A counter-current shell-and-tube heat exchanger is used to recover heat from post-combustion exhaust gases to heat a process water stream required as a heat source for an endothermic reaction. The exhaust gas (density 0.870 kg/m3 , specific heat capacity 1.100 kJ/kg.K) flows through the shell (one shell pass) at 10 m3 /s with an inlet temperature of 300 . The water inlet flow rate is 2 kg/s, the inlet temperature is 22 and this stream is required to reach 90 for the reactor. As a U-tube type exchanger, the tubes are arranged in 2-pass format, with 180 x 2m long tubes, each with 2.54 cm internal diameter and length.

(a) Determine the exit temperature of the exhaust gas 10 marks

(b) Determine the overall heat transfer coefficient (UA) for the exchanger 5 marks After 12 months of operation, soil in the process water has deposited on the inside wall of the tubes, at the rate of 2 m/day. Your boss wants to know what outlet water temperature can now be achieved, without making any changes to the inlet stream temperatures or flow rates.

(c) Determine the outlet water temperature under the fouled conditions, assuming that the Tlm is approximately the same 8 marks

(d) What simple action could you recommend maintaining the cooling water temperature before maintenance can be carried out?