1. The heat transmission by free or natural convection can be conveniently expressed as in Equation 2a: dtiM (2a) where 9c = heat transmission by convection, Btu per square foot per hour. C = a constant depending upon the surface shape. D = diameter of pipe or circular duct or height of vertical wall, inches. (Effect of diameter or height becomes constant at 24 in;) T av. average wall surface arid surrounding air temperature, degrees Fahrenheit absolute. rit = temperature excess between wall surface and surrounding air, degrees Fahrenheit. For horizontal cylinders, the value of C = 1.016 has been well estab lished by various investigations. For vertical plates, the value of C = , 1.394 has been fairly well established. A value of C = 1.79 for horizontal plates warmer than the surrounding air facing upward and 0.89 for . horizontal plates warmer than air facing downward is indicated by recent investigations4. *The particular fluid temperature to use for a given system will be noted under the discussion of that system. *The Transmission of Heat by Radiation and Convection, by Griffith and Davis (Special Report No. 9. 1922, Department of Scientific and Industrial Research, His Majesty's Stationery Office, London, England). 70 CHAPTER 3. FUNDAMENTALS OF HEAT TRANSFER The heat transmission by free convection from vertical walls 24 in. or more in height is given in Table 2 as calculated from Equation 2a for ambient air temperature of 80 F. The values in Table 2 will not be changed appreciably by a considerable change in air temperature for a given temperature excess: For instance, a change in air temperature from 80 to 40 F will increase the heat transmission given in Table 2 by only 1.3 per cent. Table 2 can also be used for calculating the free convection rate of transmission for various commercial shapes such as pipes and ducts. These calculations are simplified by the use of the factors in Tables 3 and 4. Table 3 gives factors by which the values in Table 2 must be multiplied to obtain the free convective transfer from various shapes whose characteristic dimensions are 24 in. or over, and Table 4 gives the Table 2. Heat Transmission by Free Convection for Large Vertical Surfaces Expressed in Btu per square foot per hour Deg F 0 2 3 4 5 6 7 8 9 Temperature Difference between Body and Surrounding Still Air at 80 F 0 10 20 30 40 50 60 70 80 90 100 110 120 130 0 4.4 10.4 17.4 25.0 33.2 41.8 50.6 59.9 69.4 79.4 89.2 99.4 109.8 0.3 4.9 11.1 18.1 25.8 34.1 42.6 51.5 60.8 70.3 80.4 90.2 100.4 110.9. 0.6 S.5 11.8 18.9 26.7 34.9 43.5 52.4 61.8 71.3 81.4 91.2 10L5 112.0 1.0 6.0 12.5 19.7 27.5 35.7 44.3 53.4 62.7 72.3 82.4 92.2 102.6, 113.0 1.4 6.6 13.2 20.5 28.3 36.6 45.2 54.3 63.7 73.3 83.3 93.3 103.6^ 114.1 1.8 7.3 13.9 21.2 29.2 37.4 46.1 55.2 64.6 74.3 84.2 94.3 104.7 115.2 2.3 7.9 14.6 22.0 30.0 38.3 47.0 56.1 65.6 75.3 85.2 95.3 105.7 116.3 2.8 8.S 15.3 22.7 30.8 39.1 47.8 57.1 66.5 76.3 86.2 96.3 106.7 117.3 3.3 9.1 16.0 23.5 31.6 40.0 48.7 58.0 67.5 77.4 87.2 97.4 107.8 118.4 3.8 9.7 16.7 24.3 32.4 40.9 49.7 59.0 68.4 78.4 88.2 98.4 108.8 119.5 Table 3. Free Convection Factors for Various Shapes Shapes Horizontal cylinders 24 in. in diam. or over.--:--------Long vertical cylinders 24 in. in diam. or over............. Vertical plates 24 in. in height or over........ .......... ......... Horizontal plates warmer than air facing upward____ ! Horizontal plates warmer than air facing downward... Horizontal plates cooler than air facing upward______ Horizontal plates cooler than air facing downward----- Factor 0.73 0.88 1.00 1.28 0.64 0.64 1.28 Table 4. Free Convection Factors for Various Diameter Pipes or Various Height Plates Actual o. d., or height, in......... 1 Factor_____________ ________ _ 1.88 Actual o. d., or height, in_____ 9 Factor... 1.22 2 1.64 10 1.19 3 1.52 12 1.15 4 1.43 14 1.11 5 1.37 161.09 6 1.32 18 1.06 7 1.28 20 1.04 8 1.25 22 1.02