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20 April, 07:07

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.

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  1. 20 April, 10:35
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    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
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