Water with a flow rate of 5200 Kg/min is flowing inside the tubes of a cross flow heat exchanger. Water makes one pass through the tubes and its inlet temperature is 20°C. Combustion gas at 450°C flowing at a rate of 2500 Kg/min outside the tubes. Tube inside and outside diameters are Di = 20 mm and Do =25 mm and its length is 2m. Tube bank has 2500 tubes. Use the following properties:
Water properties at an assumed average mean temperature: ρ = 987 Kg/m3, Cp = 4182 J/(KgK), μ = 5.28 x 10-4 Ns/m2, k= 0.645 W/(mK), Pr = 3.42.
Combustion Gas properties at an assumed average film temperature: ρ = 0.696 Kg/m3, Cp =1030 J/(KgK), v = 3.88x10-5 m2/s, k = 0.041 W/(mK), Pr = 0.7.
a. If tubes are made of steel with k=70 W/(mK), hi = 850 W/(m2K)-and an outlet temperature of 65°C is desired for water, using NTU-Effectiveness method calculate the overall heat transfer coefficient U. and then calculate the outside heat transfer coefficient ho.
b. What is the average heat flux based on inside area of the tubes? What is the average temperature gradient in the water on the inside surface of a tube?