The flow patterns of fluids in convoluted channels were studied experimentally. The existence of secondary flow in tubes with helical static elements was proved. In such devices the magnitude of superimposed vortical motion is comparable to that of axial flow. The double helical flow persists downstream for certain distance, until dissipated by viscous friction. For the flow visualisation colour-band experiments and measurements using laser technique were applied. On the basis of the experiments several new statements were established on the microstructure of the flow. The laminar flow condition's were compared with the behaviour of flows in coiled tubes and also in tubes with twisted tape inserts. For easier comparison a modified Dean number was proposed, which took into account the curvature of the spherical line cut out by the helical element from the tubular housing.