CONVECTION HEAT TRANSFERHeat TransferIt is the temperature difference at one end of the heat exchanger minus the temperature differenceat the other end of the heat exchanger, divided by the natural logarithm of the ratio of these twotemperature differences. The above definition for LMTD involves two important assumptions:(1) the fluid specific heats do not vary significantly with temperature, and (2) the convection heattransfer coefficients are relatively constant throughout the heat exchanger.OverallHeatTransferCoefficientMany of the heat transfer processes encountered in nuclear facilities involve a combination ofboth conduction and convection. For example, heat transfer in a steam generator involvesconvection from the bulk of the reactor coolant to the steam generator inner tube surface,conduction through the tube wall, and convection from the outer tube surface to the secondaryside fluid.In cases of combined heat transfer for a heat exchanger, there are two values for h. There is theconvective heat transfer coefficient (h) for the fluid film inside the tubes and a convective heattransfer coefficient for the fluid film outside the tubes. The thermal conductivity (k) andthickness (Dx) of the tube wall must also be accounted for. An additional term (U_{o}), called theoverall heat transfer coefficient, must be used instead. It is common practice to relate the totalrate of heat transfer ( ) to the cross-sectional area for heat transfer (A_{o}) and the overall heatQtransfer coefficient (U_{o}). The relationship of the overall heat transfer coefficient to the individualconduction and convection terms is shown in Figure 6.Figure 6 Overall Heat Transfer CoefficientHT-02 Page 20 Rev. 0

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