Geometry heat transfer surfaces

Variables It is possible to identify a large number of variables that influence the design and performance of a chemical reactor with heat transfer, from the vessel size and type catalyst distribution among the beds catalyst type, size, and porosity to the geometry of the heat-transfer surface, such as tube diameter, length, pitch, and so on. Experience has shown, however, that the reactor temperature, and often also the pressure, are the primary variables feed compositions and velocities are of secondary importance and the geometric characteristics of the catalyst and heat-exchange provisions are tertiary factors. Tertiary factors are usually set by standard plant practice. Many of the major optimization studies cited by Westerterp et al. (1984), for instance, are devoted to reactor temperature as a means of optimization. 

PF burners and fluid beds best meet requirements for dual- and triple-fuel firing including solid fuel as one option. PF burners are particularly suitable, as no static grate exists to compromise the design. They also have a combustion geometry which is similar to gas and oil, and therefore the flame can be arranged to allow full development of flame shape and maximum radiant heat transfer surface utilization. 

The factors that can affect the rate of heat transfer within a reactor are the speed and type of agitation, the type of heat transfer surface (coil or jacket), the nature of the reaction fluids (Newtonian or non-Newtonian), and the geometry of the vessel. Baffles are essential in agitated batch or semi-batch reactors to increase turbulence which affects the heat transfer rate as well as the reaction rates. For Reynolds numbers less than 1000, the presence of baffles may increase the heat transfer rate up to 35% [180]. 

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... 

The design procedure must start with a specific geometry and heat transfer surface and a specific percentage vaporization. Then the heat transfer coefficient is found, and finally the required area is calculated. When the agreement between the assumed and calculated surfaces is not close enough, the procedure is repeated with another assumed design. The calculations are long and tedious and nowadays are done by computer. 

For plate-fin heat exchangers in single-phase flow, the heat transfer coefficients are related to the developed heat transfer surface, and the area ratio must be taken into account. As related to the projected surface, the overall heat transfer coefficient is very high. Heat transfer and pressure drop can be estimated from correlations (43 44), but these correlations give only an estimate of the performance, because local modification of the fin geometry will affect heat transfer and pressure drop. 

In this chapter, emphasis will be given to heat transfer in fast fluidized beds between suspension and immersed surfaces to demonstrate how heat transfer depends on gas velocity, solids circulation rate, gas/solid properties, and temperature, as well as on the geometry and size of the heat transfer surfaces. Both radial and axial profiles of heat transfer coefficients are presented to reveal the relations between hydrodynamic features and heat transfer behavior. For the design of commercial equipment, the influence of the length of heat transfer surface and the variation of heat transfer coefficient along the surface will be discussed. These will be followed by a description of current mechanistic models and methods for enhancing heat transfer on large heat transfer surfaces in fast fluidized beds. Heat and mass transfer between gas and solids in fast fluidized beds will then be briefly discussed. 

The dependence of heat transfer on operating conditions, gas/solid properties and the geometries of both the bed and the heat transfer surface, is illustrated conceptually in Fig. 1. 

Investigators Heat transfer surface System geometry Correlation Range of Reynolds number... 

Even in. simple geometries, heat transfer problems cannot be. solved analytically if the thermal conditions are not sufficiently simple. For example, the consideration of the variation of thermal conductivity with temperature, the variation of the heat transfer coefficient over the surface, or the radiation heat transfer on the surfaces can make it impossible to obtain au analytical. solution. Therefore, analytical solutions are limited to problems that are simple or can be simplified with rea.sonable approximations. 

Figure 10.1. Agitated vessel standard geometry showing impeller, baffles, and heat transfer surfaces.

Heat transfer surfaces—helical coils, harp coils, or platecoils—are often installed inside the vessel and jackets (both side wall and bottom head) so that the vessel wall and bottom head can be used as heat transfer surfaces. Figure 10.1 gives a suggested geometry for helical coils and harp coils. 

In a heat exchanger, the heat transfer surface is fixed by the geometry of the equipment selected it is just the area of the pipe wall or the tubes. In particular, the heat transfer area does not depend on the flowrates of the hot and cold streams. On the other hand, the boundary between liquid and gas in a packed bed is very complex and very hard to measure directly. Most importantly, the area also depends on the flowrates of the gas and liquid streams (T,T). 

R. L. Webb and A. E. Bergles, Performance Evaluation Criteria for Selection of Heat Transfer Surface Geometries Used in Low Reynolds Number Heat Exchangers, in Low Reynolds Number Convection in Channels and Bundles, S. Kakac, R. H. Shah, and A. E. Bergles eds., Hemisphere, Washington, DC, and McGraw-Hill, New York, 1982. 

By altering the local geometry of the heat transfer surface to intensify the turbulence in the local flow field... 

The limitations of the Wilson plot technique may be summarized as follows. (1) The fluid flow rate and its log-mean average temperature on the fluid 2 side must be kept constant so that C2 is a constant. (2) The Re exponent in Eq. 17.77 is presumed to be known (such as 0.82 or 0.8). However, in reality it is a function of Re, Pr, and the geometry itself. Since the Re exponent is not known a priori, the Wilson plot technique cannot be utilized to determine the constant C0 of Eq. 17.77 for most heat transfer surfaces. (3) All the test data must be in one flow region (e.g., turbulent flow) FIGURE 17.40 Original Wilson plot of Eq. 17.79. on fluid 1 side, or the Nu correlation must be expressed by an... 

The heat transfer coefficients for the offset strip fins are 1.5 to 4 times higher than those of plain fin geometries. The corresponding friction factors are also high. The ratio of ///for an offset strip fin to ///for a plain fin is about 80 percent. If properly designed, the offset strip fin would require substantially lower heat transfer surface area than that of plain fins at the same Ap, but about a 10 percent larger flow area. 

Heat transfer coefficients for condensation processes depend on the condensation models involved, condensation rate, flow pattern, heat transfer surface geometry, and surface orientation. The behavior of condensate is controlled by inertia, gravity, vapor-liquid film interfacial shear, and surface tension forces. Two major condensation mechanisms in film condensation are gravity-controlled and shear-controlled (forced convective) condensation in passages where the surface tension effect is negligible. At high vapor shear, the condensate film may became turbulent. 

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