Bowl baffles

The use of appropriately designed and positioned baffles, in a decanter bowl, can direct process streams in the directions required. Processes otherwise impossible are made possible, such as three-phase separation, and the use of pond levels above the solids discharge level. Baffles are used for both liquid and cake as well as floating solids. 

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Other modifications have special but more limited apphcations. A centrifugal bowl may contain, instead of disks, several aimular baffles that take the hquid through a labyrinth path before discharge. The multiple cylinders increase cake capacity to as much as 70 L for easily sedimented sohds. This centrifuge is used for clarification of food symps and antibiotics (qv), and for recovery of heavy metallic salts and catalysts (see Fig. 14c). 

The feed slurry is introduced into the lower portion of the bowl through a small orifice. Immediately downstream of the orifice is a distributor and a baffle assembly which distribute and accelerate the feed to circumferential speed. The centrate discharges from the top end of the bowl by overflowing a ring weir. Solids that have sedimented against the bowl wall are removed manually from the centrifuge when the buildup of solids inside the bowl is sufficient to affect the centrate clarity. 

The unsorted workpieces that have been fed in are moved by triggered vibration energy to the edge of the bowl. They then run onto a sloped helical channel on the inside or outside wall of the bowl and are moved upwards over mechanical baffles. These baffles have the task of orienting the... 

To increase flexibility, mechanical baffles are being replaced more and more with optical parts-recognition systems in the vibratory bowl feeder. The geometry of the various workpieces can be programmed, stored in, and retrieved from the control system if there is a change of product. This means that retooling can be kept to a minimum. 

Figure 29 Examples of Standard and Flexible Part-feeding Devices, (u) Bowl feeders with mechanical baffles. (Source RNA, MHK, Bihler) (b) Bowl feeder with parts-recognition system. (Source MRW) (c) FlexFeeder. (Source Adept)...

In the alternative screen bowl design the conical section of the bowl is shortened and a supplementary cylindrical section is attached in an effort to promote more efficient separation. Other design variants rely on the addition of baffles, helical discs, vanes, conical disc stacks or fins, all of which alter the flow and/or residence time distributions within the centrifuge. Figure 1.15 shows an example of a decanter centrifuge fitted with a vane stack. 

Several workers " have observed that, contrary to the common assumption that there is a regular stream of liquid through the whole cross-section available to flow within the bowl, the incoming liquid tends to flow within a thin layer near the surface of the liquid whilst the bulk of the liquid is essentially stagnant. As this is clearly detrimental to the separation efficiency, because it shortens the residence time of the liquid in the bowl, some manufacturers build in special baffles which are designed to stop the surface flow and make the liquid flow nearer to the bowl wall. 

Inside, the casing is compartmentalised by several baffles, which are welded to the inside surface of the casing and fit very close to the rotating assembly. This close fit may be to the plain surface of the bowl, or to a shoulder on the bowl, or into a labyrinth groove machined into the bowl. 

The simpler and slower the decanter, the less is the baffling needed. Some baffles are fitted with a large clearance from the bowl while others penetrate grooves in the bowl to form labyrinths. 

When the centrate discharges from the bowl a lot of turbulence occurs with splashing back onto the bowl. Some of this splashing may get into the baffle grooves causing an unacceptable level of cross contamination. To prevent this, a gutter is sometimes fitted to the baffle adjacent to the centrate discharge. 

Naturally, in three-phase designs there are many possibilities for variations with the fitting of special baffles, chambers and channels into the front end of the bowl but the principle is the same. However the use of a skimmer (see Section 2.4.3) in three-phase designs is very useful in controlling the equilibrium line between the two liquid phases while the bow l is at speed. The casing design is, of course, different. 

If the conveying flight is positioned on the beach then the outside edge needs to be tapered to suit the profile of the beach. Sometimes it will be found that this tapering i.s. such that the clearance between the baffle and the bowl or beach Is made to reduce towards the rear. 

A patent by KHD (now Bird Humboldt) specifies a standard pitch in the cylindrical section of the bowl with a narrower pitch on the beach [15]. This obviates the need for a baffle at the foot of the beach as the narrow pitch produces a deeper cake forming the seal. The narrow pitch provides a back pressure on which the standard pitch can work. 

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