Document 159r6drBRapb9zz4zGL5ODE4K

758 CHAPTER 41 i 1946 Guide . For round dacta less than 30 in. diameter, increase heat transmission values by the percentages shown. Thickness op Insulation (Inches) X. \% ' ; .3% - 1. 2% . 6% IX 3% ' 7% 2` . 4% . 9% ' *" - Air Duct' Design 759 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 Determine the entering air temperature and heat loss for a duct24 X 36 in. cross-section and 70 ft in length, insulated,with in. of a material,having a con ductivity 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 airsurroiinding the duct at 40 F: Solution. Referring to Fig. 9, the over-all heat transmission coefficient is found to'be 0.49 Btu. From Table 1, Chapter 3 the density of ah at 70 F and 29.921 in. Hg is found to be 0.0749 lb per cubic foot.' Substituting these and the other given Values in- Equa tion 13: -* 28.8 X 6 X 1200 X 0.0749 y ' 0.49 X 10 X 70 120 ( 45.3+ 1) - 80 ------------- 45.3 - 1----- ^ =123-7 Substituting in Equation 11: ; Q = 0.49 X 10 X 70 (1-237 f 120^,_ 40 j = 28,100 Btu per hour. For special considerations which apply; to insulation of ducts in marine installations see Chapter 49. ` v LETTER SYMBOLS USED IN CHAPTER 41 absolute viscosity-of air under actual-(operating) conditions, any consistent units. . . {jus::= absolute.viscosity of air under standard conditions, any consistent units. Pa = density o.f air, pounds per cubic foot. .2 Po = density of air under actual (operating) conditions, any consistent units, ps =. density of air under standard .conditions, any consistent-units. ` Pv == density of air at specifiedtemperature at which velocity, Vm, is measured, pounds .per cubic foot. , ., .4 = cross-section area of duct,'square feet, f . a. ~ length of one side of rectangular duct, feet or. inches.. (Other side is b.) b -- length of one side.of.rectarigular ductjfeet 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. : jf-ik non-dimensional friction coefficient.` *- fi == surface conductance (inside). Btu per (hour)'(square foot) .(Fahrenheit degree). . fo ==- surface conductance (outside) Btu per (hour) (square foot) (Fahrenheit degree). -. . . + _ - ; g = acceleration due to.gravityTV32Ll7, feet per (second) (second). ;. Ao * friction loss under actual (operating) conditions, any consistent units. ht = friction loss, feet of fluid flowing. Aa = friction loss under standard conditions, any consistent units. Aw = velocity head or pressure, inches of water. I = 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. It = temperature of air leaving duct, Fahrenheit degrees. /* = temperature of air surrounding duct, Fahrenheit degrees. U = thermal transmittance coefficient, Btu per (hour) (square foot) (Fahrenheit degree). V = fluid velocity, feet per second, fa = velocity of air in duct, feet per second. Ym = velocity of fluid, feet per minute. ' W = weight of air through duct, pounds per hour. x -- thickness, inches.