This study conducted the second-law analysis on a saturated FC-22 vapour flowing through and condensed in horizontal cooling tubes. The optimum Reynolds number exists over the parametric range for the single tube case at which the entropy generates at a minimum rate. For the multi-tube cases, no optimal Reynolds number is identified under the constant surface-area constraint. Apparently, there exists an optimal cooling temperature that generates a minimum of entropy for a given duty, which depends strongly upon many process parameters like mass flow rate and tube geometry. Copyright (C) 2001 John Wiley & Sons, Ltd.