Chapter 11, Problem 11.26P

Hot air for a large-scale drying operation is to be produced by routing the air over a tube bank (unmixed), while products of combustion are routed through the tubes. The surface area of the cross-flow heatexchanger is $A=25{\text{m}}^2$ , and for the proposed operating conditions, the manufacturer specifies an overallheat transfer coefficient of $U=35{\text{ W/m}}^2\cdot \text{K}$ . The airand the combustion gases may each be assumed tohave a specific heat of $c_p=1040\text{ J/kg}\cdot \text{K}$ . Considerconditions for which combustion gases flowing at 1 kg/s enter the heat exchanger at 800 K, while airat 5 kg/s has an inlet temperature of 300 K.
(a) What are the air and gas outlet temperatures?
(b) After extended operation, deposits on the innertube surfaces are expected to provide a foulingresistance of $R_f^n=0.004{\text{ m}}^2\cdot \text{KW}$ . Should operation be suspended in order to clean the tubes?
(c) The heat exchanger performance may be improved by increasing the surface area and/or the overall heat transfer coefficient. Explore the effect of such changes on the air outlet temperature for $500\le UA\le 2500\text{ W/K}$ .