Corriente en paralelo, Apuntes de Biotecnología

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336 9. HEAT TRANSFER Annulus Huid in de <“ Tube A — Muid < out M Annulus fluid out FIGURE 9.3 Double-pipe heat exchanger. From A.S. Foust, L.A. Wenzel, C.W. Clump, L. Maus, and L.B. Andersen, 1980, Principles of Unit Operations, 2nd es John Wiley, New York. Double-Pipe Heat Exchanger A double-pipe heat exchanger consists of two metal pipes, one inside the other as show in Figure 9,3, One fluid flows through the inner tube while the other fluid flows in € annular space between the pipe walls. When one of the fluids is hotter than the other, he=s flows from it through the wall of the inner tube into the other fluid, As a result, the ho* fluid becomes cooler and the cold fluid becomes warmer. Double-pipe heat exchangers can be operated with countercurrent or cocurrent flow o fluid. 1f, as indicated in Figure 9.3, the two fluids enter at opposite ends of the device and pass in opposite directions through the pipes, the flow is countercurrent. Cold fluid enter- ing the equipment meets hot fluid just as it is leaving, that is, cold fluid at its lowest ten- Pperature is placed in thermal contact with hot fluid also at its lowest temperature. The changes in temperature of the two fluids as they flow countercurrently through the pipes are shown in Figure 9.4, The four terminal temperatures are as follows: Th; is the inlet term- perature of the hot fluid, Th, is the outlet temperature of the hot fluid, T.; is the inlet te perature of the cold fluid, and T., is the outlet temperature of the cold fluid leaving the system. A sign of efficient operation is T./ close to Th;, or Tho close to Tej. The alternative to countercurrent flow is cocurrent or parallel flow. In this mode of opera tion, both fluids enter their respective tubes at the same end of the exchanger and flow ix 3. PIIYSICAL PROCESSES