5. Hot water for an industrial washing operation is produced by recovering heat from the flue gas of a furnace. A cross-flow heat exchanger is used, with the gases passing over the tubes and the water making a single pass through the tubes. The steel tubes (k=60W/mK) have inner and outer diameters of D1=20mm and D0=25mm, and length of 0.3m. The aligned tube array has longitudinal and transverse pitches of ST=SL=50mm. The number of tubes in the transverse plane is NT=25. The gas properties may be approximated as those of atmospheric air, and the convection coefficient associated with water flow in the tubes is around 3000W/m2K. If 5kg/s of water is to be heated from 300 to 360K by 45kg/s of flue gas entering the exchanger at 750K, a. What is the gas outlet temperature? b. How many tube rows NL are required by NTU method? c. In practice, the flow rate of flue gas needs to be increased over the 3 months of operation period in order to meet the required water temperature at the outlet. Give a brief explanation. (Properties, constants, and NTU relations can be found in the appendix). (20 marks)