Question

Assume the water for a boiler is preheated using flue gases from the boiler stack. The flue gases are available at a rate of 0.25 kg/s at 150°C, with a specific heat of 1000 J/kg · K. The water entering the exchanger at 15°C at the rate of 0.05 kg/s is to be heated to 90°C. The heat exchanger is to be of the type with one shell pass and four tube passes. The water flows inside the tubes, which are made of copper (2.5 cm ID, 3.0 cm OD). The heat transfer coefficient on the gas side is 115 W/m² · K, while the heat transfer coefficient on the water side is 1150 W/m² · K. A scale on the water side and gas side offer an additional total thermal resistance of 0.000176 m² · K/W. a. Determine the overall heat transfer coefficient based on the outer tube diameter. b. Determine the appropriate mean temperature difference for the heat exchanger. c. Estimate the required total tube length. d. Calculate the percent over surface design and the cleanliness factor.

Answer 1

Where thermal conductivity for copper =385 W/m² K

  

  ............................... Ans

2) Logarithmic mean temperature difference

   

........................ Ans

Heat gained by water

  

4)   

Where   

...................   Ans

  

................Ans