A fresh medium with fixed flow rate of 2.5 m h is going through a continuous sterilization system by an indirect steam heat source in a heat exchanger. The medium contains a bacterial contaminant concentration of six millions cells per milliliter of medium; the activation energy and Arrhenius constant for heat destruction are 300 kJ mol and 1.5x1038 h, respectively. The risk of survival of contamination is one organism out of 100 days' operation composition. The sterilization pipe diameter is