Convective inside film coefficient

Figure 10-51. Convection inside film coefficient for gases and low viscosity fluids inside tubes—heating and cooling. (Used by permission McAdams, W. H. Heat Transmission, 2"= Ed., 1942. McGraw-Hill, Inc. All rights reserved.)...

Reactor design and scale-up, including the choice of agitator, is aided by measuring the convective inside film coefficient (/i,) using a reaction calorimeter (see Section 4.4.5.5, page 72). 

Determine the inside film coefficient by methods previously oudined for convection. 

For a straight tube immersed in a well-stirred fluid, the inside film coefficient, h, may be calculated from the Nusselt number given by the general relationship applying to forced convection in tubes when Re > 10 and0.7[Pg.30]

Film Coefficients with Fluid Inside Tubes, Forced Convection... 

For organic liquids, evaluate the natural convection film coefficient from Figure 10-103. Equation 10-29 may be used for the inside horizontal tube by multiplying the right side of the equation by 2.25 (1 + 0.010 Gr,i/")/logRe. 

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. 

Specific correlations of individual film coefficients necessarily are restricted in scope. Among the distinctions that are made are those of geometry, whether inside or outside of tubes for instance, or the shapes of the heat transfer surfaces free or forced convection laminar or turbulent flow liquids, gases, liquid metals, non-Newtonian fluids pure substances or mixtures completely or partially condensable air, water, refrigerants, or other specific substances fluidized or fixed particles combined convection and radiation and others. In spite of such qualifications, it should be... 

By what factor would the heat loss be reduced by covering the inside wall with aluminum foil Would it be better to put the aluminum foil halfway between the two walls (Correlations for natural convection at vertical surfaces give a film coefficient of 3.9 W/m - C for each wall.)... 

In cases of combined heat transfer for a heat exchanger, there are two values for h. There is the convective heat transfer coefficient h for the fluid film inside the tubes and a convective heat transfer coefficient hg for the fluid film outside the tubes. The thermal conductivity, k, and thickness. Ax, of the tube wall must also be accounted for. An additional term Uo, called the overall heat transfer coefficient, must be used instead. It is common practice to relate the total rate of heat transfer, Q to the cross-sectional area for heat transfer and the overall heat transfer coefficient Uq. The relationship of the overall heat transfer coefficient to the individual conduction and convection terms is shown in Figure 6.5. 

Tube-side Rates. The film coefficient for fluids flowing in forced convection inside tubes has been studied thoroughly by Sieder and Tate, and they found that three regions exist, each of which exhibits different film characteristics. Between Reynolds numbers of 100 to 2,100, viscous... 

Mass-Transfer Coefficient Denoted by /c, K, and so on, the mass-transfer coefficient is the ratio of the flux to a concentration (or composition) difference. These coefficients generally represent rates of transfer that are much greater than those that occur by diffusion alone, as a result of convection or turbulence at the interface where mass transfer occurs. There exist several principles that relate that coefficient to the diffusivity and other fluid properties and to the intensity of motion and geometry. Examples that are outlined later are the film theoiy, the surface renewal theoiy, and the penetration the-oiy, all of which pertain to ideahzed cases. For many situations of practical interest like investigating the flow inside tubes and over flat surfaces as well as measuring external flowthrough banks of tubes, in fixed beds of particles, and the like, correlations have been developed that follow the same forms as the above theories. Examples of these are provided in the subsequent section on mass-transfer coefficient correlations. 

The overall heat transfer coefficient, U, is a measure of the conductivity of all the materials between the hot and cold streams. For steady state heat transfer through the convective film on the outside of the exchanger pipe, across the pipe wall and through the convective film on the inside of the convective pipe, the overall heat transfer coefficient may be stated as ... 

In general, diffusion is most useful for fundamental studies where we want to know the details about the system. For example, if we were concerned with a plastisizer inside a polymer film, we might want to know where and when the plasticizer is located. Diffusion will tell us. Dispersion can be important when there is convection, as in chromatography or atmospheric pollution. Mass transfer, on the other hand, tends to be useful in less fundamental, more practical problems. For example, if we want to know how to humidify and ventilate a house, we probably will use mass transfer coefficients. 

Cox et al. [101] used several kinds of enhanced tubes to improve the performance of horizontal-tube multiple-effect plants for saline water conversion. Overall heat transfer coefficients (forced convection condensation inside and spray-film evaporation outside) were reported for tubes internally enhanced with circumferential V grooves (35 percent maximum increase in U) and protuberances produced by spiral indenting from the outside (4 percent increase). No increases were obtained with a knurled surface. Prince [102] obtained a 200 percent increase in U with internal circumferential ribs however, the outside (spray-film evaporation) was also enhanced. Luu and Bergles [15] reported data for enhanced condensation of R-113 in tubes with helical repeated-rib internal roughness. Average coefficients were increased 80 percent above smooth-tube values. Coefficients with deep spirally fluted tubes (envelope diameter basis) were increased by 50 percent. 

Vertical Surfaces. If the laminar flow direction is downward and gravity-controlled, heat transfer coefficient for internal condensation inside vertical tubes can be predicted using the correlations for external film condensation—see Table 17.23. The condensation conditions usually occur under annular flow conditions. Discussion of modeling of the downward internal convective condensation is provided in Ref. 76. 

Film heat transfer coefficients may be estimated from flow conditions 2tnd from physical properties. For forced convection and turbulent flow (i.e. conditions prevailing inside stirred tank reactors), the relation... 

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