A shell-and-tube heat exchanger consisting of one shell pass and two tube passes is used to transfer heat from an ethylene glycol-water solution (shell side) supplied from a rooftop solar collector to pure water (tube side) used for household purposes. The tubes are inner and outer diameters Di=3.66 mm and Do=3.8 mm, respectively. Each of the 100 tubes is 0.8 m long (0.4 m per pass), and the heat transfer coefficient associated with the ethlyene glycol-water mixture is ho=11,000 W/m2 * K.
a) For pure copper tubes, calculate the heat transfer rate from the ethylene glycol-water solution (m=2.5 kg/s, Th,i=80oC) to the pure water (m=2.5 kg/s, Tc,i=20oC). Determine the outlet temperatures of both streams of fluid. The density and specific heat of the ethylene glycol-water mixture are 1040 kg/m3 and 3660 J/kg * K, respectively.
b) It is proposed to replace the copper tube bundle with a bundle composed of high-temperature nylon tubes of the same diameter and tube wall thickness. The nylon is characterized by a thermal conductivity of kn=0.31 W/m * K. Determine the tube length required to transfer the same amount of energy as in part (a).