Bare-tube external surface is 0.262 ftVft. Fin-tube surface/bare-tube surface ratio is 16.9.

To convert British thermal units per hour-square foot-degrees Fahrenheit to joules per square meter-second-kelvins, multiply by 5.6783; to convert pounds-force per square inch to kilopascals, multiply by 6.895.

Bare-tube external surface is 0.262 ftVft. Fin-tube surface/bare-tube surface ratio is 16.9.

To convert British thermal units per hour-square foot-degrees Fahrenheit to joules per square meter-second-kelvins, multiply by 5.6783; to convert pounds-force per square inch to kilopascals, multiply by 6.895.

Fluidization occurs when the fluid flow rate is great enough so that the pressure drop across the bed equals the weight of the bed. As stated previously, the solids film thickness adjacent to the wall dm is difficult to measure and/or predict. Wen and Fau [Chem. Eng., 64(7), 254 (1957)] give for external walls:

where b = 0.29 (SI) or 11.6 (U.S. customary), cs = heat capacity of solid, ps = particle density, T| = fluidization efficiency (Fig. 11-31) and Nf = bed expansion ratio (Fig. 11-32). For internal walls, Wen and Fau give hi = bhQ-037 (11-51b)

where b = 0.78 (SI) or 9 (U.S. customary), hi is the coefficient for internal walls, and h is calculated from Eq. (11-51a). Gmf, the minimum fluidizing velocity, is defined by

where b = (1.23)(10~2) (SI) or (5.23)(105) (U.S. customary).

Wender and Cooper [Am. Inst. Chem. Eng. J., 4, 15 (1958)] developed an empirical correlation for internal walls:

h.Dp/k /—J043 = ^ /— \°-23/,cs 1 - €v \cp) R\ / \cj \pg/

where b = (3.51)(10~4) (SI) or 0.033 (U.S. customary) and CR = correction for displacement of the immersed tube from the axis of the vessel (see the reference). For external walls:

where x = 0.44LHcs/Dicg and f is given by Fig. 11-33. An important feature of this equation is inclusion of the ratio of bed depth to vessel diameter LH/Dt.

For dilute fluidized beds on the shell side of an unbaffled tubular bundle Genetti and Knudsen [Inst. Chem. Eng. (London) Symp. Ser. 3,172 (1968)] obtained the relation:

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