Centrate weirs

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 

For best process control, the weir width needs to be maximised to reduce the level of cresting over the weir. The crest is the extra level of liquid above the weir inner edge, necessary to effect flow, as seen over weirs in rivers. This cresting varies with feed rate, but will be an inverse function of the weir length. Thus the larger this is. the smaller is the variation due to feed rate, or more properly, centrate flow changes. 

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Imperforate Bowl Tests The amount of supernant hquid from spin tubes is usually too small to warrant accurate gravimetric analysis. A fixed amount of shiny is introduced at a controlled rate into a rotating imperforate bowl to simulate a continuous sedimentation centrifuge. The liquid is collected as it overflows the ring weir. The test is stopped when the solids in the bowl build up to a thickness which affects centrate quality. The solid concentration of the centrate is determined similarly to that of the spin tube. 

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. 

Under hindered settling conditions the profile of the top interface between the clean centrate and the centrifiiged suspension will be as illustrated in Figure 8.8. A weir set a... 

The deeper the weir plate the more important it is to secure it with a support plate. This is because the dam, being relatively thin because of the range of sizes necessary, can easily distort with the pressure of the pond at speed. The better cover plates will have a protruding lip fabricated on their inner edge. This is to ensure that the discharging centrate separates from the bowl at the smallest possible radius. This minimises power consumption. 

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. 

The crest height, the pond surface level above the actual weir height, is a function of the centrate rate, the total weir width and centrifuge g-level, as well as physical constants of the liquor such as viscosity and density. Crest height is lie. given by ... 

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

A log-probability distribution A special pipe attachment close to the bowl front hub. which enables centrate to be skimmed off the pond surface, obviating the need for a weir plate. The skimmer can be adjusted while the bowl is at speed, which effectively alters the pond height A type of feed zone that enables feed to be brought up to speed and enter the pond without turbulence A short raised step to afford mating in a recess of another component... 

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