Hot air for a large-scale drying operation is to be produced by routing the air over a tube bank (in orthogonal direction, unmixed), while products of combustion are routed through the tubes. The surface area of the heat exchanger is A = 25 m^2, and for the proposed operating conditions, the manufacturer specifies an overall exchanger of U = 35 W/m^2 K. The air and the combustion gasses may each be assumed to have a specific heat of 1040 J/kgK. Consider conditions for which combustion gasses flowing at 1 kg/s enter the heat exchanger at 800 K, while air at 5 kg/s has an inlet temperature 300 K. Required:
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 0.004 m^2K/W. Should operation be suspended in order to clean the tubes. Assume that cleaning must be performed if the exchanger performance reduces by more than 10%?