The goal of the work presented here is to analyse the influence of secondary air on the solid circulating phenomenon in a closed loop circulating fluidized bed (CFB; by "closed loop", we mean a free solid circulating system as shown in one of the figures). Variables of such a system are "gas velocity", gas distribution ratio between "primary" and "secondary" levels and solid inventory. Solid circulating flow rate, solid mean concentration in the column and local gas-solid flow properties are measured for a large set of operating conditions. The analysis of results obtained lets us develop a simple model for estimating the solid circulating flow rate in industrial CFB. In this model, the suspension flow pattern is divided into three components: "dilute", "cluster" and "wall flow" phases. The velocity and the concentration of each phase, as well as the solid flow rate, are estimated by means of a simple force balance including some basic hypothesis. The results obtained show that the model fits experimental values quite well. However, it is imprecise for low values of air velocity there < 1.5 m/s).