Reconsider Prob. 11–26. Using EES (or other) software, investigate the effects of pipe conductivity and heat transfer coefficients on the thermal resistance of the heat exchanger. Let the thermal conductivity vary from 10 W/m · K to 400 W/m · K, the convection heat transfer coefficient from 500 W/m² · K to 1500 W/m² · K on the inner surface, and from 1000 W/m² · K to 2000 W/m² · K on the outer surface. Plot the thermal resistance of the heat exchanger as functions of thermal conductivity and heat transfer coefficients, and discuss the results.

Data from problem 26

A double-pipe heat exchanger is constructed of a copper (k = 380 W/m · K) inner tube of internal diameter D_i = 1.2 cm and external diameter D_o = 1.6 cm and an outer tube of diameter 3.0 cm. The convection heat transfer coefficient is reported to be h_i = 700 W/m² · K on the inner surface of the tube and h_o = 1400 W/m² · K on its outer surface. For a fouling factor R_{f, i} = 0.0005 m² · K/W on the tube side and R_{f, o} = 0.0002 m² · K/W on the shell side, determine

(a) The thermal resistance of the heat exchanger per unit length

(b) The overall heat transfer coefficients Ui and Uo based on the inner and outer surface areas of the tube, respectively.