Table Of Contents
A disc stack centrifuge is a specifically designed industrial centrifuge with a stack of cone-shaped discs. The additional surface increases the settling area multi-fold and reduces the settling distance.
Combined with the high g-force (7,000 Gs), these features make this centrifuge a highly efficient separation device for fine particles (0.5µm) and immiscible liquids.
It is important to note that a disc stack centrifuge is also known as a disc bowl, conical plate, disk stack, or disc stack separator.
A disc-stack centrifuge separates immiscible liquids and suspended solids from contaminated fluids. G-forces up to 10,000 Gs cause the particles’ sedimentation on the disc surface where they flocculate and move toward the centrifuge bowl periphery.
The accumulated solids are self-ejected or manually removed from the disc centrifuge bowl.
Think of the discs in the centrifuge bowl splitting the liquid column into thin slices between the inter-disc space. The rotating discs impart the rotational velocity to the incoming stationary (non-rotating) fluid quickly due to the liquid’s viscosity.
The following table lists the specifications of common disc stack centrifuges manufactured by Alfa Laval.
|Centrifuge Model||Alfa Laval MOPX 205||Alfa Laval MOPX 207||Alfa Laval MOPX 213|
|Rated Capacity – GPM||21||30||90|
|Bowl Speed – RPM||7,600||6,325||4,140|
|Bowl Volume (Ltrs)||3.1||7.5||29|
|Drive Motor – kW||4||7.5||16|
|Max. Power (Startup) – kW||6.5||10.3||18|
|Running Power (Full Capacity) – kW||2.4||6.4||10.4|
|Centrifuge Weight (w/o Motor) – Kg||430||785||1,290|
|Bowl Weight – Kg||56||125||400|
The disc cone angle is the angle between the disc surface and a horizontal axis. The following image illustrates the cone angle of a small disc stack centrifuge.
The typical disc cone angle is between 40 degrees and 60 degrees, depending on the centrifuge application. For example, the cone angle of the oil centrifuge is less than that in milk centrifuges due to difference in the separation application.
The cost of a disc-stack centrifuge depends on several factors. The main factors affecting the cost of a centrifuge are listed below in order of influence.
New centrifuges from the manufacturer are the first choice for most but are often not cost-feasible. The next option is a re-manufactured centrifuge from the OEM or a reputed centrifuge company. And finally, used ‘As-Is’ centrifuges are also an option but not recommended for safety reasons.
A small-capacity disc centrifuge from name-brand OEM starts at around $15K for a bare centrifuge. The same centrifuge with all options, including a skid, control system, wiring, alarms, etc. goes up to over $40K, depending on options. On the other end, high-capacity, food-grade centrifuges cost over $750K.
A base re-manufactured disc centrifuge costs 40% to 60% of new machines of similar capacity. However, with additional options, a factory-new centrifuge’s cost rises faster than a similarly equipped re-manufactured centrifuge with new optional accessories.
The size of a centrifuge refers to the processing capacity of the machine. Bigger, higher-capacity centrifuges cost more than smaller-capacity units, obviously. However, it is important to mention that the increase in the cost of a larger centrifuge is not proportional to the capacity increase. In other words, the cost of a centrifuge is not directly proportional to its processing capacity.
For example, a disc centrifuge with a rated capacity of 10 Gallon-per-Minute costs only about 30% more than a centrifuge rated at 5 Gallons-per-Minute.
Disc centrifuges from established, name-brand manufacturers such as Alfa Laval or Westfalia (GEA) cost more than those manufactured by Asian manufacturers (China, India, Turkey, etc.). The price multiplier is anywhere from 2 to 5 times.
Alfa Laval industrial centrifuges are high-quality machines known for their durability and longevity. Such companies have decades of manufacturing experience and also offer worldwide service and parts distribution networks. These advantages more than justify the related cost premium.
Optional equipment is often integrated with disc centrifuges to enhance the separator's performance. Accessories such as feed pumps, electric pre-heaters, pre and post-filters, sludge handling systems, etc., can add significant costs to the centrifuge system.
Actual process conditions and process fluid properties often define the options that would benefit the user. For example, an in-line heater will add to the system’s cost, but it could double the centrifuge throughput.
Operating cost refers to the actual cost incurred during the regular operation of the centrifuge system. Since centrifuges do not use any consumable filters or media, the only operating cost is electricity costs to run the centrifuge.
