The local transmembrane pressure (TMP) distribution plays a crucial role in the performance of hollow fiber microfiltration and ultrafiltration membrane devices. A direct-flow filter comprises an array of hollow fibers encased within a single module. The TMP in each hollow fiber is dependent on the spacing between neighboring fibers, and evolves with time due to pore clogging by contaminants (fouling). We consider an idealized set-up in which the fibers are undeformable and equally spaced within the device, and study the impact of the pore-blocking phenomena on the TMP during the filtration process. The model is used to evaluate the optimum inter-fiber spacing that maximizes the fluid processed either after a prescribed time or before the filter blocks and its dependence on the membrane permeability and the fouling rate. We show that significant improvements can be made on the operating efficiency of a direct flow device through careful choice in the fiber spacing during fabrication. (c) 2017 Elsevier Ltd. All rights reserved.