Figure (A) shows that, above 2 NTU, thermal losses to the environment significantly reduce the effectiveness. The results of forced (pure) convection with wind speeds up to 7.9 m/s differ only slightly from natural convection. Comparing Figures ) and 7(B), counter-current operation is significantly more effective above 1 NTU. It is also noted in Fig. (B) that with thermal losses to the surroundings, the effectiveness presents a maximum that changes to a lower 𝐿 as the thermal losses increase. The maximum effectiveness highlights the importance of considering thermal losses in the design of TTHEs. The maximum effectiveness for each case described in Fig. (B) can be calculated by setting 𝑛 equal to 1 in Eq. ( ) and solving iteratively for 𝜏Ĺ (updating the 𝑈′s). The results are shown in Table .