Dolphin Centrifuge

Decanter Centrifuge Optimization | Illustrated Guide

Table of Contents

30 Second Summary Takeaway

Decanter centrifuge performance optimization is achievable through the tuning of decanter features. We address the following typical questions related to the optimization process.

  • What is decanter optimization?
  • What parameters affect optimization?
  • How does pond-depth help optimize decanter performance?
  • How does bowl speed affect performance?
  • Isn't high speed always good for separation? Not Necessarily!
  • How does conveyor speed affect performance?

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.

Parameters Affecting Performance

The following mechanical parameters of the decanter directly affect the performance. Tuning these variables helps optimize decanter performance.

Liquid Radius / Pond Depth

Pond Depth Versus Cake Dryness Torque
Pond Depth Versus Fluid Clarity, Cake Dryness, Torque

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.

Pond-depth adjustment is one of the most versatile parameters to optimize decanter centrifuge performance. Read our detailed article about Decanter Centrifuge Pond Depth.

Deep Pond Effects

Deep Pond Depth
Deep Pond Depth

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

Shallow Pond Effects

Shallow Pond Depth
Shallow Pond Depth

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

Method of Adjusting Pond-Depth

The pond depth is changed by the replacing or adjusting of weir plates on the liquid end of the bowl.

Decanter Centrifuge Weir Plate
Decanter Centrifuge Weir Plate

Bowl Speed or Bowl RPM

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.

Decanter Energy Noise Wear versus Bowl Speed
Decanter Energy Noise Wear versus Bowl Speed

Bowl Speed Effects

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.

Decanter Centrifuge Optimization Curves
Decanter Centrifuge Bowl Speed Effects
Methods of Adjusting Bowl Speed

There are a few ways to change the main bowl speed of a decanter. The most widely used methods are:

  • VFD or Inverted Operating Main Drive Motor
  • Pulleys of Different Diameters to Change Drive Ratio

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Conveyor Speed (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.

Conveyor Speed Effect

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.

Conveyor Rotation inside Decanter Bowl
Conveyor Rotation inside Decanter Bowl

Higher conveyor speeds result in:

  • High Solids Output Capacity
  • Low Torque on Conveyor
  • Reduced Drying Capacity

NOTE: Conveying speed should not usually be higher than needed to obtain satisfactory solids capture.

Methods of Changing Conveyor Speed

There are several methods to adjust conveyor speed. These include:

  • VFD on Back Drive Motor
  • Sun-Wheel Shaft Speed Variation with Pulleys
  • Hydraulic Back-Drive

Conveyor Pitch

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.

Coarse Conveyor Pitch Effect

A coarse pitch results in a fast relative moment of flights resulting in:

  • Higher Conveying Capacity
  • Tendency to Back-slip (if solids have a low angle of friction)
  • Low Torque on Conveyor

Fine Conveyor Pitch Effect

A fine pitch gives the slow relative movement of flights resulting in:

  • Improved Solids Capture (with a small angle of friction)
  • Reduced Conveying Capacity
  • High Torque on Conveyor
Method of Changing Conveyor Pitch

The only way to change the conveyor pitch is to have interchangeable conveyors of different flight-spacing.

Summary

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.

Contact

To get more details email us or call us on (248) 522-2573

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