The main objective of this work is to study the LC rening of mechanical pulps from system-design point of view. Mathematical models that describe bre length distribution changes were used to develop aframework capable of simulating LC rening { pressure screening systems. Specically, pressure screening models and comminution models for LC rening that are available in literature together with heuristic and a developed correlation to predict rening power were implemented in MATLAB Simulink toolbox.Later on, this same environment was used to simulate three basic rening systems found in TMP mills. In this study, each considered system was composed of a single rener and single pressure screen arranged in dierent congurations. Performance curves of length-weighed mean bre length were generated from simulation results andused to assess each system behaviour in terms of rener gap, reject ratio and rener power. The impact of some design aspects such as rener size, recirculation and reject ratio were also described. Furthermore,comparisons between systems and some basic recommendations and suggested applications are made. This theoretical framework was compared to industrial-scale data from a TMP mill sub-system. Simulation results showed good agreement with real data, where relative errors between 6-18% on the predicted variables were observed.