The following table highlights the example of operating costs per gallon for a typical operation such as diesel fuel purification. This table assumes a cost of 0.20 cents/kW-hr of power.
|Centrifuge Model & Capacity||MAB 103|
(2 GPM on Diesel)
(22 GPM on Diesel)
(62 GPM on Diesel)
|Motor Power (kW)||1||5||15|
|Electricity Cost / Hour||$0.20||$1.00||$2.20|
|Gallons / Hour||120||1,320||3,720|
|Electric Cost/Gal||0.16 Cents||0.075 Cents||0.06 Cents|
From the above example, the operating costs are negligible compared to filters or other media-based separation methods.
Maintenance costs for disc centrifuges are primarily related to the consumable spare parts needed for regular maintenance. The following table lists the spare parts cost for routine maintenance of the centrifuge we highlighted in the operating cost example above.
|Intermediate Service Kit (2/year)||$300||$450||$950|
|Major Service Kit (1/year)||$650||$1,400||$2,200|
|Total Spare Parts Cost per Year||$950||$1,850||$3,150|
|Gallon / Year (200 x 8 hr Days)||192,000||2,112,000||5,952,000|
|Maintenance Cost (per Gal)||0.5 Cents||0.1 Cent||0.05 Cent|
If you have simple, routine questions: We have condensed our 40+ years of disc-stack centrifuge experience into 101 Frequently Asked Questions about Disc Stack Centrifuges!
The drive motor is the main power consumption in a disc stack centrifuge. It is industry practice to calculate the kW required per m3 or gallon of fluid processed.
Several design improvements in centrifuges have led to reduced power consumption. There are certain specific areas where the design changes have been most beneficial.
The reduction of drive train losses from belt & gear drive transmissions is achieved with direct-drive configurations with inverters.
Internal bowl turbulence reduction with improved designs has also contributed to power consumption optimization.
|Centrifuge Capacity||Motor HP||Running Amps @ 460 VAC||Power Consumption||Power Consumption / Gallon|
|5 GPM||2 HP||2.5 A||1 kW||0.20 kW per GPM|
|10 GPM||4 HP||5 A||2.2 kW||0.22 kW per GPM|
|40 GPM||10 HP||9 A||7.5 kW||0.18 kW per GPM|
|60 GPM||15 HP||12 A||11 kW||0.18 kW per GPM|
A disc stack centrifuge has a rotating bowl with closely spaced conical discs. These discs split the incoming fluid into thin layers, imparting the bowl's rotation to the process fluid instantaneously. The fluid's rotation generates high centrifugal forces causing the heavier solids to separate from the fluid causing separation.
The conical discs partition the fluid column into layers. The process fluid passes radially between these discs. The close proximity of these discs decreases the settling distance for the sediment, thus enhancing settlement speed and efficiency.
The design features of a disc stack centrifuge feature a rotating bowl with a set of thin, closely spaced conical discs within. The fluid to be separated passes through the inter disc space causing the fluid to also rotate. The centrifugal force generated by the rotation causes the solids of higher density to settle on the disc surface while the separated fluid flows out of the bowl. This separation of the solids from fluid is the primary function of the disc stack centrifuge.
A set of conical plates (discs) that are vertically stacked is the disc stack in a disc stack centrifuge. The centrifuge bowl houses this stack of thin plates. Each disc has thin metal strips attached to it, and these strips act as vertical spacers creating a space between the discs.
A disc stack centrifuge has a particle size efficiency of 0.5-microns for metal particles and 1-micron for non-metal particles.
Disc centrifuge sizing is based on the flow rate of each centrifuge for a specific fluid. For example, all Alfa Laval centrifuges have an OEM published flow rate chart, which provides the expected capacity for specific fluids like diesel, fuel oil, hydraulic oil, etc.
The capacity of these centrifuges ranges from 2 gallons per minute to over 500 gallons per minute.
The user should also consider another essential fact about disc centrifuges. The ‘rated’ capacity of a centrifuge has little relevance in the practical application of centrifuges. The rated capacity is the hydraulic swallowing capacity of a centrifuge to prevent overflow.
In real terms, the actual processing capacity is lesser than the ‘rated’ capacity. Based on the discussion above, the actual processing capacity depends on the process-liquid and contaminant properties.
In other words, the same centrifuge model has different capacities for different liquids. This difference goes back to Stokes’ law, wherein the centrifuge efficiency is dependent on the fluid's viscosity, amongst other factors.
