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Decanter centrifuge performance optimization is achievable through the tuning of decanter features. We address the following typical questions related to the optimization process.
Decanter centrifuges offer an advantage over other types of separation equipment. The decanter can optimize performance based on the type of feed and output results desired.
Based on the application, the user may need:
(a) To recover most solids in the feed.
(b) To get a clear centrate (liquid) free of solids.
(c) To recover most of the liquid in a reasonably clean state.
(d) To separate the solids in the driest possible form.
It is worth noting that (a) and (b) seek the same results - capture most solids also implies the clearest liquid.
Similarly, (c) and (d) aim for the same result.
The customer often requires the separation of all the solids at their driest state. In most cases, it is the balance of the dryness of the solids discharged against liquid clarity.
The following mechanical parameters of the decanter directly affect the performance. Tuning these variables helps optimize decanter performance.
The decanter bowl generates a centrifugal force through its rotation. This force pushes the process liquid towards the bowl wall. The liquid forms a concentric layer along the bowl wall, which is known as the pond. The pond-depth is the radial thickness of this layer.
Small liquid radius (deep pond, short beach) gives solids more time to settle but less time to drain, resulting in:
- Clean centrate
- Maximum solids capture
- Low torque load on the conveyor
- Comparatively, wet solids discharge
Large liquid radius (shallow pond, long beach) gives solids less time to settle but more time to drain, resulting in:
- Maximum dryness of discharged solids
- Comparatively high torque load
- Some loss of fine solids with centrate
The pond depth is changed by the replacing or adjusting of weir plates on the liquid end of the bowl.
The bowl speed or bowl RPM is the measure of the revolutions the centrifuge bowl performs every minute. In other words, this number indicates the rotational velocity of the bowl.
Bowl speed directly correlates to the centrifugal force generated by the centrifuge. Therefore, the higher the bowl speed, the higher the g-force.
Increased bowl speed generates higher G-force and accelerates sedimentation, resulting in:
- Cleaner Centrate
- Drier Solids
- Greater Slippage
Also, in solids with a low angle of friction, the back slip of solids into the pool may contaminate the centrate. In such cases, it is possible to get a cleaner centrate by reducing the bowl speed. See the image below showing the correlation.
There are a few ways to change the main bowl speed of a decanter. The most widely used methods are:
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Decanter Centrifuge - Differential Speed
The auger or conveyor speed is the differential speed between the bowl and the conveyor. This speed is a measure of the rate at which the auger is pushing the separated sludge inside the decanter bowl.
Read our article on Calculating the Differential Speed in a Decanter for more information.
High conveyor (differential) speed shortens the retention time of the separated solids in the bowl. In other words, fast conveyor rotation leads to quicker exit for the sludge particles.
Higher conveyor speeds result in:
NOTE: Conveying speed should not usually be higher than needed to obtain satisfactory solids capture.
There are several methods to adjust conveyor speed. These include:
The conveyor pitch is the distance between the flights on the conveyor. A fine or shorter pitch means more flights, and a coarse or longer pitch means less number of flights.
A coarse pitch results in a fast relative moment of flights resulting in:
A fine pitch gives the slow relative movement of flights resulting in:
The only way to change the conveyor pitch is to have interchangeable conveyors of different flight-spacing.
The four adjustments mentioned above help optimize decanter centrifuge performance. Needless to say, each application has a different set of parameters. However, following the guidelines above, the user can optimize the decanter centrifuge under set operating conditions.