Evaporation 48 The evaporator design engineer determines the heat load, @, and the driving force, AT, from the Heat Exchanger Specification Sheet. If an overall coefficient, U, can be obtained from operating or pilot plant data(or can be calculated, as in the example above), the required evaporator surfacearea, A can be obtained. In most types of evaporators, the overall heat transfer coefficient can be a strong function of the temperature difference, AT Because the driving force is not constant at every point along a heat exchanger or evaporator surface, a LMTD(Log Mean Temperature Difference)and LMTD correction factors are used in the Fourier equation to represent AT. Figure 5 shows how the LMTD can be calculated using terminal temperatures (i.e, inlet and outlet temperatures) for a heat exchanger in the simple case where no change of phase occurs T,t LMTD-n,2(1,二出-aMa Figure Logarithmic mean temperature difference in a counterflow heat exchanger with no phase changes. (Luwa Corporation) In a steam-heated evaporator, both the heating medium and the process fluid undergo a phase change and most of the energy transferred is latent heat Some sensible heat may be involved if the feed stream is to be preheated and if the condensate undergoes some subcooling. Further, some types of evaporators(for example, a submerged tube forced-circulation evaporator involve the concept of boiling point elevation, due to the hydrostatic pressure of the liquid phase. The point to be emphasized is that the representative driving force, AT, utilized in the proper design of an evaporator involves some rather complicated computations and correction factors, compared with a simple problem of the transfer of sensible heat in the tubular exchanger illustrated in Fig. 5Evaporation 487 The evaporator design engineer determines the heat load, Q, and the driving force, AT, from the Heat Exchanger Specification Sheet. Ifan overall coefficient, U, can be obtained from operating or pilot plant data (or can be calculated, as in the example above), the required evaporator surface area, A, can be obtained. In most types of evaporators, the overall heat transfer coefficient can be a strong function of the temperature difference, AT. Because the driving force is not constant at every point along a heat exchanger or evaporator surface, a LMTD (Log Mean Temperature Difference) and LMTD correction factors are used in the Fourier equation to represent AT. Figure 5 shows how the LMTD can be calculated using terminal temperatures (Le., inlet and outlet temperatures) for a heat exchanger in the simple case where no change of phase ocsurs. Figure 5. Logarithmic mean temperature difference in a counterflow heat exchanger with no phase changes. (Luwa Corporation) In a steam-heated evaporator, both the heating medium and the process fluid undergo aphase change and most ofthe energy transferred is latent heat. Some sensible heat may be involved if the feed stream is to be preheated and if the condensate undergoes some subcooling. Further, some types of evaporators (for example, a submerged tube forced-circulation evaporator) involve the concept of boiling point elevation, due to the hydrostatic pressure of the liquid phase. The point to be emphasized is that the representative driving force, AT utilized in the proper design of an evaporator involves some rather complicated computations and correction factors, compared with a simple problem of the transfer of sensible heat in the tubular exchanger illustrated in Fig. 5