Film coefficient external

Heat transfer by nucleate boiling is an important mechanism in the vaporization of liqmds. It occurs in the vaporization of liquids in kettle-type and natural-circulation reboilers commonly usea in the process industries. High rates of heat transfer per unit of area (heat flux) are obtained as a result of bubble formation at the liquid-solid interface rather than from mechanical devices external to the heat exchanger. There are available several expressions from which reasonable values of the film coefficients may be obtained. 

Rohsenow and Hartnetd recommend the Briggs and Young convection film coefficient relation for externally finned tubes. 

In a countercurrent-flow heat exchanger, 1.25 kg/s of benzene (specific heat 1.9 kJ/kg K and specific gravity 0.88) is to be cooled front 350 to 300 K with water at 290 K. In the heat exchanger, tubes of 25 mm external and 22 mm internal diameter are employed and the water passes through the tubes. If the film coefficients for the water and benzene are 0.85 and 1.70 kW/nr K respectively and the scale resistance can be neglected, what total length of tube will be required if the minimum quantity of water is to be used and its temperature is not to be allowed to rise above 320 K ... 

When a fluid is heated, the hot less-dense fluid rises and is replaced by cold material, thus setting up a natural convection current. When the fluid is agitated by some external means, then forced convection takes place. It is normally considered that there is a stationary film of fluid adjacent to the wall and that heat transfer takes place through this film by conduction. Because the thermal conductivity of most liquids is low, the main resistance to the flow of heat is in the film. Conduction through this film is given by the usual relation (74), but the value of h is not simply a property of the fluid but depends on many factors such as the geometry of the system and the flow dynamics for example, with tubes there are significant differences between the inside and outside film coefficients. 

The rates of heat transfer between the fermentation broth and the heat-transfer fluid (such as steam or cooling water flowing through the external jacket or the coil) can be estimated from the data provided in Chapter 5. For example, the film coefficient of heat transfer to or from the broth contained in a jacketed or coiled stirred-tank fermentor can be estimated using Equation 5.13. In the case of non-Newtonian liquids, the apparent viscosity, as defined by Equation 2.6, should be used. 

Nomenclatore A = area A, = area of tank bottom = area of coil Ag = equivalent area Ag = area of sides At = area of top Ai = equivalent area receiving heat from external coils A2 = equivalent area not covered with external coils Df = diameter of tank F = design (safety) factor h = film coefficient ha = coefficient of ambient air he = coefficient of coil hh = coefficient of heating medium hi = coefficient of liquid phase of tank contents or tube-side coefficient referred to... 

The overall coefficient Kc depends on the external film coeffient, kc exl, and on an effective internal coefficient, kc int. The external film coefficient can be estimated from the correlations for packed beds given in Section 2.6.5. The internal coefficient may be estimated by (Vermeulen et al., 1973)... 

To adopt a common procedure here, the external film coefficient is expressed in terms of the Nusselt number. The internal coefficients, however, are given indirectly by the transfer efficiency, E , representing the fractional approach to the maximum possible heat transfer. Thus, by definition. 

Overall heat transfer coefficient Solid film mass transfer coefficient External fluid film mass transfer coefficient Dimensionless Henry constant... 

Mass transfer through the external fluid film, and macropore, micropore and surface diffusion may all need to be accounted for within the particles in order to represent the mechanisms by which components arrive at and leave adsorption sites. In many cases identification of the rate controlling mechanism(s) allows for simplification of the model. To complicate matters, however, the external film coefficient and the intraparticle diffusivities may each depend on composition, temperature and pressure. In addition the external film coefficient is dependent on the local fluid velocity which may change with position and time in the adsorption bed. 

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