Abstract: The effect of mass flow rate on product temperature and residence time were studied in a helically coiled, indirectly steam-heated, vertical flow, laboratory Ultra High Temperature (UHT) system equipped with automatic temperature control and recording units. With a mass flow rate of 0.54 to 3.03 kg min- 1, the Reynolds number (Re) values ranged from 1,500 to 128,000 in the various sections using water as the test product. Holding times were calculated on the basis of holding tube length and velocity. A computer program was applied to predict the temperatures of the product as a function of residence time for the preheating, heating, precooling and cooling heat exchange sections of the UHT unit.