The design of an integral condensate subcooling section is more difficult, especially if close temperature approach is required. It is generally conservative to design a pure-component desuperheater-condenser as if the entire heat load were transferred by condensation, using the saturation temperature in the LMTD. If Tsurface > Tsaturation, no condensation occurs at that point and the heat flux is actually higher than if Tsulfloe < Tsaturation and condensation did occur. H where h is the sensible heat-transfer coefficient for the vapor, U is calculated by using h, and both are on the same area basis. Tsurface Tvapor 7 (Tvapor Tcoolant) (11-26) To determine whether or not condensation will occur directly from the superheated vapor, calculate the surface temperature by assuming single-phase heat transfer. If the surface is below the saturation temperature, condensation will occur directly from the superheated vapor, and the effective coefficient is determined from the appropriate condensation correlation, using the saturation temperature in the LMTD. This occurs if the surface temperature is above the saturation temperature, and a single-phase heat-transfer correlation is used. If the vapor is superheated at the inlet, the vapor may first be desuperheated by sensible heat transfer from the vapor.
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The overall coefficient is then used with the LMTD (no FT correction is necessary for isothermal condensation) to give the required area, even though the condensing coefficient and hence U are not constant throughout the condenser.
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A mean condensing coefficient is calculated from the appropriate correlation and combined with the other resistances in Eq. Mean Temperature Difference In condensing a single component at its saturation temperature, the entire resistance to heat transfer on the condensing side is generally assumed to be in the layer of condensate.