Document bBxMeGQxgLgDnG7dr0ZwYBRZ6

756 CHAPTER 41 1946 Guide Film conductance /0 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: c- ^[pTr*)-*] (id The heat given up by the air in the duct is: Q = 0.24 W (f, - /,) = 14.4 A Vm Pv (/, - A) ' (12) Equating (11) and (12): ti -f- A -- 2/s h-~k 28.8 A Vm pv Wi ' . ' Let y = UPl" ^r rectanSu,ar ducts, and ^ ^ ^or round Air Duct Design . 757 Table 6. Weights qf Sheet Metal Used for E)uct Construction u. & Gagb Black Sheets Approximate * Thickness, In. Weight Per Square Foot Steel . Iron Ounces Pounds Galvanized Sheets6 Approximate Thickness, In. Square Foot Steel Iron Ounces Pounds 30 . 28 26 24 22 20 18 16 14 12 11 10 0.0123 0.0153 0.0184 0.0245 0.0306 0.0368 0.0490 0.0613 0.0766 0.1072 0.1225 0.1379 0.0125 0.0156 0.0188 0.0250 0.0313 0.0375 0.0500 0.0625 0.0781 0.1094 0.1250 0.1406 8 10 12 16 20 24 32 40 50 70 80 90 0.500 0.625 0.750 1.000 1.250 1.500 2.000 2.500 3.125 4.375 5.000 5.625 0.0163 0.0193 0.0224 0.0285 0.0346 0.0408 0.0530 0.0653 0.0806 0.1112 0.1265 0.1419 0.0165 0.0196 0.0228 0.0290 0.0353 0.0415 0.0540 0.0665 0.0821 0.1134 0.1290 0.1446 ' 10.5 12.5 14.5 18.5 22.5 26.5 34.5 42.5 52.5 72.582.5 92.5 0.656 0.781 0.906 1.156 1.406 1.656 2.156 2.656 3.281 4.531 5.156 5.781 Galvanized sheets are gaged before galvanizing and are therefore approximately 0.004 in. thicker. ducts, and solve for h and h . ^ h (y + 1) - 2f, k~ (y"-T) ' (13) ti (y - 1) + 2fj (y + 1) (14) For low velocities and long ducts of small cross-section, a somewhat more accurate formula may be used as follows: . ti -- <, . /_ UPl N + () KUAAfyVm/ e . Symbols used in these equations are listed at end of chapter. - In using Equations 13, 14 and 15, one of the duct air temperatures will be unknown and will.be obtained by substitution of the other known or assumed values. Heat loss coefficients for insulated ducts with various conductivities are given in Fig. 9. The conductivities of various materials, which are based on mean temperatures, ranging from, about 70 to 90 F, will be found in Table 2 of Chapter 6. For cases where the mean,temperature Table 7. , Weights and Thicknesses of Standard Copper Sheets, Rolled to Weight Weight pee Square Foot Ounces Pounds * Thickness, Inches Decimal ' Equivalent Nearest Fraction Nearest Gage No. B. &S. StUb9 U. S. STD. 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. M4 HiHi Hi H2 M2 Hi Hi ' Hi He He : Hi -Hi . Variations from these weights must be expected in practice. 27 26 25 23 22 21 20 19 17 16 15 .15 14 13 11 29 27 26 24 23 22 21 20 19 18 17 17 16 . 15 14 29 28 26 25 24 23 22 20 19 18 . 17 . 17 16 14 ' ,13 . ,