Pressure drop and cut size measurements are reported for a full scale cyclone operating within a 58 MWth CFB-combustor unit at 775℃.The paper reviews the vast number of equations to calculate the pressure drop and separation efficiency of cyclones,generally for operation at ambient temperature and at low Cs <0.5 .None of the literature correlations predicts the pressure drop with a fair accuracy within the range of experimental operating conditions.The cut size d50 can be estimated using direct empirical methods or using the Stokes number,Stk50.Both methods were used to compare measured and predicted values of d50.With the exception of Muschelknautz and Krambrock,none of the equations made accurate predictions.Finally,an alternative method to determine the friction factor of the pressure drop equation(Euler number,Eu) and of the cut size is proposed.The Eu number is determined from the geometry of common cyclones,and the derived value of Stk50 defines more accurate cut sizes.The remaining discrepancy of less than 5%,when compared with the measured values,is tentatively explained in terms of a reduced cyclone diameter due to the solids layer formed near its wall.Further measurements,mostly using positron emission particle tracking,elucidate the particle motion in the cyclone and both tracking results and the influence of the particle movement on Eu and Stk50 will be discussed in a follow-up paper. Pressure drop and cut size measurements are reported for a full scale cyclone operating within a 58 MWth CFB-combustor unit at 775 ° C. The paper reviews the vast number of equations to calculate the pressure drop and separation efficiency of cyclones, generally for operation at ambient temperature and at low Cs <0.5. Neutral of the literature correlations predicts the pressure drop with a fair accuracy within the range of experimental operating conditions. The cut size d50 can be estimated using direct empirical methods or using the Stokes number, Stk50.Both methods were used to compare measured and predicted values ​​of d50. With the exception of Muschelknautz and Krambrock, none of the equations made accurate predictions. Finally, an alternative method to determine the friction factor of the pressure drop equation (Euler number, Eu) and of the cut size is proposed. The Eu number is determined from the geometry of common cyclones, and the derived value of Stk50 defines more accurate cut sizes. The remade ining discrepancy of less than 5%, when compared with the measured values, is tentatively explained in terms of terms reduced a cyclone diameter due to the solids layer formed near its wall. Thermal measurements, mostly using positron emission particle tracking, elucidate the particle motion in the cyclone and both tracking results and the influence of the particle movement on Eu and Stk50 will be discussed in a follow-up paper.