The determination of the entrainment requires knowledge of concentrations of each particle size in the bed. Figure 10 shows a general case and the various possible combinations (Geldart, 1986). For example:

- Without feed, 100% efficient cyclone; total recycle of product on cyclone;
- Continuous feed, cyclone's efficiency varies depending on dpi; partial recycle of the fines.

No matter the arrangement, the mass balances of each component and global must be reached. As an illustration, it is assumed that R_{E}=R_{R}=0, and F and Q are different from zero. The mass balance of the fraction considering d_{pi} given by:

The global mass balance given by:

Now:

In addition, in a well-mixed bed: xQ_{i} = xB_{i}

Substituting in the mass balance for each particle size and re-arranging:

This equation cannot be directly solved because RT depends on the values from x_{Bi} for each fraction. In practice a rapid convergence and an interaction error can be avoided by using R_{T}=0 in the first attempt.

If the freeboard height is greater than 2m, and the column's diameter exceeds about 0.2 m, then for the fraction sizes will have U_{t}/U < 0,5, Isso simplifica e encurta os cálculos significativamente.