concentric tube heat exchanger of length L = 2 m is used to thermally process a pharmaceutical product flowing at a mean velocity of u m,c = 0.1 m/s with an inlet temperature of T c,i = 20 o C. The inner tube of diameter D i = 10 mm is thin walled, and the exterior of the outer tube (D o = 20 mm) is well insulated. Water flows in the annular region between the tubes at a mean