Combined parallel and serial two shell-pass shell-and-tube heat exchangers (CPTSP-STHXs & CSTSP-STHXs) whose outer shell pass are set up continuous helical baffles have been proposed to enhance the heat transfer performance. The CPTSP-STHX and CSTSP-STHX are compared with the segmental baffled shell-and-tube heat exchanger (SG-STHX) by computer simulation. The results of simulation present that, total heat transfer rate Q of the CPTSP-STHX and CSTSP-STHX-1 raise nearly 5.1% and 9.5% respectively, and the heat transfer coefficient h of the CPTSP-STHX and CSTSP-STHX-1 enhance nearly 7.6% and 14.8% than that of SG-STHX, while all of them have the same mass flow rate M, the same heat transfer area A, and the same pressure drop Delta p. Also under the same mass flow rate M and the same heat transfer area A, the Q and the Delta p of the CSTSP-STHX-2 increase nearly by 46.3% and 130.9% than those of the SG-STHX, respectively. As to the comprehensive performance, under the same heat transfer area A and overall pressure drop Delta p, the heat transfer coefficient h of the CPTSP-STHX, CSTSP-STHX-1 and CSTSP-STHX-2 raise nearly 7.6%, 14.8% and 31.2% respectively compared with the SG-STHX. Thereby the CPTSP-STHX and CSTSP-STHX have better comprehensive performance than that of SG-STHX, and furthermore the CSTSP-STHX is the best one among three. (C) 2014 Elsevier Ltd. All rights reserved.