Centrifuges: Function and Types

Michael
Nasser

Centrifuges make use of the mechanical force to separate fluids and solids of varying densities. Force is applied perpendicularly to the axis of spin. In this article, you will learn about the types of centrifuges, applications, and maintenance and repair considerations.

Centrifuges are segmented into two groups: filtration centrifuges and sedimentation centrifuges. Filtration centrifuges rely on a filter to capture heavy particles near the center of the centrifuge. Sedimentation centrifuges capture the high-density particles at the outer part of the centrifuge. The type of centrifuge employed depends on the separation type, operating conditions, and the desired result.

Filtration Centrifuges

filtration centrifuge

Filtration is carried out via a rotating basket which is equipped with a centrifugal filter. The centrifugal head puts pressure on the layered liquid that exists on top of the cake and the fluid that flows through the cake, is subject to centrifugal forces that assist in the separation process and the low-density particles rise to the top whereas the denser particles remain at the bottom.

Sedimentation Centrifuge

Sedimentation centrifuges produce an outward movement of denser particles due to the presence of centrifugal acceleration. A spinning force that is 1000-20000 times more than gravity causes lighter particles or less-dense particles to migrate to the center.

The tendency of suspended particles to settle outside of the fluid and rest against a barrier is called sedimentation.

The main types of centrifuges that work according to this principle are: tubular bowl, decanter, and disk stack separator.

Tubular Bowl

Based on very simple geometry, this centrifuge is one the oldest in practice. It has a long, rotating tube that rotates on bearings. The process stream enters from the bottom of the centrifuge. The presence of centrifugal forces separates the solid particles which adhere to the walls of the bowl whereas the liquid comes to the top of the centrifuge.

These centrifuges also have the capacity to dewater and can only accommodate a limited amount of solid particles. Skimming and pumps may be necessary to eliminate foam produced during the process.

Disk Stack Separator

A disk stack separator aggregates the solid particles in the outer part of the bowl. Solids are discharged in a number of different ways, but mostly commonly via a basic nozzle. These nozzles allow the thick slurry to pass through them, and the continuous rotation of the bowl assists in the separation of solid particles. A more complex design may include the use of valve nozzles which open up automatically when the bowl reaches a certain value and then opens up again when there is enough room for more slurry.

Newer designs include a complex bowl design, where shells open in a circumferential manner. The depth of solids controls the opening mechanism.

disk stack separator
Courtesy: ScienceDirect.com

Decanter

A decanter centrifuge works on the principle of gravitational force as particles separate by continuous rotation. This separation process takes about a few seconds because of the increased speed that can be 1,000 to 4,000 times that of gravity.

decanter centrifuge
Courtesy: Machineto.com

The working principle of a decanter can be explained in six simple steps:

  1. Slurry is inserted via the connecting pipes that go onto the conveyor.
  2. The slurry moves into the nozzle by the conveyor.
  3. High-speed rotating bowls induce a gravitational force.
  4. Particles separate in a matter of seconds.
  5. Solid particles discharge via a nozzle at the top.
  6. The purified liquid collects via a separate outlet.

There are a number of types of decanters that are in practice and are application-specific in terms of design consideration. The pitch of the conveyor, bowl surface, feed zone design, and the material that used in its construction, all play an important role in determining which decanter is more suitable to be used as per your needs.

Applications

Centrifuges are low-weight, easy to design, and easy manufacture, which makes them a favorable option for many separation applications. Compressors used in centrifuges are low maintenance, reliable and require no special foundations required. Centrifuges are sensitive to changes in gas composition, requiring caution in regards to inlet feed control. There can be issues regarding choking, stalling, and even surging if cleaning is done frequently or improperly. In high compression applications, centrifuges may be unstable.

The separation procedure is used in a number of industries such as oil refineries and in solids industries such as polystyrene and concentrate solids. Decanter centrifuges are usually employed in such industries mainly because they handle heavier particles, and work efficiently on a wide range of feed concentrations and densities.

Some common industry applications of centrifuges are hydrocarbon separation, recycling of waste, oil additives and lubrication, refining processes, and municipal wastewater and cleaning.

Maintenance and Repair

The main concern of running a centrifuge is its sensitivity to inlet feed conditions. The inlet feed must be carefully monitored to ensure its liquid to solid ratio and flowrate are as anticipated. If the inlet feed conditions vary from design conditions, the centrifuge may clog or not remove the anticipated amount of sediment. The centrifuge must be washed out prior to startup. This is particularly of concern if the centrifuge has been setting for some time.

Depending on the type of centrifuge, there is usually a shutdown procedure that involves depowering and washing down the centrifuge in a prescribed manner. Routine maintenance should be per manufacturer’s recommendations but typically involves changing differential oil and bearing grease/oil in a prescribed manner as well as daily cleaning.

Centrifuge repairs should be performed by a qualified shop that deals with the rebuild and refurbishment of large industrial equipment. With proper maintenance procedures, catastrophic failure is rare. More frequently, a centrifuge excessively vibration. This condition can usually be remedied by by rotor balancing.