The following table lists the sizing and capacity of Alfa Laval disc stack centrifuges for different fuels and oils.
|Centrifuge Type||Alfa Laval MOPX 205||Alfa Laval MOPX 207||Alfa Laval MOPX 310||Alfa Laval MOPX 213|
|Rated Capacity - GPM||25||34||68||90|
|Capacity On ↓|
|Distillate (3 cSt)||23||31||65||82|
|Diesel Fuel (13 cSt)||15||22||56||62|
|Turbine Lube Oil (ISO 32)||14||20||52||56|
|Hydraulic Oil (ISO 15)||14||20||51||54|
|Light Fuel Oil (100 cSt)||11||19||39||41|
|Medium Fuel Oil (380 cSt)||7||12||23||24|
|Heavy Fuel Oil (600 cSt)||5||8||17||16|
As with all separation equipment, disc centrifuges have a specific niche area of application where they deliver the optimum solution for separating solids from liquids or liquids from solids and liquids.
However, they also have limitations to their applications, and we have discussed the disadvantages of disc centrifuges in a different article.
There are several benefits of disc centrifuges over decanter centrifuges. The following table highlights key benefits with a brief explanation of each.
|Feature||Description||Why Does it Matter||Advantage|
|Particle Size Efficiency||A disc centrifuge can separate particles as small as 1 micron as compared to a decanter centrifuge, limited to particles larger than 50 microns.||A disc centrifuge's higher g-force (~7,000 Gs) allows it to separate much smaller particles than a decanter with a lower g-force (~ 3,000 Gs).||This particle size efficiency makes a disc centrifuge uniquely suitable for removing small particles for polishing applications, which is impossible with a decanter centrifuge.|
|Liquid / Liquid Separation||With its high g-force, a stack disc centrifuge can easily separate immiscible liquids. Though there are three-phase decanters (tricanters), they are not suitable for separating liquids with similar specific gravities.||A higher g-force is required to separate liquids with similar specific gravities. A disc centrifuge has a higher g-force, while a decanter centrifuge does not.||The disc-centrifuge can separate one liquid from another, even if their specific gravities are very similar. Again, some decanter centrifuges that can separate liquids from others need their specific gravities to be significantly different.|
|Adjusting Sludge Dryness||It is impossible to change the separated solid wetness in the decanter centrifuge while operating. However, it is possible to adjust the sludge dryness in a disc-stack centrifuge.||A decanter centrifuge continuously ejects the separated sludge, and the sludge dryness depends on the decanter’s pond depth. A disc centrifuge ejects the separated sludge intermittently based on a timer.||The decanter needs to be stopped to adjust the pond depth to change the cake's dryness. In a disc centrifuge, adjusting the ejection cycle frequency can help change the sludge moisture content while the centrifuge is operating.|
|Physical Size or Footprint||A disc centrifuge has a smaller footprint than a decanter centrifuge of similar capacity.||Decanter centrifuges are oriented horizontally, whereas disc centrifuges are vertically oriented. This orientation allows the disc centrifuge to have a smaller footprint than decanters.||Disc centrifuges fit within limited space-constrained areas. This ability is advantageous for installing a centrifuge in an already existing facility. A decanter would be tougher to fit under similar conditions.|
Disc centrifuges are capital-intensive equipment. However, reduced maintenance costs and higher production levels quickly make up for initial expenses. Typically the upfront cost is recovered within 1 to 2 years of operation.
A disc centrifuge has several benefits compared to filters. The following list highlights some of these benefits.
The following table compares disc-stack centrifuges’ features and costs with other conventional separation methods, i.e., filters and coalescers.
|Type of Separator||Disc Centrifuge||Filtration||Coalescing|
|Free Water Separation||Yes||No||Yes|
|Dissolved Water Separation||No||No||No|
|Total Cost of Ownership||Low||High||High|
* Chemically bonded emulsions need demulsifiers before centrifugation.
Compared to the disc-stack bowl, the rotating surface is the bowl wall in the chamber bowl (disc-less) centrifuge. Depending on the fluid layer’s depth, the inner fluid particles (away from the bowl wall) rotate much slower than the liquid particles in contact with the bowl wall.
This reduced rotational velocity decreases the separation efficiency considerably in chamber bowl centrifuges.
The liquid pond in the decanter centrifuge has similar limitations. The rotating bowl body contacts the fluid adjacent to it. This outer layer contact limits the separation efficiency of the decanters to bulkier and denser particles.
That is one reason the decanter centrifuge has a particle separation range higher than the one for disc-stack centrifuges.
The discs in the disc-stack centrifuge bowl also increase the settling area for sedimentation. Considering the g-force and the discs’ surface area, the effective settling area in a disc-stack centrifuge equals the size of multiple football fields!
