Disc Stack Centrifuge | Operation, Benefits, Sizing, Specs, Cost …


Alfa Laval MOPX 205 Duplex Centrifuge System Diesel

Disc Stack Centrifuge (aka Disc Centrifuge)

A disc stack centrifuge is a specifically designed centrifuge that uses a stack of cone-shaped discs. The additional surface increases the settling area multi-fold while also reducing the sedimentation distance. Combined with the high g-force (7,000 Gs), these features make the disc 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.

Operation

A disc 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, stationery (non-rotating) fluid quickly due to the liquid’s viscosity. 

Alfa Laval Disc Stack Centrifuge Bowl Parts
Alfa Laval Disc Stack Centrifuge Bowl Section

Difference Between Decanter and Disc Centrifuge

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 at a much slower speed 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 Decatenr and Disc Centrifuge.

Area Effectiveness

Chamber Decanter Disc Centrifuge Ae

Referring to the illustration above, the Area Equivalent (Ae ) is the settling area available to the separating liquid times the centrifugal acceleration.

Ae = As g

Where:
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

Specifications

Most disc-stack centrifuge specifications are generic as they apply to centrifuges manufactured by established manufacturers. However, some specifications are manufacturer specific. In the following table, we list the crucial specifications of centrifuges manufactured by Alfa Laval. 

  Alfa Laval MOPX 205 Alfa Laval MOPX 207 Alfa Laval MOPX 310 Alfa Laval MOPX 213
Rated Capacity – GPM 21 30 61 90
Bowl Speed – RPM 7,600 6,325 5,175 4,140
Bowl Volume (Ltrs) 3.1 7.5 13 29
Drive Motor – kW 4 7.5 11 16
Max. Power (Startup) – kW 6.5 10.3 16 18
Running Power (Full Capacity) – kW 2.4 6.4 9.5 10.4
Centrifuge Weight (w/o Motor) – Kg 430 785 1,060 1,290
Bowl Weight – Kg 56 125 220 400

Cost

The cost of a disc-stack centrifuge depends on several factors. The main factors affecting the price are listed below in the order of influence. The table below provides cost estimates for Alfa Laval and similar disc-centrifuges, depending on the factors listed below.

New, Remanufactured, and Used

New centrifuges from the manufacturer are the first choice for most but often not cost feasible. The next option is a remanufactured centrifuge from the OEM or a reputed centrifuge company. And finally, used ‘As-Is’ centrifuges are also an option but not recommended due to 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. go up to over $40K depending on options. On the other end, high-capacity, food-grade centrifuges cost over $750K.

A base remanufactured disc centrifuge costs between 40% to 60% of similar capacity new machines. However, with additional options, a factory-new centrifuge’s cost rises quickly than a similarly equipped remanufactured centrifuge with new optional accessories.

Size / Capacity

In general, disc-stack centrifuges have a higher throughput capacity as compared to chamber bowl centrifuges. In other words, chamber bowl centrifuges with their limited efficiency have a maximum capacity closer to the smallest disc centrifuges. 

For example, a typical chamber bowl centrifuge (with 3,000 Gs RCF) processes 10 to 40 Gallons per hour. The smallest Alfa Laval disc-stack centrifuge (with 8,000 Gs RCF) processes over 60 Gallons per hour. 

Manufacturer

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. 

Centrifuges manufactured by Alfa Laval 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.

Options & Accessories

Optional equipment is often integrated with disc centrifuges to enhance the performance of the separator. 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

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.

  MAB 103 (2 GPM on Diesel) MOPX 207 (22 on GPM Diesel) WHPX 513 (62 GPM on Diesel)
Motor Power (kW) 1 5 11
Electric Cost per Hour $0.20 $1.00 $2.20
Gallons Processed per Hour 120 1,320 3,720
Electricity Cost (per 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

Maintenance costs for disc centrifuges are primarily related to the consumable spares parts needed for regular maintenance. In the following table, we have listed the spare parts cost for routine maintenance of the centrifuge we highlighted in the operating cost example above.

