Disc Stack Centrifuge | Difference Between Purifier and Clarifier

Purifier Centrifuge Bowl Section

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The primary difference between a purifier and clarifier centrifuge is that a purifier separates a liquid from another liquid and solids simultaneously. Therefore a purifier is a three-phase separator. A clarifier centrifuge separates a liquid from solids, which is also known as two-phase separation.

This illustrated article explains the technical differences between these two types of disc-stack centrifuges.

Purifier Centrifuge Operation

A purifier is a disc-stack centrifuge that separates one liquid from another liquid and solid particles (if any). A purifier centrifuge is capable of continuously separating two liquid phases and one solid phase.

The illustration below shows the bowl cross-section of a purifier centrifuge. The incoming process fluid enters the disc-stack through the hole in the distributor. The corresponding holes on the bowl discs line up and create a vertical passage for the process liquid.

As it passes through the discs, the centrifugal force causes the solids to move towards the bowl periphery. The solids being the heaviest, they accumulate in the sludge space at the outer edge of the bowl. These separated solids are intermittently ejected from the bowl by the self-cleaning operation of the bowl. In manual cleaning centrifuges, the operator needs to manually remove the sludge from the bowl.

The dense phase (water) also moves radially outward from the bowl rotation axis. The water collects in the water space, as shown in the diagram. The incoming separated water displaces and pushes this accumulated water over the top disc into the water chamber at the top of the bowl assembly. The water gets pressurized by the water paring disc and escapes the bowl through the heavy phase outlet.

The separated light phase (oil) moves towards the center of the rotating bowl. It rises through the oil passage on the distributor. The oil then enters the light phase chamber inside the top disc. The paring disc pump expels the separated oil under pressure from the bowl.

Purifier Centrifuge Bowl Cross Section
Disc Centrifuge - Purifier Bowl Cross Section

Purifier Centrifuge Applications

In most purifier centrifuge applications, the light phase in the predominant phase and the heavy-phase is a much smaller proportion of the process fluid. The following are some real-world purifier centrifuge applications.

  • Lube oil separation from water and solids
  • Diesel fuel separation from water and sludge
  • Hydraulic oil purification from water and sludge
  • Machining coolant separation from tramp oil and metal fines
  • Waste vegetable oil separation from water and food particles
  • Crude oil recovery from salt-water and tank bottom sludge
  • Clarified butter separation from water and milk solids
  • Milk classification from fat and solids
  • Animal fat purification from water and particles
  • Fish oil purification from water and fish-meal

Clarifier Centrifuge Operation

A clarifier centrifuge separates a liquid phase from the solids in the fluid. In other words, it is a liquid-solid separation device. Therefore a clarifier continuously separates the solids from a liquid carrying solids.

The diagram below shows the cross-section of a clarifier centrifuge bowl.

The process fluid enters the bowl through the distributor to the underside of the distributor. A blind-disc (without the holes) is assembled at the bottom of the disc-stack. This closure prevents the process fluid from entering the disc stack through the bottom holes or fluid passage mentioned above.

The incoming liquid enters the disc-stack from the outside. As the fluid passes through the disc-stack, the heavier solids move outward due to the centrifugal force.

Also, in a clarifier centrifuge, the gravity disc is replaced by a sealing ring (clarifier disc), closing the passage of water to the water chamber in the bowl top. This closure of the heavy phase pathway essentially blocks one fluid passage from the bowl.

All the processed fluid moves towards the only exit in the bowl. The liquid, therefore, passes through the disc-stack into the fluid passage between the distributor’s ribs. The fluid then enters the light-phase chamber inside the top-disc.

The centripetal pump (paring disc) converts the liquid’s rotational energy into pressure, and the pressurized fluid exits the bowl through the light phase pathway.

Disc Centrifuge Clarifier Bowl Section
Disc Centrifuge - Clarifier Bowl Cross Section

Clarifier Centrifuge Applications

Clarification applications involved the separation of solids from one liquid phase. Clarifier centrifuges are used in all of these applications.

  • Beer clarification from yeast
  • Wine clarification from solids
  • Ethanol clarification from waxes and lipids for CBD oil
  • Ethanol from marijuana biomass for THC recovery
  • Fruit juice separation from pulp
  • Chemical and catalyst separation
  • Pyrolysis oil separation from biochar
  • Quench oil clarification from ash and scale particles
  • Algae cell harvesting from water
  • Humus biomass separation from liquid humus fertilizer
  • Silicon dioxide from water in wafer grinding

Conversion Between Purifier and Clarifier Centrifuges

As mentioned above, two essential components are required to convert a purifier to a clarifier centrifuge. The first and most crucial part is a clarifier disc or a sealing ring. A standard purifier has a gravity disc installed in it, which regulates the bowl’s oil-water interface location. A seal ring, which is a gravity disc with the smallest opening, replaces the gravity disc.

The second component that needs to be replaced is the bottom bowl-disc in the disc-stack. A blind disc replaces the bottom disc. This replacement closes out the rising fluid pathway through the disc stack.

It is important to note that though it is possible to convert all purifiers to a clarifier, it is not always possible to switch a clarifier to a purifier centrifuge. This inability to modify is because some centrifuges designed as clarifiers do not have the water chamber. They may also lack the water paring disc pump and not have a provision to install a gravity disc.

Operational Differences Between Purifier and Clarifier Centrifuges

A clarifier is a simplified version of a purifier centrifuge. This difference between the centrifuge configurations reflects in the operation of either design.

Firstly, due to the absence of the gravity ring, a clarifier does not require sealing liquid. Since the heavy phase outlet is closed out by the sealing ring, the incoming process liquid fills the bowl and can only exit the bowl through the light phase outlet.

In a purifier, sealing liquid is needed before feeding the process fluid to the bowl. This liquid is also known as the priming fluid and typically consists of the heavy phase of the process fluid. The priming liquid prevents the incoming process fluid from escaping the bowl through the heavy phase outlet.

Secondly, back-pressure on the light phase outlet is essential for the operation of a purifier. The back-pressure stabilizes the liquid column within the bowl and helps keep the paring disc pump immersed in the light phase. Backpressure is not essential to the operation of a clarifier centrifuge.

Thirdly, selecting the correct gravity ring is crucial for effectively separating the two phases in a purifier centrifuge. A clarifier does not have a gravity ring, and therefore problems associated with incorrect gravity disc do not apply to clarifiers.

Fourthly, liquid seal breaking is a common occurrence in purifier centrifuges. When the heavy phase or sealing liquid is not present in the bowl or escapes the bowl during operation, the light phase exits the bowl through the heavy phase outlet. This issue is known as a “liquid seal break” condition. Since there is no sealing liquid in a clarifier, this problem does not occur in clarifiers.

Concentrator Centrifuge

A concentrator centrifuge is a purifier centrifuge specially configured to separate fluids with a large proportion of the heavy phase (water) from a smaller percentage of the light liquid (oil). It is also a three-phase centrifuge, which implies it simultaneously separates the solid particles as well.

An example of a concentrator application is the separation of water-based machining fluid from tramp oil and metal fines. In this case, the coolant is mainly water and is the majority phase that the user wants to separate from the tramp oil.

A purifier centrifuge with minor modifications is known as a concentrator. The main change is the position of the holes in the distributor and disc-stack. The holes in a concentrator disc-stack are moved radially inward towards the axis of rotation. This change allows for more settling area for the coolant, which is the desired fluid.


In summary, a clarifier centrifuge is a simplified version of a purifier centrifuge. The clarifier version is modified to separate the liquid from solids, whereas the purifier centrifuge can separate the primary liquid from another liquid and solids.