In open heart surgery under hypothermic conditions, the

Chapter , Problem 11.83

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In open heart surgery under hypothermic conditions, the patients blood is cooled before the surgery and rewarmed afterward. It is proposed that a concentric tube, counterflow heat exchanger of length 0.5 m be used for this purpose, with the thin-walled inner tube having a diameter of 55 mm. The specific heat of the blood is 3500 J/kg K. (a) If water at Th,i 60C and m . h 0.10 kg/s is used to heat blood entering the exchanger at Tc,i 18C and m . c 0.05 kg/s, what is the temperature of the blood leaving the exchanger? The overall heat transfer coefficient is 500 W/m2 K. (b) The surgeon may wish to control the heat rate q and the outlet temperature Tc,o of the blood by altering the flow rate and/or inlet temperature of the water during the rewarming process. To assist in the development of an appropriate controller for the prescribed values of m . c and Tc,i , compute and plot q and Tc,o as a function of m . h for 0.05 m . h 0.20 kg/s and values of Th,i 50, 60, and 70C. Since the dominant influence on the overall heat transfer coefficient is associated with the blood flow conditions, the value of U may be assumed to remain at 500 W/m2 K. Should certain operating conditions be excluded? 11.