Heat transfer, annular tubes

The reactor, or Contactor, is basically a special type of a continuous-flow stirred tank reactor, as shown in Figure 1 (6). It is a cylindrical vessel positioned horizontally in which the acid/hydrocarbon dispersion is repeatedly circulated over and around heat transfer coils (tube bundle). The impeller employed to promote the dispersion of the feed mixture of isobutane and olefins in the acid phase is located at one end of the reactor. The impeller causes the dispersion to enter the annular region between the shell of the reactor and the tube bundle the dispersion flows rapidly in this region, which extends over most of the length of the reactor. As the dispersion reaches the exit end of the annulus, a small portion of the dispersion is withdrawn and fed to the decanter, which is discussed later. The remainder of the dispersion leaving the annular region makes a 180° turn at the end of the reactor and flows back toward the impeller. As it returns, the dispersion passes over and around U-shaped heat transfer coils that remove the exothermic heats of reaction and the energy added to the reactor by the impeller. 

In high heat flux (heat transfer rate per unit area) boilers, such as power water tube (WT) boilers, the continued and more rapid convection of a steam bubble-water mixture away from the source of heat (bubbly flow), results in a gradual thinning of the water film at the heat-transfer surface. A point is eventually reached at which most of the flow is principally steam (but still contains entrained water droplets) and surface evaporation occurs. Flow patterns include intermediate flow (churn flow), annular flow, and mist flow (droplet flow). These various steam flow patterns are forms of convective boiling. 

A2. Adorni, N., Bertoletti, S., Lesage, J., Lombardi, C., Peterlongo, G., Soldaini, G., Weckermann, F. J., and Zavattarelli, R., Results of wet steam cooling experiments pressure drop, heat transfer and burnout measurements in annular tubes with internal and bilateral heating, CISE-R.31 (1961). 

Experiments in annular flow were performed by Hetsroni et al. (2003b) to study the flow regimes and heat transfer in air-water flow in 8° inclined tubes of inner diameter 49.2 mm and 25 mm. 

In Fig. 5.39a-d the local heat transfer coefficients derived in the horizontal tube are compared to those obtained in the 8° upward inclined pipe and presented by Hetsroni et al. (2006). The results show a clear improvement of the heat transfer coefficient with the pipe inclination. Taitel and Dukler (1976) showed that the flow regimes are very sensitive to the pipe inclination angle. In the flow regime maps presented in their work, the transition from stratified to annular flow in the inclined tube occurs for a smaller air superficial velocity than for the case of the horizontal tube. 

Flow patterns and heat transfer were also investigated by Ghajar et al. (2004) in slug and annular flow. The different flow regimes depicted in Fig. 5.40 illustrate parameters in their experiments in the tube of t/ = 25.4 mm. 

Chen s method was developed from experimental data on forced convective boiling in vertical tubes. It can be applied, with caution, to forced convective boiling in horizontal tubes, and annular conduits (concentric pipes). Butterworth (1977) suggests that, in the absence of more reliable methods, it may be used to estimate the heat-transfer coefficient for forced convective boiling in cross-flow over tube bundles using a suitable cross-flow correlation to predict the forced-convection coefficient. Shah s method was based on data for flow in horizontal and vertical tubes and annuli. 

Longitudinal fins can also be used, but their application is restricted to small heat exchangers in the form of a concentric pipe heat exchanger, similar to the schematic in Figure 15.5a. In this arrangement, the inner tube would be the extended surface tube with the fins in the annular space to enhance the heat transfer. Longitudinal fins can increase the surface area by a factor of 14 to 20 relative to plain tubes. 

Macbeth (M5) has recently written a detailed review on the subject of burn-out. The review contains a number of correlations for predicting the maximum heat flux before burn-out occurs. These correlations include a dependence upon the tube geometry, the fluid being heated, the liquid velocity, and numerous other properties, as well as the method of heating. Sil-vestri (S6) has reviewed the fluid mechanics and heat transfer of two-phase annular dispersed flows with particular emphasis on the critical heat flux that leads to burn-out. Silvestri has stated that phenomena responsible for burn-out, due to the formation of a vapor film between the wall and the liquid, are believed to be substantially different from phenomena causing burn-out due to the formation of dry spots that produce the liquid-deficient heat transfer region. It is known that the value of the liquid holdup at which dry spots first appear is dependent on the heat flux qmi. The correlations presented by Silvestri and Macbeth (S6, M5) can be used to estimate the burn-out conditions. 

Three main flow patterns exist at various points within the tube bubble, annular, and dispersed flow. In Section I, the importance of knowing the flow pattern and the difficulties involved in predicting the proper flow pattern for a given system were described for isothermal processes. Nonisother-mal systems may have the added complication that the same flow pattern does not exist over the entire tube length. The point of transition from one flow pattern to another must be known if the pressure drop, the holdups, and the interfacial area are to be predicted. In nonisothermal systems, the heat-transfer mechanism is dependent on the flow pattern. Further research on predicting flow patterns in isothermal systems needs to be undertaken... 

The tube-in-tube or multitube-in-tube heat exchangers are useful in small Linde lique-fiers or in the final Joule-Thomson stage of any liquefier. The performance of Linde-type exchangers is easy to calculate, and their realization is simple. In the examples shown in Fig. 5.12 (a)-(c), the tubes are concentric and the outer wall contributes appreciably to the pressure drop in the outer stream without contributing to the heat transfer. Usually, the smaller inner tube is used for the high-pressure stream and the low-pressure stream flows through the outer annular space. The tubes in Fig. 5.12 (d) and (e) are solder bonded while that in (f) is flattened and twisted before insertion into an outer tube. 

The heat transfer characteristics of liquid-solid fluidised systems, in which the heat capacity per unit volume of the solids is of the same order as that of the fluid are of considerable interest. The first investigation into such a system was carried out by Lemlich and Caldas193, although most of their results were obtained in the transitional region between streamline and turbulent flow and are therefore difficult to assess. Mitson194 and Smith(20) measured heat transfer coefficients for systems in which a number of different solids were fluidised by water in a 50 mm diameter brass tube, fitted with an annular heating jacket. 

Figure 17.18. Heat transfer in fixed-bed reactors (a) adequate preheat (b) internal heat exchanger (c) annular cooling spaces (d) packed tubes (e) packed shell (f) tube and thimble (g) external heat exchanger (h) multiple shell, with external heat transfer (Walas, 1959).

Fukada, S., S. Morimitsu, N. Shimoozaki (2004a), Heat and Mass Transfer in a Concentric-annular Tube Bed Packed with ZrV19Fe01 Particles , Journal of Alloys and Compounds, 375, 305-312. 

Apparatus construction companies specializing in these reactors have developed a detailed and comprehensive know-how as regards flow control of the heat transfer medium [28, 29], This concerns the uniform supply and removal of the heat transfer medium, which generally takes place vial external annular channels, as well as the flow control within the reactor. Some recent publications illustrate the major differences in the behavior of different tube sections that can arise due to an inadequate design and layout of the heat transfer medium circuit [30-32]. 

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