Document Z4YXXKVOr5pBgpwrJqZBXeLpp

830 CHAPTER 41 1949' Guide not only if the duct passes through unconditioned space, but 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 = 1 4-1 / '/. ' For uninsulated non-metallic ducts, U = (19) (20) Air Duct Design where ,. . : - U = overall coefficient of 'heat transfer, Btu per (hour) (square foot) (degree Fahrenheit).. /; = surface conductance . (inside) Btu per (hour) (square' foot) (degree Fahrenheit). /,, = surface conductance (outside) Btu per (hour) (square foot) (degree - . Fahrenheit). , x = thickness, inches k = unit conductivity of material, Btu per (hour) (square foot) (degree Fah renheit per inch thickness). ' Table 10. Weights and Thicknesses of 2S Aluminum (Density 0;Q9,lb/cu in.) B. & S. Gags Thickness, Inches Decimal Nearest Frsctioa . Weight peb Squai.'eroot ' . Ounces Pounds 28 26 24 22 20 . .. - 18 16 14 0.012 0.016 . 0.020 0.025 :< 0.032 ' 0.040 : ; 0.051 0.064 1/64 1/64. 1/64 1/32 1/32 3/64 3/64 1/16 2.7 3.6 4.5 5.4 7.2 . . 9.0 11.5 ' " ' .14.4 . .169' .226 .282 .353 .452 .563 J .720. > .903. : Where x is small and k is large, however, this factor ^ is of little impor tance and may be neglected. . . .. Film conductance f\ 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 : , 0.32V;-" /" (21) where V* = velocity of. air in duct, feet per second, D = inside'diameter of duct, feet... : . , .. .... - 1 ! Film conductance/,, depencfa.on a number of variables irichiding" tem perature; diameter, and'emissivily of the outer surface'anid "can be "calcu lated-'from data in Chapter 5. From this" explanation, it is seen thatdt'is unwise to recommend a given value of [/ for all uninsulated metal ducts: The heat loss from a given length of duct can be expressed by: '; ' Qw = UPl (22> where * Qw = heat lbss through duct walls, Btu per hour; P = perimeter of duct', feet, *' ' *y.;' J, -t * .* - i -.I'v r: i