A mathematical model to predict an equilibrium agglomerate size in agglomerating fluidized beds of Geldart group C powders was derived based on the balance of bed expansion force caused by bubbles and agglomerate-to-agglomerate cohesive rupture force. The bed compaction force caused by bubbles of ordinary sizes was found not sufficient to break agglomerates. Among the two solutions the one corresponding to a stable equilibrium was calculated by a simple iteration. The model was successfully validated by experimental data in the literature and additional new data in the present work. Previous models are also critically reviewed and evaluated.