Grid Pressure-Drop Criteria

For a grid, achieving equal distribution of gas flow through many parallel paths requires equal resistances and sufficient resistance to equal or exceed the maximum value of any unsteady-state pressure fluctuation. It has been determined experimentally that the head of solids in some fluidized beds above an upwardly-directed grid port can vary momentarily by as much as 30%. This is due to large fluctuations in the jet penetration for an upwardly-directed jet as discussed in the previous section. The equivalent variation downstream of a downwardly-directed port is less than 10%. Thus, as a rule of thumb, the criteria for good gas distribution based on the direction of gas entry are  

Several investigators (Hiby, 1964 Zuiderweg, 1967 Whitehead, 1971 Siegel, 1976 and Mori and Moriyama, 1978) have found the ratio of pressure drops to be in the range of 0.015 to 0.4. 

If turndown is desired, the grid pressure-drop criteria (Eqs. 3 and 4) must apply at the minimum gas flow rate. Also, if the grid is curved, i.e., concave, convex, or conical, the criterion must apply with respect to the 

A pressure balance across the curved grid can be written as  

(6) APh (Highest Hole) = Al (Lowest Hole) + pBg(Hhigh - Hlow) i.e., 

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If turndown is desired, the grid pressure drop criteria (Eqs. 3 and 4) should apply at the minimum gas flow rate. This can be a problem for circulating fluidized bed combustors, since this means that under full load the grid pressure drop will be unacceptably high. Also, if the grid is curved, i.e., concave, convex, or conical, the criterion must apply with respect to the lowest hole on the grid. Take an example of a fluid bed with curved grid, as shown in Fig. 3. 

Therefore the lowest grid hole has the lowest pressure drop, and hence the pressure drop criterion must apply with respect to the lowest hole on the grid. 

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