A double-wall heat exchanger is used to transfer

Chapter 8, Problem 8.86

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A double-wall heat exchanger is used to transfer heatbetween liquids flowing through semicircular coppertubes. Each tube has a wall thickness of t?3 mm andan inner radius of ri?20 mm, and good contact ismaintained at the plane surfaces by tightly woundstraps. The tube outer surfaces are well insulated.(a) If hot and cold water at mean temperatures ofTh,m?330 K and Tc,m?290 K flow through the adjoining tubes at m?h?m?c?0.2 kg/s, what is therate of heat transfer per unit length of tube? The wallcontact resistance is 10?5m2?K/W. Approximate theproperties of both the hot and cold water as??800?10?6kg/s?m, k?0.625 W/m?K, andPr?5.35. Hint: Heat transfer is enhanced by con-duction through the semicircular portions of the tubewalls, and each portion may be subdivided into twostraight fins with adiabatic tips.(b) Using the thermal model developed for part (a),determine the heat transfer rate per unit length whenthe fluids are ethylene glycol. Also, what effect willfabricating the exchanger from an aluminum alloyhave on the heat rate? Will increasing the thicknessof the tube walls have a beneficial effect?