A counter-flow double-pipe heat exchanger with A_s = 9.0 m² is used for cooling a liquid stream (c_p = 3.15 kJ/kg·K) at a rate of 10.0 kg/s with an inlet temperature of 90°C. The coolant (c_p = 4.2 kJ/kg·K) enters the heat exchanger at a rate of 8.0 kg/s with an inlet temperature of 10°C. The plant data gave the following equation for the overall heat transfer coefficient in W/m²·K: U = 600/(1/m_c^0.8 + 2/m_h^0.8), where m_c and m h are the cold- and hot-stream flow rates in kg/s, respectively.

(a) Calculate the rate of heat transfer and the outlet stream temperatures for this unit.

(b) The existing unit is to be replaced. A vendor is offering a very attractive discount on two identical heat exchangers that are presently stocked in its warehouse, each with A_s = 5 m². Because the tube diameters in the existing and new units are the same, the above heat transfer coefficient equation is expected to be valid for the new units as well. The vendor is proposing that the two new units could be operated in parallel, such that each unit would process exactly one-half the flow rate of each of the hot and cold streams in a counterflow manner; hence, they together would meet (or exceed) the present plant heat duty. Give your recommendation, with supporting calculations, on this replacement proposal.