A shell and tube heat exchanger must be designed to heat 2.5 kg/s water from 15 to 85 degrees celsius. The heating is to be accomplished by passing hot engine oil, which is available at 160 degrees celsius, through the shell side of the exchanger. The oil is known to provide an average convection coefficient of h0 = 400 W/mK 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 degrees celsius, what is its flow rate? How long must the tubes be to accomplish the desired heating?