Water at a rate of 45,500 kg/h is heated from 80 to 150°C in a heat exchanger having two shell passes and eight tube passes with a total surface area of 925 m2. Hot exhaust gases having approximately the same thermophysical properties as air enter at 350°C and exit at 175°C. Determine the overall heat transfer coefficient.

U = 129.053 KJ/m². h

Explanation:

overall heat transfer coefficient (U):

U = (mCp (ToutH2O - TinH2O)) / [A ((ToutAir - TinH2O) - (TinAir - ToutH2O)) / (Ln (ToutAir - TinH2O) / (TinAir - ToutH2O)) ]

∴ A = 925 m²

H2O:

∴ mass flow H2O (m) = 45500 Kg/h

∴ Cp H2O = 4186 J/Kg°C

∴ Tin = 80°C

∴ Tout = 150°C

Air:

∴ Tin = 350°C

∴ Tout = 175°C

⇒ U = ((45500Kg/h) (4186J/Kg°C) (150-80)) / (925m²) [ ((175-80) - (350-150)) Ln ((175-80) / (350-150)) ]

⇒ U = (1.333 E10 J/h) / (925m²) [ (-150) Ln (0.475) ]

⇒ U = 129052.73 J/m². h = 129.053 KJ/m². h