Question

Problem 4: Consider the heat exchanger design illustrated. Hot air flows at speed of r0.6 m/s through the center pipe. The center pipe has an outer diameter of D=7 cm and length 4-2 m Cold water flows at 20-25 cm3/s through a smaller helical pipe having an outer diameter d = 1 cm and wall thickness of mm. The helical pipe is wrapped around the center pipe to form a heat exchanger. The center pipe has a thermal conductivity of k,-15 W/m/K and the helical pipe has a thermal conductivity of kc-170 W/mK. The exposed outer surfaces of center pipe and helical pipe are insulated to prevent heat loss to the ambient. If the heat exchanger were cut along the dashed line shown, the wall of the center pipe could be unraveled to reveal the “a-a" section view shown. Find an expression for the net Section "a-a" 2t Tt 3d thermal resistance between the hot and cold fluids in the heat exchanger. To evaluate this expression, assume the water in the helical pipe is at a temperature 15 °C, and the air is at atmospheric pressure and a temperature of 200 °C. Use appropriate correlations to determine the convective coefficients for the hot and cold flows. Finally, determine the net thermal resistance between the hot and cold sides of the heat exchanger. (hint: review text discussing Fig. 4-15)

Answer 1

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