Submerged heat-transfer tubes

It is sometimes necessary to cool or heat dense bubbling fluidized beds, and this is usually accomplished by the insertion of heat transfer tubes carrying cooling or heating fluids into the bed. Heat transfer occurs between the fluidized particle/gas medium (often referred to as the bed ) and the submerged tube surfaces (often referred to as walls ). For this situation, one requires a heat transfer coefficient based on the surface area of the submerged wall ... 

If a heat transfer tube of a 2.9 cm diameter is submerged horizontally within the fluidized medium at the above operating conditions, what would be the surface heat transfer coefficient estimated by various empirical correlations ... 

Submerged heat-transfer area,. 4, is prc rtional to liquid level above the bottom of the lowest tube. The change in condensing area,. A is then the negative of the change in A. This assumption is not bad if there are many tubes and if they are not layered. To make this assumption valid may require that in some cases the tube bundle be sUghtly rotated about its axis. 

In a submerged-tube FC evaporator, all heat is imparted as sensible heat, resulting in a temperature rise of the circulating hquor that reduces the overall temperature difference available for heat transfer. Temperature rise, tube proportions, tube velocity, and head requirements on the circulating pump all influence the selec tion of circulation rate. Head requirements are frequently difficult to estimate since they consist not only of the usual friction, entrance and contraction, and elevation losses when the return to the flash chamber is above the liquid level but also of increased friction losses due to flashing in the return line and vortex losses in the flash chamber. Circulation is sometimes limited by vapor in the pump suction hne. This may be drawn in as a result of inadequate vapor-liquid separation or may come from vortices near the pump suction connection to the body or may be formed in the line itself by short circuiting from heater outlet to pump inlet of liquor that has not flashed completely to equilibrium at the pressure in the vapor head. 

This design is not well adapted to free-convection heat transfer outside a tube or coil therefore, for this discussion only agitation is considered using a submerged helical coil, Oldshue and Kern . 

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... 

Dou, S., Herb, B., Tuzla, K., and Chen, J. C., Heat Transfer Coefficients for Tubes Submerged in Circulating Fluidized Bed, Experimental Heat Transfer, 4 343-353 (1991)... 

---