Vertical baffles

Figure 3.3 shows a simple type of classifier. In this device, a large tank is subdivided into several sections. A size range of solid particles suspended in vapor or liquid enters the tank. The larger, faster-settling particles settle to the bottom close to the entrance, and the slower-settling particles settle to the bottom close to the exit. The vertical baffles in the tank allow the collection of several fractions. 

The Oldshue-Rushton column (Eig. 15d) was developed (162) in the early 1950s and has been widely used in the chemical industry. It consists essentially of a number of compartments separated by horizontal stator-ring baffles, each fitted with vertical baffles and a turbine-type impeller mounted on a central shaft. Columns up to 2.74 m in diameter have been reported in service (162—167). Scale-up is reported to be reliably predictable (168) although only limited performance data are available (169). A detailed description and review of design criteria are available (170). 

A. Solid particles suspended in agitated vessel containing vertical baffles, continuous phase coefficient -2 + 0.6Wi f,.Wi D Replace Osi p with Vj = terminal velocity. Calculate Stokes law terminal velocity [S] Use log mean concentration difference. Modified Frossling equation K, -< T.d,P. [97] [146] p.220... 

Calvert found that reentrainment from the baffles was affected by the gas velocity, the liquid-to-gas ratio, and the orientation of the baffles. Horizontal gas flow past vertical baffles provided the best drainage and lowest reentrainment. Safe operating regions with vertical baffles are shown in Fig. 14-112. Horizontal baffles gave the poorest drainage... 

In design of separating chambers, static vessels or continuous-flow tanks may be used. Care must be taken to protect the flow from turbulence, which coiild cause back mixing of partially separated fluids or which could cany unseparated hquids rapidly to the separated-hquid outlet. Vertical baffles to protect rising biibbles from flow currents are sometimes employed. Unseparated fluids should be distributed to the separating region as uniformly and with as little velocity as possible. When the bubble rise velocity is quite low, shallow tanks or flow channels should be used to minimize the residence time required. 

The Oldshue-Rushton (Mixco) extractor is similar in construction to the RDC in the fact that it is a relativelv open design, consisting of a series of compartments separated by horizontal stator baffles. The major difference from the RDC is that the height/diameter ratio of the compartments is greater, each compartment is fitted with vertical baffles, and the mixing is accomplished by means of a turbine impeller rather than a disc. 

Axial-Flow Fluidfoil Impellers For vessel volumes of 4 to 200 m (1000 to 50,000 gal), a turbine mixer mounted coaxiaUy within the vessel with four or more baffles should be the initial choice. Here also the vessel straight-side-height-to-diameter ratio should be 0.75 to 1.5. Four vertical baffles should be fastened perpendicularly to the vessel wall with a gap between baffle and wall equal to Df/24 and a radial baffle width equal to Df/12. 

The most comprehensive correlation for heat transfer to vertical baffle-type coils is for a disk flat-blade turbine over the Reynolds number range lO to (2)(10 ) ... 

Vortex formation leads to a considerable drop in mixing efficiency and should be suppressed as much as possible in practical applications to increase the homogenizing effects of mixers. The preferable method of vortex suppression is to install vertical baffles at the walls of the mixing tank. These impede rotational flow without interfering with the radial or longitudinal flow. Figure 11 illustrates such a system. 

Proportions of a stiiTcd tank relative to the diameter D liquid level = D turbine impeller diameter = D/3 impeller level above bottom = D/3 impeller blade with = D/15 four vertical baffles with width = D/10. 

The helical coil relationship was developed using four vertical baffles of Xs T, located either outside the coil or inside the coil [29]. 

The addidon of a second bank of similar coils for additional heat transfer area, either outside or inside the first bank of coils will not provide twice the transfer of heat but the effecdveness is estimated to be 70%-90% of the first bank of duplicate coils (assuming the same heat transfer area per bank) [29]. The additional coils will provide additional baffling therefore, the need for the dimension of the vertical baffles is reduced, or they might be removed entirely. 

The results of Petree and Small are summarized in [29]. These coils present a solid vertical face, with the coils vertical but impressed in the plates for flow of the heating or cooling medium. They take the place of vertical baffles, and are more solid obstructions to through flow in die vessel than individual vertical coils. Usually four or six banks are used. 

Radius of Bend = 2 V2 times tube O.D. for all pitches and tube sizes Pie shape layout Pie shape layout Vertical baffle layout Vertical baffle layout... 

For heat transfer performance, horizontal baffles to isolate tube-side passes in horizontal bundles are preferred over vertical baffles that isolate groups of tubes in vertical columns. The expansion of capacity by adding more tube bundles or sections in parallel is easier, and the MTD is better with the horizontal pass plates. The fan drive may be by any of the available means, including ... 

Figure 7.11. Flow patlem in vessel with vertical baffles...

The crystallizer was an agitated vessel with an Inside diameter of 9.0 cm and a volume of about 1 1. It was equlppet with four vertical baffles, a water jacket to keep the solution temperature constant, and a nozzle through which nitrogen gas was Introduced In several experiments to suspend a speed crystal more effectively In the solution. Agitation was accomplished with a 5.0 cm stainless steel marine propeller having three blades driven by a variable speed motor. 

When this type of impeller is used, typically four vertical baffle plates, each one-tenth of the tank diameter in width and the total liquid depth in length, are fixed perpendicular to the tank wall so as to prevent any circular flow of liquid and the formation of a concave vortex at the free liquid surface. 

Very large beaters (capacity 25 m3), may be used (Figs. 155 and 156). Here, the cutting drum is situated at one end of the tub which is made either of cast iron, from a couple of units to be assembled together, or of concrete. Concrete vessel are lined internally with glazed tiles. The trough is divided into two by means of a vertical baffle plate. In these compartments the bottom is inclined in opposite directions in order to facilitate the circulation of the pulp. 

Wave breakers. In long horizontal vessels it is necessary to install wave breakers, which are simply vertical baffles spanning the gas-liquid interface perpendicular to the flow ... 

Figure 5.3 shows a once-through thermosyphon reboiler with a vertical baffle. This looks quite a bit different from Fig. 5.2, but processwise, it is the same. Note that the reboiler return liquid goes only to the hot side of the tower bottoms. Putting the reboiler return liquid to the colder side of the tower bottoms represents poor design practice. 

Figure 5.3 A once-through reboiler, with vertical baffle plate. 

We said before that it was wrong to return the effluent, from a once-through reboiler, with a vertical baffle, to the cold side of the tower s bottom. Doing so would actually make the once-through thermosyphon reboiler work more like a circulating reboiler. But if this is bad, then the once-through reboiler, must be better than the circulating reboiler. But why ... 

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