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30079_45b

# 30079_45b - Fig 45.31 Gas radiation(Hr and convection(Hc...

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Fig. 45.31 Gas radiation (Hr) and convection (Hc) coefficients for flue gas inside radiant tubes.1 perature. The gas radiation factor depends on temperature and inside diameter. The effect of flame luminosity has not been considered. 45.9 FLUID FLOW Fluid flow problems of interest to the furnace engineer include the resistance to flow of air or flue gas, over a range of temperatures and densities through furnace ductwork, stacks and flues, or re- cuperators and regenerators. Flow of combustion air and fuel gas through distribution piping and burners will also be considered. Liquid flow, of water and fuel oil, must also be evaluated in some furnace designs but will not be treated in this chapter. To avoid errors resulting from gas density at temperature, velocities will be expressed as mass velocities in units of G = Ib/hr ft2. Because the low pressure differentials in systems for flow of air or flue gas are usually measured with a manometer, in units of inches of water column (in. H2O), that will be the unit used in the following discussion. The relation of velocity head hv in in. H2O to mass velocity G is shown for a range of temperatures in Fig. 45.32. Pressure drops as multiples of hv are shown, for some configurations used in furnace design, in Figs. 45.33 and 45.34. The loss for flow across tube banks, in multiples of the velocity head, is shown in Fig. 45.35 as a function of the Reynolds number. The Reynolds number Re is a dimensionless factor in fluid flow defined as Re = DGI jx, where D is inside diameter or equivalent dimension in feet, G is mass velocity as defined above, and JJL is viscosity as shown in Fig. 45.9. Values for Re for air or flue gas, in the range of interest, are shown in Fig. 45.36. Pressure drop for flow through long tubes is shown in Fig. 45.37 for a range of Reynolds numbers and equivalent diameters. 45.9.1 Preferred Velocities Mass velocities used in contemporary furnace design are intended to provide an optimum balance between construction costs and operating costs for power and fuel; some values are listed on the next page:

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Fig. 45.32 Heat loss for flow of air or flue gas across tube banks at atmospheric pressure (ve- locity head) x F x R Velocity Head Medium Mass Velocity G (in. H2O) Cold air 15,000 0.7 800°F air 10,000 0.3 2200°F flue gas 1,750 0.05 1500°F flue gas 2,000 0.05 The use of these factors will not necessarily provide an optimum cost balance. Consider a furnace stack of self-supporting steel construction, lined with 6 in. of gunned insulation. For G = 2000 and hv = 0.05 at 1500°F, an inside diameter of 12 ft will provide a flow of 226,195 Ib/hr. To provide a net draft of 1 in. H2O with stack losses of about 1.75 hv or 0.0875 in., the effective height from Fig. 45.38 is about 102 ft. By doubling the velocity head to 0.10 in. H2O, G at 1500°F becomes 3000. For the same mass flow, the inside diameter is reduced to 9.8 ft. The pressure drop through the stack increases to about 0.175 in., and the height required to provide a net draft of 1 in. increases to about 110 ft. The outside diameter area of the stack is reduced from 4166 ft2 to 11 X 3.1416 x 110 = 3801 ft2. If the cost per square foot of outside surface
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