Chapter 11, Problem 11.5P

A heat recovery device involves transferring energyfrom the hot flue gases passing through an annular
region to pressurized water flowing through the innertube of the annulus. The inner tube has inner and outerdiameters of 24 and 30 mm and is connected by eightstruts to an insulated outer tube of 60-mm diameter.
Each strum is 3 mm thick and is integrally fabricatedwith the inner tube from carbon steel $\left(k=50\text{ W/m}\cdot \text{K}\right)$ .

Consider conditions for which water at $\text{300 K}$ flows through the inner tube at $\text{0}\text{.161 kg/s}$ while fluegases at $\text{800 K}$ flow through the annulus, maintaining a convection coefficient of $100{\text{ W/m}}^2\cdot \text{K}$ on both thestruts and the outer surface of the inner tube. What is the rate of heat transfer per unit length of tube from gas to the water?