Document RpBv889nvY3yGLmqayv63oepa

Air Duct Design 583 A = cross-sectional area of duct, feet. D = diameter of round duct, feet. ' V = velocity of air in the duct, feet per minute,.at specified temperature.- p = density of air, pounds per cubic foot, at the specified temperature at which V is measured. e = Naperian base of logarithms = 2.718. In using Kquations 13,14 and 15, one of the duct air temperatures will be unknown and will be solved for 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 4. For cases where the mean temperature is other than that on which the test was conducted, a correction should be made. However, in most cases the effect of this factor will be small and may be neglected. :--e'" 7. TTM au LcwperdLure ana neat loss lor a duct 24 x do in. cross-section and 70 ft in length, insulated with in. of a material having a con- Ouctivity of 0.35 Btu at 86 F mean temperature, carrying air at a velocity of 1200 fpm. measured at 70 F, to deliver air at 120 F with air surrounding the duct at 40 F. n Aau>i0n'~ig' 9'the over-all heat transmission coefficient is found to be ?A4LBn nv,.nFfum Tab'e 6, Chapter 1 the density of air at 70 F and 29.92 in. Hg is found to be 0.0749 lb per cubic foot. Substituting these and the other given values in Equa- 28.8 X 6 X 1200 X 0.0749 y = 0.49 X 10 X 70 = 45.3 120 ( 45.3+ 1) - 80 45.3 - 1 Substituting in Equation 11: 123.7 Q = 0.49 X 10 X 70 (--?2+ 12) - 40 J Q = 28,100 Btu per hour. For special considerations which apply to insulation of ducts in marine installations see Chapter 38. Fn^rWA c1?7etIf?ining Rectangular Equivalents and Weights of Ducts,.by Peter 1940).k (A S H-V'E- Journal Section, Heating, Piping and Air Conditioning, December, p. 279) FIW f Liquids' fay w- H- McAdams (Refrigerating Engineering, February, 1925, r,,,A,,^v of the Data on the Flow of Fluids in Pipes, by Emory Kemler (A.S.M.E. transactions. Hydraulics Section, August 31, 1933, p. 7). . Air Conditioning and Engineering, American Blower Corp. Fan Engineering, Buffalo Forge Co. Wi^& Sonl)r Engineering- hy Barnard, Elienwood, and Hirshfeld, 'Part III (John Mechanical Engineers' Handbook, by Lionel S. Marks (McGraw-Hill Book Co.).