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 cp = 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.="">