A numerical analysis was carried out to study the thermal-hydraulic characteristics of a 3-D laminar in-lined and staggered plate-fin and tube heat exchangers with block type vortex generators (VG) mounted behind the tubes. In this study, the optimization of the VG span angle (theta) and the VG transverse location (L-y) was investigated numerically along with a simplified conjugate-gradient method (SCGM). The area reduction ratio using a vortex generator relative to the plain surface is the objective function to be maximized. A search for the optimum span angle (theta) and transverse location (L-y), ranging from 30 degrees < theta < 60 degrees and 2 mm < L-y < 20 mm, respectively, was performed. The effect of different inlet conditions, (Reynolds number Re-Dh = 400-1200), was also investigated. The results showed that the maximum area reduction ratios reached 14.9-25.5% combined with the optimal design of (theta, L-y) at Re-Dh = 400-1200 for the in-lined arrangement, and 7.9-13.6% of the maximum area reduction ratio was achieved for the staggered arrangement. (C) 2013 Elsevier Ltd. All rights reserved.