Problem 2 (4 points) Use NTU Method

A shell-and-tube heat exchanger must be designed to heat 2.5 kg/s of water from 15 to 85°C. The heating is to be accomplished by passing hot engine oil, which is available at 160°C, through the shell side of the exchanger. The oil is known to provide an average convection coefficient of ho = 400 W/m² K on the outside of the tubes. Ten tubes pass the water through the shell. Each tube is thin walled, of diameter D = 25 mm, and makes eight passes through the shell. If the oil leaves the exchanger at 100°C, what is its flow rate? How long must the tubes be to accomplish the desired heating?

** Given properties for:

From Table A.5, unused engine oil: (Th= 130°C): cp = 2350 J/kg. K.

Form Table A.6, water (T. = 50°C): cp=4181 J/kg. K, μ = 548 x 106 N. s/m², k = 0.643 W/m. K,Pr = 3.56.