11.24 A counterflow, concentric tube heat exchanger used for engine cooling has been in service for an extended period of time. The heat transfer surface area of the exchanger is 5 m2, and the design value of the overall convection coefficient is 38 W/m2 · K. During a test run, engine oil flowing at 0.1 kg/s is cooled from
110°C to 66°C by water supplied at a temperature of 25°C and a flow rate of 0.2 kg/s. Determine whether fouling has occurred during the service period. If so, calculate the fouling factor, R"f (m2 · K/W).