Heat transfer mechanical design

Design Methods for Direct-Heat Rotary Dryers Direct drying in a direct-heat rotary dryer is best expressed as a heat-transfer mechanism as follows ... 

As was shown before, the Leidenfrost temperature is the second transformation of heat transfer mechanisms. Empirical correlations have been established by film boiling data obtained from water at high pressure levels. For a wide range of steam-water mixture velocities, the correlation for hFB reported by Bishop et al. (1965), as shown in Eq. (4-37), is recommended for use in design. 

Figure 33 Comparison of heat transfer mechanisms for glass vials on a polished stainless steel shelf. Pressure is in millitorr. The vial identification numbers are manufacturers designations. (From Ref. 1.)...

In order to achieve passive safety with reactive material, the radius of the reactor tube is designed to be small to avoid any thermal explosion inside the tube. Using the Frank-Kamenetskii approach (see Chapter 13), the radius remains below the critical radius. Thus, even assuming a purely conductive heat transfer mechanism, corresponding to a worst case, no instable temperature profile can develop inside the reaction mass. The reactor can be shut down and restarted safely. 

Work is still required to optimize the design of such PHEs because the heat transfer mechanisms are complex. On the feed side, a complex mixture is vaporized on the effluent side, the mixture is condensed with the presence of noncondensable gases. 

Design methods for vacuum-shelf dryers Heat is transferred to the wet material by conduction through the shelf and bottom of the tray and by radiation from the shelf above. The critical moisture content will not be necessarily the same as for atmospheric tray drying, as the heat-transfer mechanisms are different. 

Stages (a), (b) and (e) involve direct heat transfer between a gaseous medium and the particles. As a unit process, the heat transfer mechanism is well understood and not specific to the calcination of limestone. Stages (c) and (d), however, are specific to limestone, are influenced by the design of lime kiln, and influence the properties of the quicklime. They are described in detail below. 

Table 5-9. Indication of fluid mechanics, mass and heat transfer and design of fluidized beds.

There are obvious tradeoffs that must be made in controlling the various heat-transfer mechanisms. Figure 5.167 illustrates the contribution to total conductivity of each mechanism at three different temperatures. The most efficient insulation design, both thermally and economically, will vary depending on the application conditions. References 1 and 2 are basic texts on heat transfer for further study. 

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