Decanter centrifuges process characteristics
The separation process in a decanter centrifuge relies on a few process characteristics such as centrifugal force or G-force, sedimentation rate and separating factor, differential speed between the conveyor and bowl, and clarity of the liquid discharge.Decanter centrifuges require a centrifugal force for the separation of the solids from the liquid. This characteristic is dependent on the radius of the centrifuge and its angular rotational speed. A decanter centrifuge applies a force equivalent to several thousand G’s, which reduces the settling time of the particles. It is also favored to maintain a large G-force, which will result in an improved separation.
The rate at which sedimentation occurs is an important characteristic of the decanter centrifuge separation process. The
sedimentation rate is influenced by the particle size, the shapes of the particles, their differential densities and the viscosity between the particles and the liquid. This process characteristic can be improved by utilizing flocculating agents. The sedimentation rate is also dependent on the separating factor of the decanter centrifuge, which is related to the centrifugal force.
The exterior bowl and the scroll conveyor rotate at different high speeds. This differential speed between the two is accountable for the sedimentation throughout the decanter centrifuge cylinder. A high differential speed results in a smaller residence time of the cake settlement, so it is necessary to keep the cake thickness to a minimum to avoid impairing the discharge quality.
Keeping the cake thickness to a minimum also aids in the improvement of the cake dewatering process. For this reason, it is necessary to obtain an optimal differential speed to balance the cake thickness and quality.
The characteristic above all affects the clarity of the liquid output which is dependent on the volumetric throughout rate, where a higher flow rate will result in a poor liquid clarity. Another characteristic that influences the clarity of the liquid output is the differential speed. A low differential speed results in a better clarity, therefore, aiding in the separation process. The G-Force also plays a role in the clarity of the liquid discharge. Higher G-force results in an increase in the separation of the solid particles from the liquid and yields a better clarity.

How does it work
The feed product is pumped into the decanter centrifuges through the inlet. Feed goes into a horizontal bowl, which rotates. The bowl has is composed of a cylindrical part and a conical part. The separation takes place in the cylindrical part of the bowl. The fast rotation generates centrifugal forces up to 4000 x g. Under these forces, the solid particles with higher density are collected and compacted on the wall of the bowl. A scroll (also screw or screw conveyor) rotates inside the bowl at a slightly different speed. This speed difference is called the differential speed. This way the scroll is transporting the settled particles along the cylindrical part of the bowl and up to the end conical part of the bowl. At the smallest end of the conical part of the bowl, the dewatered solids leave the bowl via discharge opening. The clarified liquid leaves through a paring disc (internal centripetal pump).
3-phase separation with a decanter

With a 3 phase decanter centrifuge, it is possible to separate 3 phases from each other in one process step only. For example, two liquids which cannot be mixed because of different densities (e.g. oil and water) are separated from a solids phase. The heavy liquid (water) collects in the middle between the oil and the solids layer. Thus the two liquids separated from each other can be drawn off from the decanter. The solids are transported via the scroll to the discharge openings as it happens also in 2-phase separation.
Typical applications of 3-phase separation are the production of edible oils such as olive oil, oil sludge processing, the
production of biodiesel etc.
Parameters and influencing factors of the separation

Feed, throughput and residence time

Through the feed, the separation medium to be processed can be input into the center of the infeed chamber of the scroll, where it is accelerated. The throughput will have an influence on the residence time.
Decanter Centrifuges Acceleration
The separation medium reaches its maximum speed in the decanter bowl, causing the solids to settle on the bowl inner diameter. A characteristic feature of the bowl is its cylindrical/conical shape.
Differential speed

There is a differential speed between the decanter bowl and the scroll, which is created by a gear unit on the industrial decanter centrifuges. The differential speed determines the solid content in the outfeed.
Filling volume / Weir discs or overflow weir
The clarified liquid flows to the cylindrical end of the bowl in the decanter centrifuge, from where it runs out through openings in the bowl cover. These openings contain precisely adjustable weir discs/weir plates by means of which the pond depth in the bowl can be set. The weir discs determine the filling volume of the bowl.

Advantages and limitations over competitive processes
Generally the decanter centrifuge has more advantages than disadvantages; however, there are some limitations when compared to other processes.
Decanter Centrifuges Advantages:
* Decanter centrifuges have a clean appearance and have little to no odor problems.
* Not only is the device easy to install and fast at starting up and shutting down but also only requires a small area for
operation compared to other competitive processes.
* The decanter centrifuge is versatile as different lengths of the cylindrical bowl section and the cone angle can be selected for different applications. Also, the system can be pre-programmed with various design curves to predict the sludge type, while some competitive processes, such as a belt filter press, cannot change the belt type to operate for different sludge types. Its versatility allows the machine to have various functions such as operating for thickening or dewatering.
* The machine can operate with a higher throughput capacity than smaller machines. This also reduces the number of units required.
* The device is simple to optimize and operate as it has few major variables and reliable feedback information.
* The decanter centrifuge has reduced labor costs compared to other processes, as it requires low continuous maintenance and operator attention.
* Compared to some competitive process such as the belt filter process, the decanter centrifuge has more process flexibility and higher levels of performance.

Decanter Centrifuges Limitations:
* The decanter centrifuges cannot separate biological solids with very small density differences, such as cells and viruses. A competitive process that is capable of separating these difficult-to-separate solids is the tubular-bowl centrifuge.
* The machine can be very noisy and can cause vibration.
* The device has a high-energy consumption due to high G-forces.
* The decanter centrifuge has high equipment capital costs. Hard surfacing and abrasion protection materials are required for the scroll to reduce wear and therefore reduce the maintenance of the scroll wear.