Document 6bp2zX16y5r5nO3YwdgrKEGV9

HEATING VENTILATING AIR CONDITIONING GUIDE 1944 CHAPTER 3. FUNDAMENTALS OF HEAT TRANSFER Table 1. Approximate Unit Thermal Conductivities of Miscellaneous Materials3 Material kConductivity, Btu per Hour per Sq Ft per Deg F for One Inch Thickness Air... Aluminum...... Brass (70 - 30)_______________________________________ Cast-Iron Copper. . Glass Lead . Nirll SoiL ......... Steel, mild ...... ........... ........ ....... ............. ........................ Water, liquid___________ ___________ ___ _____ _____ 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-Hill Co.. 1942) for additional values. the temperature difference, (ta -- tf) which is the temperature of the surface less that of the fluid'. The proportionality factor is termed the unit convection conductance (sometimes called the film coefficient for convection), he, 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 for objects sur rounded by air can be conveniently expressed as in Equation 2a: where ( te)1 N"-1 / 1 \0.1M -w) ' TM 2c = Heat transmission by convection, Btu per square foot per hour. C .= a constant depending upon the surface shape. D -- diameter of pipe dr circular duct or height of vertical wall, inches: (Effect of diameter or height becomes constant at 24 in.) T av. = average wall surface and surrounding air temperature, degrees Fahrenheit absolute. h -- tf = 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 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 hr 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 factors to be used in conjunction with the factors in Table 3 for obtaining the free convection from Table 2 for pipes and ducts whose characteristic dimensions are less than 24 in. For example, the free convection transfer from a 3 in. O.D. horizontal cylinder for a temperature difference of 40 F = 25.0 X 0.73 X 1.52 = 27.7 Btu per square foot'per hour. Table-2. Heat Transmission by Free Convection for Large Vertical Surfaces Expressed in Btu per square foot per hour Temp Deg F 0 i 2 3 4 5 6 7 8 9 Temperature Difference between Body and Surrounding Still Air at 80 F 0 10 20 30 40 60 ' 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 5.5 11.8 18.9 26.7 34.9 43.5 52.4 61.8 71.3 81.4 91.2 101.5 112.0 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 8.5 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 2 1.64 3 1.52 4 1.43 5 1.37 6 1.32 7 1.28 8 1.25 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, ji 1922, Department of Scientific and Industrial Research, His Majesty's Stationery Office, London, England). 74 Actual O.D., or height, in_____ 9 Factor 1.22 10 1.19 12 1.15 14 1.11 16 1.09 18 1.06 20 1.04 22 1.02