Heat transfer in fluidized bed freezers

The gas velocity in a fluidized bed freezer is fixed by the particle size and by the requirements of fluidization and cannot be varied to increase 

Despite this, the expected heat transfer coefficients obtainable in a fluidized bed are greater than those for forced convection in a gas (Ditchev and Richardson, 1999) although not as high as in the dynamic dispersion medium (DDM) method described by these authors. Comparative data are presented in Table 3.3. 

For heat transfer to a single sphere in turbulent flow, McAdams (1954) suggested 

Equation 3.7 is valid only for particle Reynolds numbers below 100 and the Reynolds number of, say, a pea in a fluidized bed freezer is of the order of 1000 and thus the correlation tends to overestimate the heat transfer coefficient considerably. Kelly (1965) proposed, specifically for the fluidization of peas. 

There are considerable problems in measuring heat transfer coefficients where only sensible heat is transferred because of the difficulties in measuring particle surface temperatures in a fluidized bed (Vazquez 

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The use of the Plank/Nagaoka model can be illustrated with a simple example. Consider a fluidized bed 0.75 m wide and 5 m long which is used to freeze peas 8 mm in diameter at a rate of 6000kgh k Assume that the peas enter the bed at 12°C, have a freezing temperature of -2°C and that the fluidizing air enters the bed at -35°C and at a velocity such that the heat transfer coefficient (see Heat transfer in fluidized bed freezers, below) is 170Wm K k What is the necessary bed depth ... 

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