Document 3e4XLD8ZgnvZwz0RyVy0eXw2D

766: CHAPTER 41 .1946 Guide ducts, and solve for t\ and W- h (y + l)"- 2i, (y - i). (13) h- ~ (14> For low velocities and long ducts of small cross-section, a somewhat more accurate formula may be used as follows:. h = k -1, ' ( UPl Y+ h ^\14.4i4pv Vra) (15> Symbols used in these equations are listed at end of chapter. For round duct* less than 30 in. diameter, increase heat transmission va.nes by the percental shown. Thickness of Insulation (Inches) 21 CO 30 ID. JJQCt DiamMrr 12 to 21 in. Duct Diarriitpr H 1 1H 2' 1% 2% 3% 4% 3% 3% 7% 9% Air Duct Design 767 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. 10. 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 is other than that at 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. Example 5. Determine the entering air temperature and heat loss for a duct 24 X 36 in. cross-section and 70 ft in length, insulated with Vi in. of a material having a conductivity 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. Solution. Referring to Fig. 10, the over-all heat transmission coefficient is found to be 0.49 Btu. From Table 1, Chapter 3 the density of air at 70 F and 29.921-in. Hg is found to be 1/13.348 = 0:0749 lb per cubic foot. Substituting these and the other given values in Equation 13: 28.8 X 6 X 1200 X 0,0749 ,, _ y------------ 0.49 X 10 X 70 _ = 45 3 h 120 ( 45.3+ 1) - 80 45.3 - 1 = 123.7 Substituting in Equation 11: Q = 0.49 X 10 X 70 (123~7 ) - 40 = 28,100 Btu per hour. For special considerations which apply to insulation of ducts in marine installations see Chapter 49. LETTER SYMBOLS USED IN CHAPTER 41 Mo = absolute viscosity of air under actual (operating) conditions, any consistent units. Ms = absolute viscosity of air under standard conditions, any consistent unite. ' Pa = density of air, pounds per cubic foot. Po = density of air under actual (operating) conditions, any consistent units, Ps = density of air under standard conditions, any consistent unite, Pv " density of air at specified temperature at which velocity, Vm, is measured, pounds per cubic foot. . A = cross-section area of duct, square feet. o.= length of one side of rectangular duct, feet or. inches. (Other side is 6.) b = length of one side of rectangular duct, feet or inches. (Other side is a.) . D = inside diameter of conduit; feet, d = equivalent diameter, feet or inches. e -- Naperian base of logarithms = 2.718.: f = non-dimensional friction coefficient. Ji = surface conductance (inside) Btu per (hour) (square foot) (Fahrenheit degree), Jo = surface conductance (outside) Btu per (hour) (square foot) (Fahrenheit degree). g = acceleration due to gravity, 32.17, feet per (second) (second), K = friction loss under actual (operating) .conditions, any consistent units/ ' hs = friction loss, feet of fluid flowing. X = friction loss under standard conditions, any consistent units. ' Aw = velocity head or pressure, inches of water. l = length of conduit, feet. * P = perimeter of duct, feet. Q = heat loss through duct walls, Btu per hour. h = temperature of air entering duct, Fahrenheit degrees.