Document 5k8GzXo04vnG1nGM2Bw4pEV5

750 CHAPTER 32 1955 Guide Table 11. Weights and Thicknesses of Standard Copper Sheets* Rolled to Weight Weight peb Square Foot Thickness, Inches Nearest`Gage No. Ounces Pounds Decimal Equivalent Nearest Fraction B. &S. 10 12 14 16 18 20 24 28 32 36 40 44 48 56 64 0.625 0.750 0.875 1.000 1.125 1.250 1.500 1.750 2.000 2.250 2.500 2.750 3.000 .3.500 4.000 0.0135 0.0162 0.0189 0.0216 0.0243 0.0270 0.0324 0.0378 0.0432 0.0486: 0.0540 0.0594 0.0648 0.0756 0.0864 H* 54* Ht Ha 54o 54> Ha Ha H* 54. H* Ho He W4 54. * Variations from these weights must be expected in practice. 27 26 25 23 22 21 20 19 . 17 16 15 15 14 13 . 11 Stubs 29 27 26 24 23 22 21 20 19 18 17 17 16 15 14 U. S. Std. 29 28 26 25 24 . 23 22 20 19 18 17 17 16 14 13 stiffened. Joints can be of any of the standard designs, and can be fabri cated in the same manner as iron. Repeated sharp bending and rebending should be avoided, as aluminum has a tendency to crack under such treat ment. Aluminum of 16 B. & S. gage or heavier can readily be welded by the metallic arc or acetylene process. Soldering is difficult and is not gen erally recommended. Riveting is done in the same manner as in iron or steel sheet. Self-tapping screws tend to loosen because of the softness of aluminum. HEAT LOSSES FROM DUCTS In designing duct systems, the heat gains or losses of ducts should not be neglected. Heat gains in large duct systems can be quite considerable, not only if the duct passes through unconditioned space, blit also on long duct runs within conditioned space. Proper insulation will remedy this situation considerably, but sometimes a redistribution of the supply air to the various supply outlets is necessary in order to compensate for the heating effect of the duct surface. The thermal transmittance U for ducts can be found as follows: For uninsulated metal duct, U = ------- (20) For uninsulated non-metallic ducts, U= lil f, + k + Jo (21) where U = overall coefficient of heat transfer, Btu per (hour) (square foot) (Fahren heit degree). /1 = surface conductance (inside) Btu per (hour) (square foot) (Fahrenheit degree). Air Duct Design 751 /o surface conductance (outside) Btu per (hour) (square foot) (Fahrenheit degree). x = thickness, inches. k = unit conductivity of material, Btu per (hour), (square foot) (Fahrenheit degree per inch thickness). Where x is small and k is large, however, the factor 5 is of little impor.. tance and may be neglected. . * Film conductance /i for air flowing in ducts apparently depends only on the velocity of the air and the diameter of the duct. A fairly reliable inside coefficient can be calculated from Schultz's modified equation: , 0.32'8 -- ^jo.js ! (22) where v = velocity of air in duct, feet per second. D = insi de diameter of duct, feet. Film conductance f0 depends on a number of variables including tem perature, diameter, and emissivity of the outer surface, and can be calcu lated from data in Chapter 5. From this explanation, it is seen that it is unwise to recommend a given value of U for all uninsulated metal ducts. The heat loss from a given length of duct can be expressed by: Q'= upi [04^) -,s] (23) where Q* ~ heat loss through duct walls, Btu per hour. P = perimeter of duct, feet. I = length of duct, feet. h -- temperature of air entering duct, Fahrenheit degrees, h = temperature of air leaving duct, Fahrenheit degrees. 6 = temperature of air surrounding duct, Fahrenheit degrees. The heat given up by the air in the duct is: where <?w = 0.24,0(1, - U) = 14.4/1 Fmpv(i, - h) (24) w -- weight of air through duct, pounds per hour. A ~ cross-sectional area of duct, square feet. = mean velocity of fluid, feet per minute, pv = density of air at specified temperature at which velocity Vm, is measured pounds per cubic foot. 1 Equating (23) and (24): t, + t, - 2u 28.&4FmpT fj + UPI -