Consider a shell and tube heat exchanger in a milk pasteurizing unit in which the milk flowing is to be heated from 20°C by hot water initially at 140°C and flowing at a rate of 5 kg/s. The milk flows through 30 thin-walled tubes with an inside diameter of 20 mm with each tube making 10 passes through the shell. The average convective heat transfer coefficients on the milk and water side are 450 W/m² · K and 1100 W/m² · K, respectively. In order to complete the pasteurizing process and hence restrict the microbial growth in the milk, it is required to have the exit temperature of milk attain at least 70°C. As a design engineer, your job is to decide upon the shell width (tube length in each pass) so that the milk exit temperature of 70°C can be achieved. One of the design requirements is that the exit temperature of hot water should be at least 10°C higher than the exit temperature of milk.

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Water 140°C, 5kg/s 20°C Milk