484 fermentation and Biochemical Engineering Handbook Flow≡f( potential× conductance) a potential or driving force in a process heat exchanger or evaporator is a gure conduction through composite walls or slabs having different thickness and composition. The conductance, also known as the wall coefficienf, is giver by: hw=k/k,(e.g Btu/hr f2F). 6I By selecting a conducting material, such as copper or carbon steel, which has a relatively high value of thermal conductivity, and by designing a mechanically rigid but thin wall, the wall coefficient could be large. Fouling problems at surfaces xo and xy must be understood and accounted for. a stagnant oil film or a deposit of inorgani salts must be treated as a composite wall, too, and can seriously reduce the performance ofan evaporator or heat exchanger over time. This phenomenon has been accounted for in good evaporator design practice by assigning fouling factor, f, for the inside surface and the outside surface based upon experience. I7] The fouling coefficient is the inverse of the fouling factor I/fo outside fouling coefficient hi= l/f inside fouling coefficient T Distance, r Figure 3. Heat conduction through a composite wall, placed between two fluid streams T and T.(From Transport Phenomena by R. B. Bird, W. E. Stewart, and E. N Lightfoot, 1960, p. 284. Used with permission of John Wiley Sons, Inc484 Fermentation and Biochemical Engineering Handbook Flow = f (potential x conductance) A potential or driving force in a process heat exchanger or evaporator is a local temperature difference, AT. Figure 3 illustrates an example of conduction through composite walls or slabs having different thickness and composition. The conductance, also known as the wall coeficient, is given by: h, = k/x, (e.g. Btu/hr ft2 By selecting a conducting material, such as copper or carbon steel, which has a relatively high value of thermal conductivity, and by designing a mechanically rigid but thin wall, the wall coefficient could be large. Fouling problems at surfaces x, and x3 must be understood and accounted for. A stagnant oil film or a deposit of inorganic salts must be treated as a composite wall, too, and can seriously reduce the performance ofan evaporator or heat exchanger over time. This phenomenon has been accounted for in good evaporator design practice by assigning a fouling factor,J for the inside surface and the outside surface based upon experience.[’] The fouling coeficient is the inverse of the fouling factor: hf, = l/f, outside fouling coefficient hf, = 14 inside fouling coefficient 0 XO XI x2 x3 Distance, x ----ir Figure 3. Heat conduction through a composite wall, placed between two fluid streams T, and Tb. (From Transport Phenomena by R. B. Bird, W. E. Stewart, and E. N. Lightfoot, 1960, p. 284. Used with permission of John Wiley & Sons, Inc.)