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[siretb]

There is a direct relationship between efficiency and the flowrate that goes into a given cyclone. Dietz has developped a model. Search the net for it.
As a very first approximation, the d50 (50% of the particules of this diameter are collected) is inversely proportional to the square root of the flow. Increasing the flow increases the efficiency.
A cyclone will have a limited efficiency and the d50 can be anything between 5 microns and 20 microns. In terms of design, it is usually not a good idea to design too large cyclones. For a given inlet velocity (a very important parameter for a cyclone), the d50 is proportional to the square root of the diameter. So it is better, and more expensive, to use multiple cyclones.
Remember that a cyclone has a poor turndown ratio. If specific flowrate decreases too much, efficiency becomes very poor.
Also, many "improvements" lurk around for cyclone. My own experience is that there is a very direct relationship between the pressure drop (what you pay) and the efficiency. In other words, if one plots efficiency versus pressure drop for different designs, not much difference. So, for me: the simpler the better and I like the the good old plain cyclone.
One design procedure could be
1) calculate the number of cyclones you want, using 15-20 m/s as inlet velocity
2) the first estimate of the diameter will be between sqrt(Q1/1.5) and sqrt (Q1/3)
This gives an idea, not more. Q1 is the flowrate (m3/s) into one cyclone.