For more details, please read the Difference between Decanter and Disc Centrifuge.
There are two types of calculations associated with disc stack centrifuges. These calculations are summarized below.
The first calculation defines the efficiency of a disc stack centrifuge through the relevant formula.
The second calculation helps a potential centrifuge user to gauge the effectiveness of the types of industrial centrifuges based on a combination of factors i.e., settling area and G-force.
The governing principle of disc stack centrifuges is based on Stoke's law which determines the separation of particles within a fluid acted upon by gravitational forces.
In the above formula, it is clear that the settling velocity (V) of the particle moving in the direction of the gravitational force is directly proportional to its density and size. This velocity is inversely proportional to the viscosity of the fluid.
A higher settling velocity allows the particles to reach and accumulate on the disc surface quickly, thereby enhancing the separation efficiency and settling minute particles. Conversely, a thicker (more viscous) fluid reduced the settling velocity, thereby increasing the time required by the particles to reach the settling surface, which reduces process efficiency.
Referring to the illustration above, the Area Equivalent (Ae ) is the settling area available to the separating liquid at centrifugal acceleration. It is calculated by the following formula.
Ae = As x g
As = Surface Area
g = G-Force
The following table compares the Ae of the disc centrifuge versus the decanter centrifuge and a chamber bowl centrifuge. This table illustrates the comparative effectiveness of the disc-stack centrifuge versus these other ‘low – G’ centrifuges.
|Chamber Bowl Centrifuge|
300 mm x 250 mm @ 1000 G-Force
|Decanter Centrifuge (NX-314)|
350 mm x 300 mm @ 3,500 G-Force
|Disc Stack Centrifuge|
Multi-Disc Area 10 m2 @ 7,000 G-Force
|Area Effective (Ae)||250||780||19,000|
Disc-stack centrifuges are available with two bowl configurations. A ‘self-cleaning’ (aka auto ejecting) bowl has the built-in ability to eject the separated sludge periodically with the machine stoppage. A ‘manual-cleaning’ (aka solids retaining) bowl retains the separated sludge and needs to be manually cleaned periodically. This needs the centrifuge to be stopped for the operator to open and clean the sludge from the bowl.
The process fluid enters the bowl through the feed tube. The disc stack imparts the rotating velocity to the incoming liquid. The heavy phase (water) moves outward due to the centrifugal force.
It displaces the lighter oil (light phase) inwards towards the bowl center. The clean oil rises to the top of the bowl and exits the bowl through the paring disc pump (4).
The heavy phase (water) travels over the top disc (5) and exits the bowl through the water-paring disc (6).
The heaviest phase (Solids) collects at the bowl periphery. The ‘self-cleaning’ bowl opens intermittently to discharge the separated solids periodically. The mechanism operates as follows.
The hydraulic operating system opens and closes the bowl at predetermined intervals. The sliding piston (7) is pushed down, which opens the bowl body’s sludge-ejection ports. The extremely high centrifugal g-force within the bowl instantly ejects out the separated sludge.
Due to their intermittent sludge ejection feature, ‘self-cleaning’ disc centrifuges are limited to solids content up to about 8% (%v/v). Higher solids content leads to a higher possibility of bowl clogging and frequent sludge ejection cycles. Both of these conditions are not desirable from the centrifuge perspective.
In the ‘manual-clean’ bowl, the process fluid comes into the bowl from the top (1). The distributor (2) imparts rotation to the fluid before the fluid enters the disc stack (3). The separation happens between the discs of the stack.
The high g-force pushes the heavier fluid (heavy-phase) and solids toward the bowl periphery (wall). While the solids collect on the bowl watt, the water (heavy-phase) passes through the passage over the top disc (4) to the heavy-phase outlet (5).
The oil (light-phase) is displaced towards the inner center of the bowl. It rises through the distributor channels to reach the paring disc pump (6). This pump pumps the clean light oil out of the centrifuge bowl.
The operator must manually remove the collected solids from the bowl by stopping the centrifuge at periodic intervals.
These solid-bowl centrifuges are much simpler than the ‘self-cleaning’ bowls described above. They do not have the hydraulic sludge ejection system built into the bowl.
This simplified design does not require the operating water system (like the ‘self-cleaning’ centrifuges) for hydraulic operations underneath the bowl. This makes these ‘manual-clean’ centrifuges easier to install, maintain, and operate.
However, this ‘solids-retaining’ design also limits these centrifuges’ applicability to processing fluids with no solids or negligible amounts of solids. In other words, these centrifuges are ideal for liquid/liquid separation applications.
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