In some membrane processes, such as electrodialysis and crossflow microfiltration, flat-channel systems can be used, The fluid flow takes place in channels with rectangular cross-sections, where the membranes form the walls, and where the channels are filled with a woven spacer material, These spacers act as mechanical stabilisers for the channel geometry, and they also promote turbulence, which reduces the polarisation effect near the membrane walls, However, increased turbulence results in a higher pressure drop on the retentate side, and thus also in a higher power consumption, Furthermore, in crossflow microfiltration which is a pressure-driven process the driving potential is reduced by a high pressure drop in the channel, Thus the permeate channel has to be designed to guarantee the channel geometry without leading to a high pressure drop, The work presented in the paper is aimed at finding a simple, analytical model to predict the pressure drop before installing spacers in a membrane module.