Centrate discharge

It is usual to cone down the centrate end of the casing in order to mate with an off take pipe. The pipe size needs to be sufficient to allow free flow of the copious quantities of air at this end. Alternatively a separate vent pipe will be introduced. The centrate tends to continue its circular motion on leaving the bowl and swirls around in the casing. It is therefore sometimes seen that the centrate discharge is offset from the centre line to take advantage of this tangential flow. 

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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. 

Figure 9.6 Liquid seal of solids ejecting DSC - Westfalia separator. 1, Feed 2, sealing-liquid discharge 3, sealing-liquid feed 4, centrate discharge.

In a basic decanter the centrate discharges from the front hub over weirs. These weirs, sometimes called dam plates, cause a pond to form in the bowl. The level of liquid in the bowl, the distance from the bowl wall to the inner... 

The design of the front (centrate discharge) hub varies considerably from machine to machine. However these variations are generally for mechanical considerations, having little effect, by themselves, on process performance. 

An alternative to the solid helical flight is one with windows" to allow axial flow of the clarified liquor from the feed zone area to the centrate discharge. The way these windows are manufactured can vary. The simplest design has holes cut out of the flights before they are welded onto the hub. A more sophisticated design (see also Section 2.4.7.1) has a ribbon flight welded to the extremities of pillars hanging from the conveyor hub. 

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. 

This disc is the same as the cake baffle disc but fitted close to the centrate discharge. It is only suited to the countercurrent decanter. 

Many manufacturers have experimented with variable conveyor pitches. These have been used to enhance dryness in effluent dewatering. The pitch starts wide at the centrate discharge end. narrowing progressively along the... 

An alternative use for the variable pitch is to improve centrate quality. For this the pitch of the flight adjacent to the front hub is increased, as much as double that for the rest of the conveyor. This design is particularly useful for applications in which the decanter can become heavily loaded with solids. The increased pitch tends to keep the centrate discharge region of the bowl relatively free of solids, thus reducing re-enlrainment of solids as the centrate streams towards the weir. 

If the vanes do not extend the full length between the feed zone and the centrate discharge, then the Sigma of the plain section needs to be added to Ey to obtain the total Sigma value for the centrifuge. 

A type of weir plate, made in one piece, which covers all centrate discharge points to the same level... 

Decanter centrifuges, for process applications, are typically solid bowl scroll-discharge machines, capable of continuous sohd and centrate discharge. They are ideally suited to handling high feed solids concentrations and delivering dry solid products. They are generally able to convey the soft solids often encountered in the pharmaceutical and fermentation industries. 

Tubular bowl centrifuges usually incorporate continuous centrate discharge and batch solids recovery, and operate at variable speeds up to 20,000 G. Although primarily intended for liquid-liquid separation, they can also be used for recovery or classification of ultra-fine solids. These sedimenting centrifuges (disc, decanter and tubular bowl) are described further in Section 7. 

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