Describe in brief about the following problem
Problem - A counter-flow, double-pipe heat exchanger is used to heat 1.25 kg/s of water from 35°C to 80°C by cooling an oil (heat capacity = 2.0 kJ/kg K) from 150°C to 85°C. The overall heat transfer coefficient is 850 W/m² K. A similar installation is to be built at another location but it is desired to compare the performance of the single exchanger with two identical smaller counter-flow heat exchangers connected in series on the water side and in parallel on the oil side.
The oil flow is to be split equally between the two exchangers and it may be assumed that the overall heat transfer coefficient for the smaller exchangers is the same as that for the larger one. If the smaller exchangers cost 20 per cent more per unit surface area than the bigger ones, comment on the economics of the choice between the two systems.
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