A multiphase flow profile inside a helically coiled tubular device (HCTD) was observed by using a high-speed camera. Gas liquid slug flow observations revealed that the Taylor vortices are influenced by secondary flow due to the centrifugal force acting perpendicular to the flow direction. Hence, mixing inside the liquid slug is enhanced by the combination of Dean and Taylor vortices in HCTD. The modular design of a specific type of HCTD, that is, the coiled flow inverter (CFI) is elucidated by the representation of a new design space diagram. Continuous precipitation of calcium carbonate (CaCO3) was investigated for modular CFI made of polyvinyl chloride (PVC) tubes (d(i) = 3.2 mm) with slug flow patterns. CaCO3 was continuously precipitated along CFI with a conversion of ca. 90%. CFI provided a narrower particle size distribution with median particle diameters around 28 mu m and more uniform morphology in comparison to a batch reactor.