An engineer designs a counterflow, concentric tube heat exchanger to cool a hot fluid from 80C to 55C. The mean velocity of hot fluid through the inner tube (Di=25mm) is 1.3m/s. The water flows through the outer annulus (D0=40mm) with inlet and outlet temperatures being 15C and 35C, respectively. Calculate the length of the heat exchanger, Show all equations, steps. Write your assumptions and discuss the result. Notes: - The engineer assumes the inner wall temperature is uniform. - The engineer assumes flow are in fully developed region. - While calculating Reynolds and Nusselt numbers for annulus (water) flow, use the hydraulic diameter (Dh=DoDi). - Pr=kcp Hot fluid properties