CONVECTION HEAT TRANSFERHeat TransferQkDrAlm (T2T3)Qh2A2 (T3T4)DTo can be expressed as the sum of the DT of the three individual processes.DTo(T1T2)(T2T3)(T3T4)If the basic relationship for each process is solved for its associated temperature difference andsubstituted into the expression for DTo above, the following relationship results.DToQ1h1A1Drk Alm1h2A2This relationship can be modified by selecting a reference cross-sectional area Ao.DToQAoAoh1A1Dr Aok AlmAoh2A2Solving for results in an equation in the form .QQUo Ao DToQ1Aoh1A1Dr Aok AlmAoh2A2Ao DTowhere:(2-10)Uo1Aoh1A1Dr Aok AlmAoh2A2Equation 2-10 for the overall heat transfer coefficient in cylindrical geometry is relativelydifficult to work with. The equation can be simplified without losing much accuracy if the tubethat is being analyzed is thin-walled, that is the tube wall thickness is small compared to the tubediameter. For a thin-walled tube, the inner surface area (A1), outer surface area (A2), and logmean surface area (A1m), are all very close to being equal. Assuming that A1, A2, and A1m areequal to each other and also equal to Ao allows us to cancel out all the area terms in thedenominator of Equation 2-11.HT-02 Page 22 Rev. 0
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