Reverse-Flow Baffles

Low liquid rates. With the aid of serrated weirs, splash baffles, reverse-flow trays, and bubble-cap trays, low liquid rates can be handled better in trays. Random packings suffer from liquid dewetting and maldistribution sensitivity at low liquid rates. 

Another type of classifier directs an air stream across a stream of the particles to be classified. An example is the radial-flow classifier Kennedy Van Saun Corp.), which Features adjustable elements to control the flow and classification. A further development on this principle is the Vari-Mesh classifier Kennedy Van Saun Corp.), which controls classification by adjustable flow baffles. A change in direction of air flow is the operating principle of the reverse-flow Superfine classifier Hosokawa Mineral Processing Systems). 

Reverse flow Liquid flowing from the inlet on one side of the tray (around a center baffle) reverses its direction at the other side of the tray, and flows back to the downcomer on the same side of the tray where the inlet is. 

In a reverse-flow ("half-pass ) tray (Fig. 6.9), liquid is forced to flow around a central baffle. Both the downcomer and the downcomer seal area are on the same side of the tray, and the liquid flow path is quite long. This tray is mainly suitable for low-liquid-flow-rate applications. Making the baffle at least twice the height of the highest calculated clear liquid height on the tray has been recommended (48) in order to avoid short-circuiting of liquid. For the same reason, leakage under the bafile should also be minimized. 

In the time or temporal domain, periodicity in operation is incorporated to realize all four principles of PI. A combination of adsorption-reaction-desorption on catalyst surface by periodic forcing of temperatures and pressures demonstrates the application of first principle. Oscillatory baffled flow reactor enhances uniformity, and illustrates the second PI principle. The application examples for third and fourth PI principles are pulsation of feed in trickle bed reactors enhancing the mass transfer rates, and flow reversal in reversed flow reactors shifting the equilibrium beyond limitations respectively. Switching from batch to continuous processing also result in realization of second and third PI principles. 

Another design, shown ia Figure 5, functions similarly but all components are iaside the furnace. An internal fan moves air (or a protective atmosphere) down past the heating elements located between the sidewalls and baffle, under the hearth, up past the work and back iato the fan suction. Depending on the specific application, the flow direction may be reversed if a propeUer-type fan is used. This design eliminates floorspace requirements and eliminates added heat losses of the external system but requires careful design to prevent radiant heat transfer to the work. 

Performance Data for Direct-Heat Tray Dryers A standard two-truck diyer is illustrated in Fig. 12-48. Adjustable baffles or a perforated distribution plate is normally employed to develop 0.3 to 1.3 cm of water-pressure drop at the wall through which air enters the truck enclosure. This will enhance the uniformity of air distribution, from top to bottom, among the trays. In three (or more) truck ovens, air-reheat coils may be placed between trucks if the evaporative load is high. Means for reversing air-flow direction may also be provided in multiple-truck units. 

The gravity and baffle chambers provide the simplest of gas collection techniques. The gas is allowed to travel through a large chamber or a long tunnel to reduce its velocity, causing the dust to drop out by the action of gravity. The gas flow is deflected by baffles, or the flow direction may be reversed in order to enhance the separation. 

For reasons of compactness of equipment, the paths of both fluids may require several reversals of direction. Two of the simpler cases of Figure 8.3 are (b) one pass on the shell side and two passes on the tube side and (c) two passes on the shell side and four on the tube side. On a baffled shell side, as on Figure 8.4(c), the dominant flow is in the axial direction, so this pattern still is regarded as single pass on the shell side. In the cross flow pattern of Figure 8.5(c),... 

B0 = roughly the number of tube crosses = number of baffles + 1=6. Nr = number of tube rows across which shell fluid flows = 23 minus the tube rows that pass through the cut portions of the baffles. With 25 percent cut baffles, the fluid will flow across approximately one-half of the tubes. In this case, Nr will be taken as 14. This gives some allowance for neglecting friction due to reversal of flow direction and friction due to flow parallel to the tubes ... 

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