The variation of the thermal performance enhancement factor (TEF) with the Re values for all the 45°Z-baffles is depicted in Fig. 10. The data obtained from the Nu and f measurements are compared at an equal pumping power condition. It is observed that the TEFs for the in-phase Z-baffles generally are found to be above unity and to be much higher than those for employing the out-phase Z-baffles at similar baffle and flow conditions. This indicates that the use of the in-phase Z-baffles leads to the advantage over that of the out-phase ones. The TEF tends to decrease with the increase of Re, e/H and P/H for all Z-baffles applied. It is worth noting that the TEF of the in-phase Z-baffle at P/H=1.5 is higher than that at higher P/H and the TEF of the in-phase Z-baffle at e/H=0.1 is higher than that at larger e/H. The optimum TEF is around 2.2 for the in-phase Z-baffle with e/H=0.1 and P/H=1.5 at lower Re values. The present TEF result is found to be much better than that using the ribs as reported in previous investigations [9,26,27,29].