Chapter 11, Problem 11.72P

A shell-and-tube heat exchanger consisting of oneshell 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 (or household purposes. The tubes areof inner and outer diameters $D_i=3.6$ mm and $D_o=3.8$ mm, respectively. Each of the 100 tubes is0.8 in long (0.4 in per pass), and the heal transfer coefficient associated with the ethylene glycol—water mixture is $h_o=11,000$
${\text{W/m}}^2\cdot \text{K}$ .
(a) For pure copper tubes, calculate the heat transferrate from the ethylene glycol—water solution $\left(\stackrel{\dot{}}{m}=2.5{\text{ kg/s, T}}_{h,i}\text{=8}0°C\right)$ to the pure water $\left(\stackrel{\dot{}}{m}=2.5{\text{ kg/s, T}}_{c,i}\text{=2}0°C\right)$ . Determine the outlet temperatures of both streams of fluid. The density andspecific heat of the ethylene glycol—water mixture are $1040{\text{ kg/m}}^3$ and $3660\text{ J/kg}$
$3660\text{ J/kg}\cdot \text{K}$ ,respectively.
(b) It ¡s proposed to replace the copper tube bundlewith a bundle composed of high-temperature
nylon tubes of the same diameter and tube wall thickness. The nylon is characterized by a thermalconductivity of $k_n=0.31\text{ W/m}\cdot \text{K}$ . Determine thetube length required to transfer the same amountof energy as in part (a).