Theoretically, the 2 concept allows performance comparison between similar centrifuges operating on the same feed material. Since 2 reflects a fixed maximum bowl capacity, for any flow rate (Q), the Q/2 ratio represents the hydraulic loading.
In theory, any two similar centrifuges treating the same sludge should show the same settling performance at the same value of Q/2, and differences reflect nontheoretical factors. Since these efficiency factors are nearly identical in hydrodynamically similar centrifuges, i.e., in two sizes of disc centrifuge bowls of identical dimensional ratios, the clarification performance in one bowl can be directly related by the 2 ratio to the anticipated performance of a different size bowl.
When the centrifuges are not similar, as in a conveyor bowl compared to a disc bowl, the Q/2 plots indicate relative performance, but environmental engineers cannot use the performance curve of one type for scaleup to the other type. Nevertheless, a Q/2 plot showing flow ratios against solids recovery represents a generalized correlation and is a standard method of evaluating centrifuge performance.
Figure 7.48.11 is a generalized plot for a municipal excess activated sludge in disc, conveyor, and knifing centrifuges. The low Q/2 value for the disc centrifuge results from its high shear with consequent feed degradation. The conveyor and basket centrifuges cause less floc degradation; the former has a scrolling inefficiency on a soft cake. Even considerable polymer addition cannot entirely overcome this effect.
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