However, to achieve the proper working condition of a plate heat exchanger with a rough surface, the criterion for performance evaluation as used by Webb [17] and Xiao-wei [18] is used to evaluate
the heat transfer enhancement. The ratio of Nusselt number and friction factor ((Nu/Nus)/(f/fs)1/3) of rough surface plate is compared with that obtained from a smooth surface plate as shown in Fig. 12.