One method of pasteurizing milk is to heat it above 71°C for a minimum of 15 sec

Question

One method of pasteurizing milk is to heat it above 71°C for a minimum of 15 seconds. In a particular process being designed, this will be accomplished by heating it from 30°C to 73°C in a heat exchanger. Furthermore, the heat will be supplied by an oil which must be at least 20°C hotter, but not more than 30°C hotter, than the milk at all points in the exchanger (if the oil is more than 30°C hotter than the milk anywhere, there is a risk of scorching the Milk)/ The specifications for the process are: Oil: flow rate 99 kg/min Heat capacity 2.60 kJ/(kg°C) Inlet temperature 93°C Outlet temperature ? Milk: Flow rate 50 kg/min Heat capacity 4.17 kJ/(kg°C) Inlet temperature 30°C Outlet temperature 73°C (a) The above application calls for a counter-current shell and tube heat exchanger. Why is this preferable to a co-current exchanger in this case? (b) For the stainless-steel exchangers used in this industry, the overall heat transfer coefficient is typically 260 W/((m^2) °C). For such an exchanger as specified in part A, what heat exchanger surface area is required for this process?

Explanation / Answer

a)This preferable to a co-current excahnger since it there is any possibility of temperatrue increase, the temperature can be easity decreased and this will prevent the milk from scorching(by use of water)

b)Output power = 50*4.17*(93-30)/60

=13135.5/60 kW

=218.925 kW

so surface area = 218.925/0.2

=1094.625 m^2

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