Water flowing at a rate of 10 kg/s through 50 tubes in a double-pass shell-and-tube heat exchanger heats air that flows on the shell side. The tubes are made of brass with outside diameters of 2.6 cm and are 6.7 m long. Surface coefficients on the inside and outside tube surfaces are 470 and 210 W/m² · K, respectively. Air enters the unit at 15°C with a flow rate of  16 kg/s. The entering water temperature is 350 K. Determine the following:
a. Heat-exchanger effectiveness
b. Heat-transfer rate to the air
c. Exiting temperatures of the water and air streams If, after a long period of operation, a scale has been built up inside the tubes resulting in an added fouling resistance of 0.0021 m² · K/W, determine the new results for parts (a), (b), and (c), above.