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In this article, you will get to learn more about the following aspects related to the differential speed of a decanter centrifuge.
Speed is often referenced in the context of decanter centrifuges. In technical terms, speed refers to the rotational speed or RPM (Rotations Per Minute) of the decanter’s two main rotating components. These are the decanter bowl and the scroll, which rotates independently within the decanter bowl.
As shown in the cross-sectional diagram above, the decanter centrifuge is a horizontal bowl that rotates. The decanter bowl’s rotation is powered by the main drive motor coupled to the bowl by pulleys and belts. The actual RPM of the bowl is referred to as the bowl speed.
This bowl’s rotation causes high centrifugal forces to act on the liquid and solids within the bowl. The high centrifugal force leads to accelerated sedimentation of the solid particles in the liquid along the bowl wall.
Within the decanter, the bowl is a scroll (aka auger), which also rotates. However, the scroll rotates at a slightly different speed than the bowl. The rate of rotation of the scroll is known as the scroll speed.
Read more about decanter centrifuge operating principles.
The differential speed of a decanter centrifuge is the difference between the bowl’s rotational speed and the scroll. Therefore the differential speed is a measure of the scroll speed in relation to the decanter bowl.
This relational scroll speed indicates how fast or slowly the scroll is rotating about the bowl. The differential speed enables the scroll to move the separated solids toward the solid’s bowl end for the solids’ eventual evacuation.
The differential speed is directly related to the amount of time the solids remain in the bowl before being ejected. This time in the bowl is known as retention time or residence time. Longer residence time causes compression of the solids under the high centrifugal force.
The decanter bowl connects to the scroll through a gearbox assembly. This gearbox conveys the bowl rotation to the scroll through a set of gears. The ratio of different gears within the decanter gearbox determines the differential speed between the bowl and the scroll.
A decanter differential speed can be changed by the controlled rotation of the sun wheel shaft extension of the gearbox. The direction of rotation and speed of rotation of the sun wheel shaft determines the actual differential speed of the decanter.
Three decanter parameters determine the differential speed of the scroll versus the bowl. These are (1) Bowl Speed, (2) Sun-Wheel Shaft Speed, and (3) Gearbox Ratio.
Differential Speed = (Bowl Speed – Sun Wheel Shaft Speed) / Gear-Box Ratio.
An example of a decanter speed difference calculation is as follows.
Bowl Speed: 3,500 RPM
Gear-Box Ration: 200 : 1
Sun-Wheel Shaft Speed: 1,500
Using the above formula, the differential speed of the decanter is (3,500-1,500)/200 = 10 RPM
|Gear Box Ration||159||Sun-Wheel Shaft Speed||Varying|
|Bowl Speed (RPM)||4000|
|BD Freq (Hz)||BD Motor Speed||Scroll Speed||Bowl Speed||Differential RPM|
Higher differential speed reduces the retention time of solids in the decanter bowl. This reduced residence time leads to:
It is essential to note that the scroll conveying speed should not be higher than that required for satisfactory sludge separation.
The following chart summarizes the effects of scroll speed on decanter performance.
Learn more about other types of industrial centrifuges....
There are tangible benefits of scroll speed variation, which are listed below.
Process fluid consistency cannot be assumed or expected to remain constant. Changes in solids concentration are common when solids settle in the feed tank during the process. As the solids load increases, varying the differential speed to move the separated solids quicker is essential for efficient separation of solids.
Separated solid's cake dryness can be optimized by tuning the decanter scroll speed under varying sludge load conditions.
In case of a sudden solids increase, the scroll can be subjected to excessive torque. Increasing the differential speed with increasing load can push the solids out faster, reducing the scroll load. This variation of scroll speed is essential for overload protection.
A back-drive motor driven by an inverter or Variable Frequency Drive (VFD) automatically controls the decanter differential speed. An electronic controller monitors the current drawn by the blacked-rive motor, which is a direct indicator of the torque on the scroll.
In a simple execution, any increase in the back-drive motor’s current causes a proportional increase in the back-drive speed. This increased scroll speed pushes out the sludge faster, reducing the scroll's load.
A significant increase in the scroll torque (as determined by the motor current) may trigger a temporary feed shut-off to allow the scroll to catch up with the bowl’s sludge. Any further increases in the motor current (torque) trigger an automatic decanter shut-down with an ‘over-torque alarm.
Dolphin Centrifuge decanter control systems feature these advanced functions for automatic scroll speed adjustment during the process.
The differential speed between the decanter bowl and scroll is the key to the function of a decanter centrifuge. The differential speed has important functional benefits to the equipment user. In advance decanter control, the differential speed is key to fine-tuning the decanter operation and safety features.
by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge