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686 CHAPTER 31 1951 Guide 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 7. Determine the entering air temperature and heat loss for a duct 24 X 36 in. cross-section and 70 ft in length, insulated with J in. of a material having a * Fot round duotalMa than 90 la. diameter, increase heat tranamimion values by the percentages shown below. . Thickness or Insulation (Inohcs) t 1 li 2- - 1% 5% 7%' t% 2% *% 4% 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 i. Solution. Referring to Fig. 16, the'overall 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 Air-Duct -Design 687 is found to be 1/13.348 = 0.0749 lb per cu f . . Substituting these and the other given values in Equation 25, y and h will be as follows: _ 28.8 X 6 X 1200 X 0.0749 V 0.49 X 10 X 70 ~ 45-3 120(45.3 + 1) - 80 h 45.3 - 1 123.7F Substituting in Equation 23: 0. = 0.49 X 10 X 70 ^(123 72+ 12) " 40J = 28.HX) Btu per hr. For special considerations which apply to insulation of ducts in marine installations see Chapter 47. MAINTENANCE Ducts should be designed in such a manner as to enable easy mainte nance.*3 They should have enough access doors, not only to enable in spection, but also to facilitate cleaning of the ducts.*4 The periodic clean ing of the ducts should be part of the regular maintenance schedule. It should be done efficiently and competently to avoid difficulties or hazards in the operation of the system.25 *6 LETTER SYMBOLS USED IN CHAPTER 31 a = coefficient of contraction. = absolute roughness, feet. Po = density of air under actual (operating) conditions, any consistent units. p* -- density of air under standard conditions, any consistent units. pr = density at which Vm is measured, pounds per cubic foot. A cross-section area of duct, square feet. A\ = area of inlet duct, square feet. 'At = area of outlet duct, square feet. A't ~ area of vena contracts, square feet. a = length of one side of rectangular duct, inches. (Other side is b.) b = length of one.side of rectangular duct, inches: (Other side is a.) C and Ci -- dynamic loss coefficients, dimensionless. Ci'.=-regain constant,.dimensionless.,, : D == inside diameter of duct, feet. <4=. circular equivalent of a rectangular duct for equal friction and . capacity, inches. . e *= Naperian base of logarithms = 2.718. /= non-dimensional friction coefficient. . ft = surface conductance (inside) Btu per (hour) (square foot) (Fahren heit degree). ' / = surface conductance-(outside) Btu per (hour) (square foot) .(Fah renheit degree). g = acceleration due to gravity, 32.17 feet per (second) (second). * H =.duct dimension perpendicular to plane of bend, feet. . HJWaspect ratio, dimensionless. - . .. . Hc = .pressure lpss due to. sudden contraction,. based'on standard air, inches of water. . H9 pressure loss, due to`sudden enlargement; based on standard air, inches of water. =.pressure loss due to gradual enlargement, based on standard air, inches of water. Hr = dynamic loss for standard air, inches of water. Hi and Ht = static pressure head at given points (1) and (2), inches of water. Hr ---regain static pressure for standard air, inches of water. AH = total pressure loss, inches of water.