Dead flux curve

A typical grade efficiency curve for the product classification step is given in Figure 4. A value of nearly 100 percent is attained at large sizes, whereas normally a value equal to or larger than the so-called dead flux is attained at small sizes. This is caused by the diluted discharge of the coarse fraction. It represents the minimum amount of residual fines in the product after one separation stage. 

Grade efficiency curve (a) without taking into account dead flux effect, and (b) taking into account dead flux effect. 

Note that the S-shaped grade efficiency curves do not necessarily start from the origin—in applications with a considerable underflow to throughput ratio (by volume) R, the grade efficiency curves tend to the value G x) = / f as X —> 0. This is a result of the splitting of the flow, or dead flux that carries even the finest solids into the underflow in proportion to the volumetric split of the feed. Section 3.4 discusses possible modifications to the efficiency definitions which account for the volumetric split and illustrate only the net separation effect. Such reduced efficiencies are widely used for hydrocyclones and nozzle-type disc centrifuges where large diluted underflows occur. 

The effect of flow splitting (or dead flux ) in applications with appreciable and dilute underflow, as is common with some separators, is to modify the shape of the grade efficiency curve to make it appear as if the performance of the separator were better than it actually is. As shown in Figure 3.15, the curve does not start from the origin (as it should for inertial separation) but has an intercept, the value of which is usually equal to the underflow-to-throughput ratio Rf. This is because the very fine particles simply follow the flow and are split between the underflow and the overflow in the same ratio as the fluid. The R ratio is defined as the fraction of the volumetric feed rate which turns up in the underflow, i.e. the underflow rate, divided by the feed rate. 

First experiments were conducted with day suspensions at 1 g in dead-end mode. The TMP was constant and equal to 80kPa. Figure 11.16 gives an example of results obtained with the acoustic method applied on the filtration module. The measured thickness and the flux are plotted versus time. The flux curve is a typical result encountered in dead-end filtration a rapid decrease at the beginning of the experiment and then more slowly until a stabilized value (which was not reached at the end of this experiment). The final thickness is 180 pm for a total deposited mass of 120 gm and a relative flux decrease of 66%. Reproducibility of the measurement was tested and gave good results. 

The collimator impact for 81 keV is shown in Fig. 11. The counts are normalized to the counts at 90°. The 81 keV lines are almost completely absorbed by either the steel or tungsten. The upturn in the uncollimated curve at 0° and 180° is due to the difference in absorption by the endcap, cup and germanium dead layer for the 0° angle of incidence and the 10° angle of incidence. The combined circular (front face) and cylindrical surface area of the detector perpendicular to the gamma ray flux has a maximum at about 45°, but this is reduced by the change in... 

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