The method to calculate the pressure drop of acetate filter rod which contains charcoal (charcoal filter) has been estimated by extending the method for acetate filter rod without charcoal (acetate filter). Chemical Engineering theory of air flow is applied to the estimation of pressure drop. The basic idea of the method is same as the case of acetate filter ; total pressure drop (PD) of charcoal filters are calculated as the sum of friction drag (DP_{V) on surface of paper, acetate fibers and charcoal, and form drag (DPF) of acetate fibers and charcoal (1).
PD = DPV + DPF (1)}
By using this method, the pressure drops can be calculated in good agreement with measured pressure drops of actual charcoal filters in a wide range of pressure drops (R = 0.99 at 150~750 mmWG), within the effect of various factors (filament denier, total denier, size of plug, etc.) to the pressure drop. The method is very useful to design charcoal filters and to select the adapted acetate tow for a charcoal filter with desired pressure drop.