The screw-thread flow promoters presented in this work are designed to enhance filtration in a tubular geometry using a standard half-inch diameter. Two separate experimental studies were undertaken to evaluate the ultrafiltration of BSA and the microfiltration of bovine blood. The convective mixing in each system was augmented through a combination of two vortex patterns : helical how around a semi-circular cross section and flow through a sudden expansion. The screw-threads are simple to construct and operate well under laminar, quasi-steady flow conditions such that scale-up and the processing of shear sensitive fluids should be possible. The internal screw-thread generated 75% of the clean water flux at a high BSA concentration of 60 g/l. The superposition of an oscillatory flow component on the mean flow did not significantly improve the performance. The internal screw-thread design performed poorly when applied to the separation of plasma from whole blood because the centrifugal forces appeared to complement concentration polarization, An external screw-thread design was found to be an effective anti-fouling technique and tripled the microfiltration performance relative to an internal screw-thread. The lower pitch of 3.5 mm gave a plasma flux of order 0.1 cm/min, moderate flux decline and no signs of haemolysis.