2.5. Comparison of separators of different size
Before talking about different separation capability of separators an explanation is needed of how to make a fair comparison between separators of different physical size. In figure 3 are shown three separators of different size and their respective separation efficiency curve for a typical case.
If we want to know which separator has the best separation efficiency relative to its size we need to eliminate all size-related parameters. To do that we can make an analogy with a farmer who wants to know which of his fields gives the best yield relative to its size. He divides of course the total yield per field with the area of the field to get yield per square meter.
Luckily enough also separators have a size related area called areaequivalent, (Ae), which is equal to the liquid surface of a settling tank having the same capacity as the centrifugal separator.
Dividing the separator throughput with Ae gives the specific load on the disc stack making it possible to compare separators of different size with respect to performance per square meter disc area. This comparison reveals which separator has the best disc stack design, figure 4.
Alfa Laval has modified the theoretical Ae in order to adapt it to reality. This modified areaequivalent is called KQ-value which is used in fig.4.
If the disc stacks in figure 3 are of the same conceptual design all curves should fall on top of each other when using Flow Rate/KQ.
Figure 4 shows the separation efficiency plotted versus the flow rate divided by respective KQ-value. The three curves do not fall exactly on top of each other. The small differences in the curves can most probably be attributed to the actual condition of the disc stacks rather than conceptual deviations.
Fig. 4 Separation efficiency plotted versus flow rate per KQ for the same separators as in fig. 3
2.6. Tests on separators of different size and makes
In figure 5 are shown the efficiency curves obtained from three different separators. The curves are normalized by dividing with the KQ-value (Ae) in order to exclude any difference in size of the disc stack. In accordance with what we have learnt from figure 3 and 4 all three curves should be very close or lay on top of each other if the disc stacks are performing equally well. The big dots show the recommended load (flow rate/KQ) given by the manufacturer.
First of all, the graph clearly demonstrates that the separators represent three different technical levels of disc stack design. At the same liquid load the three separators have completely different efficiency levels. The upper curve is far more efficient compared with the middle curve which in turn is far more efficient than the lower curve. The differences in separation capability are not reflected in the recommended capacity of the three separators. The reason is that the separation efficiency is not an issue on the market. When the separators are presented to prospective customers nothing is normally mentioned about the separation efficiency. Recommended capacity is the sole size related parameter of the separator for a given duty implying that all separators perform equally well. The recommended specific load should reflect the level and the shape of the efficiency curve ensuring a reasonable sludge removal capability.
