Evaporation 485 Note that the bulk fluid temperatures( designated Ta and Tb in Fig 3) are different than the wall or skin temperatures(To and T3). Minute layers of stagnant fluid adhere to the barrier surfaces and contribute to relatively important resistances which are incorporated into a film coefficient h, outside film coefficient h,= inside film coefficient The magnitude of these coefficients is determined by physical proper- ties of the fluid and by fluid dynamics, the degree of turbulence known as the Reynolds number or its equivalent. Heat transfer within a fluid, due to its motion,occurs by convection; fluid at the bulk temperature comes in contact with fluid adjacent to the wall. Thus, turbulence and mixing are important factors to be considered, even when a change in phase occurs as in condensing steam or a boiling liquid The development of heat transfer equations for the tubular surface in Fig 4 is similar to that for the composite walls of Fig 3 except for geometr It is quite important to differentiate between the inner surface area of the tubing and the outer surface area, which could be considerably greater particularly in the case of a well-insulated pipe or a thick-walled heat xchanger tubing. Unless otherwise specified, the area A, used in determin- ing evaporator sizes or heat transfer coefficients, is the surface through which the heat flows, measured on the process or inside surface of the heat The derivation of specific values for the inside and outside film coefficients, h, and ho, is a rather involved procedure requiring a great deal of applied experience and the use of complex mathematical equations and correlations; these computations are best left to the staff heat transfer specialist, equipment vendor, or a consultant. Listed are four references that deal specifically with evaporation and the exposition and use of semi quations If steady-state conditions exist(flow rates, temperatures, composition fluid properties, pressures), Fourier's equation applies to macro-systems in which energy is transferred across a heat exchanger or an evaporator surface Q=UA△T The term U is known as the overall heat transfer coefficient and is defined by the following equationEvaporation 485 Note that the bulk fluid temperatures (designated To and T, in Fig. 3) are different than the wall or skin temperatures (To and T3). Minute layers of stagnant fluid adhere to the barrier surfaces and contribute to relatively important resistances which are incorporated into afilm coeflcient. h, = outside film coefficient hi = inside film coefficient The magnitude of these coefficients is determined by physical proper￾ties of the fluid and by fluid dynamics, the degree of turbulence known as the Reynolds number or its equivalent. Heat transfer within a fluid, due to its motion, occurs by convection; fluid at the bulk temperature comes in contact with fluid adjacent to the wall. Thus, turbulence and mixing are important factors to be considered, even when a change in phase occurs as in condensing steam or a boiling liquid. The development of heat transfer equations for the tubular surface in Fig. 4 is similar to that for the composite walls of Fig. 3 except for geometry. It is quite important to differentiate between the inner surface area of the tubing and the outer surface area, which could be considerably greater, particularly in the case of a well-insulated pipe or a thick-walled heat exchanger tubing. Unless otherwise specified, the area A, used in determin￾ing evaporator sizes or heat transfer coeficients, is the surface through which the heat flows, measured on the process or inside surface of the heat exchanger tubing. The derivation of specific values for the inside and outside film coefficients, hi and h,, is a rather involved procedure requiring a great deal of applied experience and the use of complex mathematical equations and correlations; these computations are best left to the staff heat transfer specialist, equipment vendor, or a consultant. Listed are four references that deal specifically with evaporation and the exposition and use of semi￾empirical equations for heat transfer coefficients.[*]-["] If steady-state conditions exist (flow rates, temperatures, composition, fluid properties, pressures), Fourier's equation applies to macro-systems in which energy is transferred across a heat exchanger or an evaporator surface: Q = UAAT The term U is known as the overall heat transfer coefficient and is defined by the following equation: