An oil preheater consists of a single tube of 10 mm diameter and 5 m length, with its surface maintained at 175^o C by swirling combustion gases. The engine oil (new) enters at 75^o C. What flow rate must be supplied to maintain an oil outlet of 100^o C? What is the corresponding heat transfer rate?

Engine oil flows at a rate of I kg/s through a 5-mm-diameter straight tube. The oil has an inlet temperature of 45°C and it is desired to heat the oil to a mean temperature of 80°C at the exit of the tube. The surface of the tube is maintained at 150°C. Determine the required length of the tube. Hint: Calculate the Reynolds numbers at the entrance and exit of the tube before proceeding with your analysis.