Milk is Pasteurized by first heating it to 71oC, and keeping it at that temperature for 17 seconds before cooing it rapidly for bottling. Fresh milk at 5oC enters the Pasteurization unit where it flows through a thin-walled stainless steel (SS) tube of 2.0 cm diameter. The inside surface of the tube is such that the flow is turbulent to promote good mixing and uniform temperature across the flow at every point along the tube. Heating is provided by passing the tube in a closed container that is filled with saturated steam at atmospheric pressure and 100oC. Hence, we can assume that the thin-walled SS tube temperature is maintained at 100oC all along its length by condensing steam over the outside surface. The flow-rate of milk is 40kg per minute (i.e. approximately 10 gallons per minute). a) What is the mean velocity (m/s) and ??? for the milk flowing through the tube between 1-2? b) What is the convective heat transfer coefficient h (W/m2K) over the inside surface of the tube? c) What is the length of the tube that is necessary to bring the milk to the Pasteurization temperature of 71oC? d) How long would it take milk between entering and coming out of the heating section? i.e. find the residence time for milk in the heating section?
Milk is Pasteurized by first heating it to 71oC, and keeping it at that temperature for 17 seconds before cooing it rapidly for bottling. Fresh milk at 5oC enters the Pasteurization unit where it flows through a thin-walled stainless steel (SS) tube of 2.0 cm diameter. The inside surface of the tube is such that the flow is turbulent to promote good mixing and uniform temperature across the flow at every point along the tube. Heating is provided by passing the tube in a closed container that is filled with saturated steam at atmospheric pressure and 100oC. Hence, we can assume that the thin-walled SS tube temperature is maintained at 100oC all along its length by condensing steam over the outside surface. The flow-rate of milk is 40kg per minute (i.e. approximately 10 gallons per minute).
a) What is the mean velocity (m/s) and ??? for the milk flowing through the tube between 1-2?
b) What is the convective heat transfer coefficient h (W/m2K) over the inside surface of the tube?
c) What is the length of the tube that is necessary to bring the milk to the Pasteurization temperature of 71oC?
d) How long would it take milk between entering and coming out of the heating section? i.e. find the residence time for milk in the heating section?
e) Next, the tube enters a pool of water that is maintained at a constant temperature of 71oC, that is the pasteurization temperature of milk, for 17 seconds while it flows through the tube. Note that there is no heat exchange between the milk and the hot water tank since both are at the same temperature. What should be the diameter of the tube if we want to limit the length of the tube to 2.0 meters?
Use the following properties for milk:
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