A correlation has been developed that allows the characterization and design of net-like feed channel spacers in any combination of geometric characteristics : angle, mesh size, thickness, strand size and voidage. Flow visualization was used to determine the flow path in a spacer-filled channel and to assess the effect of spacer characteristics on fluid mixing. Pressure losses in the spacer-filled channel have been modelled by inclusion of viscous drag on the channel walls and spacer, form drag of the spacer and kinetic losses due to directional flow change. This semi-empirical model permits evaluation of spacer performance. Grober’s mass transfer laminar flow correlation for developing concentration and velocity profiles was modified to account for spacer geometry. The equation obtained predicts mass transfer in spacer-filled channels with a deviation of less than 10% for the majority of spacers tested. The penalty for improving flux is increased pressure loss along the channel. An illustrative economic analysis which includes operating costs (which are pressure drop related) and capital costs (which are flux related) is used to identify optimal spacer designs for ultrafiltration.