Shell-side flow patterns in shell and tube heat exchanger with helical baffles (STHXsHB) have been experimentally investigated by particle image velocimetry (Ply), in which a series of velocity vector graphs are obtained in three typical cross-sections. Compared with the original heat exchanger with plain baffles, the flushing angle a of improved heat exchanger with fold baffles reduces in cross-section A and B, which is beneficial to heat transfer enhancement according to field synergy principle. Moreover, the vortex vanishes and the circumfluence obviously decreases in the cross-section C. The axial velocity u significantly decreases and the radial velocity v increases at the external of tube bundle because the external triangle leakage flows are blocked after the configuration improvement of baffles. The local pressure drops under the same distance in the middle part of tube bundles are basically the same in both original and improved heat exchangers. It illustrates that the flow patterns in shell side have an obvious periodicity. The further heat transfer experiments show that the integrative performance of the improved heat exchanger is enhanced by 2-8% under the identical operation conditions, which can be well explained by the results of PIV experiments where the field synergy angle alpha of velocity and heat flow fields is smaller in the improved heat exchanger. Therefore, this study is of great significance to help understand and analyze the reasons for the heat transfer enhancement of STHXs with fold baffles. (C) 2016 Elsevier Ltd. All rights reserved.