Cleaning of heat transfer surfaces fouled by dairy milk was investigated in order to determine the change of thermal resistance in a shell and tube heat exchanger during fouling and cleaning. Fouling resistance was calculated by the Logarithmic Mean Temperature Differences (LMTD) method. Foulant (whole milk) and cleaning fluids (0,4 g/l NaOH and 0,4 g/l HNO3 applied by two-stage alkali-acid "CIP", Cleaning In Place, process) passed through 3 different shell and tube heat exchangers, which were designed for 30degrees, 60degrees, and 90degrees, arrangement. In this paper only the effects of fluid velocity are presented. Both, the fluid velocity of foulant and cleaning fluid were 500, 750,1000,1250 and 1500 ml/min. Reynolds number varied from 128 to 428 for 500 ml/min and 1500 ml/min flow velocity, respectively. Optimum fluid velocity and design arrangements were determined.