Document omwmznKEpX41EREk61VBQ3a6w

716 CHAPTER 32 1953iGuide 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 PER DEG FAHR PER INCH THICKNESS) Fig. 16. Heat Loss Coefficients for Insulated Ducts* * For round ducts less than 30 in. diameter, increase beat transmission values by the percentages shown below. Thickness of Insulation (Inches) .. i 3% 1% 1 11 5% : 7% 2% 3% /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 40,3?. 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 ,717 is found to be 1/13.348 = 0.0749 lb per cu ft. Substituting these and the other given values in Equation 25, y and it will be as follows: 28.8 X 6 X 1200 X 0.0749 V = -------0,,.49 .X. 16 .X. 70------- = 45.3 . 120(45.3 + 1). - 80 t, = 45.3 123.7F Substituting in Equation 23V K 123.7 + 120\ ~| ------ -------- 1 - 40 = 28,100 Btu per hr. For special considerations which apply to insulation of ducts in marine installations see Chapter 48. MAINTENANCE Ducts. should be designed, in such a manner as to enable easy mainte nance.23 They should have enough access doors, not only to enable in spection, but also to facilitate cleaning of the ducts.24 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.26'28 LETTER SYMBOLS USED IN CHAPTER 32 a = coefficient of contraction, e = absolute roughness; feet. So = density of air under actual (operating) conditions, any consistent units. . pt = density of air under standard conditions, any consistent units. P, = density at which Vm is measured, pounds per cubic foot. A = cross-section.area of duct, square feet. , . , At = area of inlet duct, square feet. A, = area of outlet duct, square feet. A'i = area of vena contracta, square feet. a = length of one side of rectangular duct, inches. (Other side is 6:) 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. . fi = 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. - H/W = aspect ratio, dimensionless. He = pressure loss due to sudden contraction, based on standard air, inches of water. H, -- pressure loss due to sudden enlargement, based on standard air, inches of water. H,, = pressure loss due to gradual enlargement, based on standard air, inches of water. Hr = dynamic loss for standard air, inches of water. Hi and Hi -- static pressure head at given points (1) and (2), inches of water. H, = regain static pressure for standard air, inches of water. AH = total pressure loss, inches of water.