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 heat exchanger is A = 25 m², and for the proposed operating conditions, the manufacturer specifies an overall heat transfer coefficient of V = 35 W/m² ∙ K. The air and the combustion gases may each be assumed to have a specific heat of c_p = 1040 J/kg ∙ K. Consider conditions for which combustion gases flowing at 1 kg/s enter the heat exchanger at 800 K, while air at 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 inner tube surfaces are expected to provide a fouling resistance of R'_f = 0.004 m² ∙ K/W. 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 < UA < 2500 W/K.