Spouted beds heat transfer

It should be noted that Macchi et al. (1999) reported an increase of heat transfer to wet particles with a diameter of 2000 pm by about 15% in a spouted bed. Brown et al. (1998) used so-called phaseparticle diameters ranging from 142 to 585 pm, and reported an enhancement of wall-to-bed heat transfer by up to 30% in comparison to respective inert particles. 

Employing wood chips, Cowan s drying studies indicated that the volumetric heat-transfer coefficient obtainable in a spouted bed is at least twice that in a direct-heat rotaiy diyer. By using 20- to 30-mesh Ottawa sand, fluidized and spouted beds were compared. The volumetric coefficients in the fluid bed were 4 times those obtained in a spouted bed. Mathur dried wheat continuously in a 12-in-diameter spouted bed, followed by a 9-in-diameter spouted-bed cooler. A diy-ing rate of roughly 100 Ib/h of water was obtained by using 450 K inlet air. Six hundred pounds per hour of wheat was reduced from 16 to 26 percent to 4 percent moisture. Evaporation occurred also in the cooler by using sensible heat present in the wheat. The maximum diy-ing-bed temperature was 118°F, and the overall thermal efficiency of the system was roughly 65 percent. Some aspec ts of the spouted-bed technique are covered by patent (U.S. Patent 2,786,280). 

Silva, E. M. V., Ferreira, M. C., and Freire, J. T., Mean Voidage Measurements and Fluid Dynamics Analysis in a Circulating Fluidized Bed with a Spouted Bed Type Solids F eeding System, Proc. of2nd European Thermal-Sciences and 14th UIT National Heat Transfer Conference, Rome (1996)... 

This section discusses the behavior of heat transfer between gas and particles, and between the bed and the surface in a spouted bed. 

The heat transfer behavior in a spouted bed (see 9.8) is different from that in dense-phase and circulating fluidized bed systems as a result of the inherent differences in their flow structures. The spouted bed is represented by a flow structure that can be characterized by two regions the annulus and the central spouting region (see Chapter 9). The heat transfers in these two regions are usually modeled separately. For the central spouting region, the correlation of Rowe and Claxton (1965) can be used for Repf > 1,000... 

Compared to the fluidized bed, a spouted bed with immersed heat exchangers is less frequently encountered. Thus, the bed-to-surface heat transfer in a spouted bed mainly is related to bed-to-wall heat transfer. The bed-to-immersed-object heat transfer coefficient reaches a maximum at the spout-annulus interface and increases with the particle diameter [Epstein and Grace, 1997]. 

Since the solid particles in the spouted bed are well mixed, their average temperature in different parts of the annulus can be considered to be the same, just as in the case of a fluidized bed. The maximum value of the heat transfer coefficient in the h-U plot is also similar to that in a dense-phase fluidized bed [Mathur and Epstein, 1974]. 

The importance of the intraparticle heat transfer resistance is evident for particles with relatively short contact time in the bed or for particles with large Biot numbers. Thus, for a shallow spouted bed, the overall heat transfer rate and thermal efficiency are controlled by the intraparticle temperature gradient. This gradient effect is most likely to be important when particles enter the lowest part of the spout and come in contact with the gas at high temperature, while it is negligible when the particles are slowly flowing through the annulus. Thus, in the annulus, unlike the spout, thermal equilibrium between gas and particles can usually be achieved even in a shallow bed, where the particle contact time is relatively short. 

Bed-to-Surface Heat Transfer. The heat transfer between the bed and the surface in spouted beds is less effective than in fluidized beds. The heat transfer primarily takes place by... 

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