The rotor shaft of an ice cream freezer consists of 3 scraper blades. The temperature difference between the ice cream mix and the refrigerant during freezing of ice cream is 30°C. Density and latent heat of fusion of ice are 917 kg m^-3 and 335 kJ kg^-1, respectively. The overall heat transfer coefficient is 2000 kJ m^-2 h^-1 °C^-1. If the maximum thickness of ice formed before being scraped off is 10 μm, the minimum speed of the scraper shaft in rpm is