Page 265 - Mechanical Engineers' Handbook (Volume 4)

254 Furnaces

Figure 32 Heat loss for flow of air or flue gas across tube banks at atmospheric pressure (velocity
head) F R.

The Reynolds number Re is a dimensionless factor in fluid flow defined as Re DG/
, where D is inside diameter or equivalent dimension in feet, G is mass velocity as defined
above, and is viscosity as shown in Fig. 9. Values for Re for air or flue gas, in the range
of interest, are shown in Fig. 36. Pressure drop for flow through long tubes is shown in Fig.
37 for a range of Reynolds numbers and equivalent diameters.

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:
Velocity Head
Medium Mass Velocity G (in. H O)
2
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