Heat transfer, freeboard

Freeboard Heat Transfer. In some applications, e.g., fluidized combustors, heat exchanger tubes are located in the freeboard space above the bed, as well as within the bed. The coefficients / , h, and h, defined by Eqs. (15, 16, and 42), are also appropriate to represent the heat transfer process at the surface of such freeboard tubes. If the tube is placed so that it sees mostly bed material (not the vessel walls), Eqs. (44) and (45) may be used to estimate the radiative... 

Biyikli S, Tuzla K, Chen JC. Freeboard heat transfer in high-temperature fluidized beds. Powder Tech 53 187-194, 1987. 

The heat transfer from tubes in the freeboard was also measured for the 20 MW model. Figure 45 shows a comparison of the measured overall heat transfer coefficient in the 20 MW pilot plant versus that predicted from the scale model test. When the bed height is lowered, uncovering some tubes, the heat transfer is reduced because there are fewer particles contacting the tube surface. Although the scale model did not include proper scaling for convective heat transfer, the rate of change of the overall heat transfer should be a function of the hydrodynamics. 

A fluidized-bed reactor consists of three main sections (Figure 23.1) (1) the fluidizing gas entry or distributor section at the bottom, essentially a perforated metal plate that allows entry of the gas through a number of holes (2) the fluidized-bed itself, which, unless the operation is adiabatic, includes heat transfer surface to control T (3) the freeboard section above the bed, essentially empty space to allow disengagement of entrained solid particles from the rising exit gas stream this section may be provided internally (at the top) or externally with cyclones to aid in the gas-solid separation. A reactor model, as discussed here, is concerned primarily with the bed itself, in order to determine, for example, the required holdup of solid particles for a specified rate of production. The solid may be a catalyst or a reactant, but we assume the former for the purpose of the development. 

Operational troubles occur due to particle adhesion (Decrease of heat transfer, excess reaction at walls in freeboard, and clogging in pipes and cyclones will take place.)... 

In the transition zone and the freeboard region, heat transfer between bed and wall is a function of bed density. Shirai et al. (S9, Sll) studied heat transfer from a sphere immersed in the fluidized bed and showed the trend of decreasing heat-transfer coefiicient with decreasing bed density. 

Gas Convective Component. The gas convective component is caused by the gas percolating through the particulate phase and the gas bubbles coming in contact with the heat transfer surface. For small particles, though the contribution of gas convective component is small in the in-bed region, it could be important in the freeboard region. The gas convective... 

In the freeboard region for bubbling and turbulent beds, the heat transfer coefficient falls off rapidly with increasing height as the particles, responsible for enhancing heat transfer, are disengaged and fall back onto the bed surface. If reactor temperatures exceed approximately 600°C, radiation also contributes appreciably to the overall bed-to-surface heat transfer. For start-up. 

All parts of the fluidized bed unit are subject to erosion by the solid particles. Heat transfer tubes within the bed or freeboard are particularly at risk and erosion here may lead to tube failure. Erosion of the distributor may lead to poor fluidization and areas of the bed becoming deaerated. 

Heat Transfer Coefficients for Radiation in the Freeboard of a Rotary Kiln... 

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