A helically coiled flow tube crystallizer was investigated as a novel device for the shape-selective generation of crystals with potassium alum as the model solid. The shape evolution of the crystal population was analyzed for growth-dominated seeded cooling crystallization. The macro-mixing behavior was characterized by residence time distribution measurements for the fluid phase and the crystals. At the crystallizer outlet, a flow-through microscope was used to analyze the crystal size and shape distribution, where the 3D shape of individual crystals was estimated from 2D projections. The experiments showed a separation of the particle population according to the crystal size. This process allows the controlled shaping of crystals under continuous operating conditions.