Heat transfer, fluidized beds horizontal tubes

Chandran, R., Chen, J. C., and Staub, F. W., Local Heat Transfer Coefficients Around Horizontal Tubes in Fluidized Beds, J. of Heat Transfer, 102(2) 152-157 (1980)... 

Figure 3 Heat transfer coefficients for horizontal tube in bubbling beds of glass spheres, fluidized by air at atmospheric pressure.

Experimental data and models for the heat transfer coefficients in freeboards space are limited. Biyikli et al. (1983) measured heat transfer coefficients on horizontal tubes in freeboards of fluidized beds with particle mean diameters ranging from 275 to 850 pm. These investigators discovered that data for particles of different sizes and materials could be unified into a family of curves by plotting the normalized ratios of heat transfer coefficients against dimensionless velocity ... 

Biyikli S, Tuzla K, Chen JC. Heat transfer around a horizontal tube in freeboard region of fluidized beds. AIChE J 29, no. 5 712-716, 1983. 

Chandran R, Chen JC, Staub FW. Local heat transfer coefficients around horizontal tubes in fluidized beds. AIChE J 102, no. 2 152 157, 1980. 

Jonassen, O., 1999. Heat transfer to immersed horizontal tubes in gas fluidized bed dryers. Diss., Norwegian University of Science and Technology, Trondheim. 

Tamarin, A. I., Zabrodsky, S. S., and Yepanov, G., Heat Transfer Between a Horizontal Staggered Tube Bundle and a Fluidized Bed, Heat Transfer-Soviet Research, 8(5) 51—55 (1976)... 

Heat Transfer Heat-exchange surfaces have been used to provide the means of removing or adding heat to fluidized beds. Usually, these surfaces are provided in the form of vertical or horizontal tubes manifolded at the tops and bottom or in a trombone shape manifolded exterior to the vessel. Horizontal tubes are extremely common as heat-transfer tubes. In any such installation, adequate provision must be made for abrasion of the exchanger surface by the bed. The prediction of the heat-transfer coefficient for fluidized beds is covered in Secs. 5 and 11. 

Figure 1. Average heat transfer coefficients at surface of horizontal tube in bubbling fluidized bed. (From Chandran, Chen and Staub, 1980.)...

General Characteristics. Energy addition or extraction from fast fluidized beds are commonly accomplished through vertical heat transfer surfaces in the form of membrane walls or submerged vertical tubes. Horizontal tubes or tube bundles are almost never used due to concern with... 

Biyikli, S., and Chen, J. C., Effect of Mixed Particle Sizes on Local Heat Transfer Coefficients Around a Horizontal Tube in Fluidized Beds, 7th Int. Heat Transfer Conf, Munich, Germany (1982)... 

Bock, H. J., and Schweinzer, J., Heat Transfer to Horizontal Tube Banks in a Pressurized Gas Solid Fluidized Bed, German Chem. Eng., 9(1) 16-23 (1986)... 

Figure 12.14. Variation of local heat transfer coefficient around a horizontal tube immersed in a fluidized bed of alumina (from Botterill et al., 1984).

H.A. Vreedenberg, Heat transfer between a fluidized bed and a horizontal tube, Chem.Eng. Sci. 9, 52-60 (1958) Vertical tubes, Chem. 

The influence of surface location and orientation on the bed-to-surface heat transfer coefficient in circulating fluidized bed combustors is summarized in Table 13.6. The geometric construction of the combustor and the heat transfer surface is shown in Fig. 13.17. Besides the location and orientation, differences in local heat transfer can also be found on the heat transfer surface/tube. For example, the upper part of the horizontal tube shows the smallest value for the heat transfer coefficient in dense-phase fluidized beds due to less frequent bubble impacts and the presence of relatively low-velocity particles. 

Good heat transfer is one of the most attractive features of the fluidized bed. From the standpoint of its use as a chemical reactor, the most important mode of heat transfer is that from a fluidizing bed to a bank of tubes (with a circulating fluid) immersed within it. The value of the heat transfer coefficient will depend on whether the tube bank is vertical or horizontal. A number of correlations are available for predicting these and other modes of heat transfer in a fluidized bed, and good reviews of these correlations can be found in Botterill (1966), Zabrodsky (1966), Muchi et al. (1984), and Kunii and Levenspiel (1991). Most of them are restricted to relatively narrow ranges of variables. Two useful correlations are listed in Table 12.6. It is important to note that reactions such as the chlorination of methane (Doraiswamy et al., 1972) are entirely heat transfer controlled. The rate of heat removal and design of reactor internals become the crucial considerations in such cases. 

Recent studies have made it easier to design reactors with vertical tubular inserts. This arises from the observation (Gunn and Hilal, 1994, 1996, 1997) that the heat transfer coefficients for these systems are almost equal to those for the corresponding open fluidized beds of the same diameter operating with the same particles. Hence, correlations for the latter (which are readily available) can be used for vertical inserts without significant loss of accuracy. Vertical inserts have an additional advantage over horizontal inserts. In horizontal inserts there is accumulation of particles on top of the tubes and depletion of particles at the bottom, a situation that does not exist in the vertical orientation. 

Fluidized beds equipped with internal heaters or immersed tubes transfer heat indirectly to the drying material. Horizontal tube bundles (Figure 8.15) are used extensively compared to vertical type. Tube pitch is an important design parameter. Fluidizing gas stream fluidizes the material and carries over the evaporated moisture. As a result, total sensible heat of gas and thus quantity of gas required are reduced. Immersed tubes or internally heated FBDs are used to dry smaller-size or fine powders. This is because the heat transfer coefficient decreases with increasing particle size. Instead of tubes, vertical plates are also used as immersed heaters. 

Wear (also called wastage and erosion) of surfaces is a serious operational issue in some fluidized-bed reactors. Wear occurs when hard particles (e.g., silica-supported catalyst particles) continually strike fixed surfaces such as heat transfer tubes, reactor walls, or cyclone inner surfaces. The most damaging collisions tend to be those which are oblique (e.g., at 60 ) to the surface, for example, at about the 5 and 7 o clock positions, when bubble wakes slam into the underside of horizontal heat transfer tubes. If corrosion is also a factor, then the combined damage from erosion and corrosion can be considerably more extensive than estimated from the summation of the individual effects. 

---