  MAB 103 (2 GPM on Diesel) MOPX 207 (22 on GPM Diesel) WHPX 513 (62 GPM on Diesel)
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
Based on the above, we can calculate the estimated maintenance cost per gallon of diesel processed, as follows.
Maintenance Cost (per Gal) 0.5 Cent 0.1 Cent 0.05 Cent

 

Sizing and Capacity

Disc centrifuges sizing is based on the flow rate of each centrifuge for a specific fluid. For example, all Alfa Laval disc centrifuges have an OEM published flow rate chart, which provides the expected capacity for specific fluid like diesel, fuel oil, hydraulic oil, etc.

The user should also consider another essential fact about disc centrifuges. The ‘rated’ capacity of a centrifuge has not much 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 is totally dependent on the process-liquid properties 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 viscosity of the fluid, amongst other factors.

The following table lists the sizing and capacity of Alfa Laval disc centrifuges for different fuels and oils.

  Alfa Laval MOPX 205 Alfa Laval MOPX 207 Alfa Laval MOPX 310 Alfa Laval MOPX 213
Rated Capacity – GPM 25 34 68 90
Capacity (GPM) 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

 

Benefits

As with all separation equipment, disc centrifuges have a specific niche area of application where they deliver the optimum solution for the separation of 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.

Benefits over Decanter Centrifuge

Decanter and Disc Centrifuge Particle Size Efficiency
Decanter and Disc Centrifuge Particle Size Efficiency

There are several advantages of disc centrifuges over decanter centrifuges. The following table highlights key benefits with a brief explanation of each.

Feature What? Why? Benefit

Particle Size Efficiency

A disc centrifuge can separate particles as small as 1 µ as compared to a decanter centrifuge, which is limited to particles larger than 50 microns. The higher g-force (~7,000 Gs) in a disc centrifuge 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 disc stack centrifuge can separate immiscible liquids very efficiently. 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 liquid even if their specific gravities are very similar. Again, some decanter centrifuges that can separate liquids from each other need their specific gravities to be significantly different.

Adjusting Sludge Dryness

It is not possible to change the separated solid wetness in the decanter centrifuge. 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 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-stack centrifuge has a smaller footprint compared to 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 compared to decanters. Disc centrifuges fit within limited space-constrained areas. This ability is an advantage for installing a centrifuge in an already existing facility. A decanter would be tougher to fit under similar conditions.

 

Benefits over Filtration

Disc centrifuges are capital intensive equipment. However, reduced maintenance costs and higher production levels make up for initial expense quickly. 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.

  • Particle and water separation in a single operation
  • Removal of particles down to a 1-micron level
  • Consistent throughput without flow reduction (filters clog)
  • Water separation is continuous
  • Minimized O&M cost (Operation and Maintenance)
  • Negligible disposal costs (no filter or membrane media to dispose of)
  • No loss of lubricant additives

The following table compares disc-stack centrifuges’ features and costs with other conventional separation methods, i.e., filters and coalescers.

  Disc Centrifuge Filtration Coalescing
Solids Separation Yes Yes No
Free Water Separation Yes No Yes
Dissolved Water Separation No No No
Emulsion Breaking Yes* No Yes*
Costs
Operating Low High High
Equipment High Low Medium
Total Cost of Ownership Low High High
* Chemically bonded emulsions need demulsifiers before centrifugation. 

Disc Centrifuge Bowl Types

Disc-stack centrifuges are available with two bowl configurations. A ‘self-cleaning’ (aka auto ejecting) bowl has the built-in ability to ejected 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 on a periodic basis. This needs the centrifuge to be stopped for the operator to open and clean the sludge from the bowl.

Self-Cleaning Centrifuge Bowl Operation

Disc Stack Centrifuge Bowl
Self Cleaning Centrifuge 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) collect 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.

 

Manual Clean Centrifuge Bowl Operation

Industrial Centrifuge by Alfa Laval
Solid Bowl Centrifuge Bowl

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 disc of the disc-stack.

The high g-force pushes the heavier fluid (heavy-phase) and solids towards 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 out the clean light-phase out of the centrifuge bowl.

The collected solids must be manually removed from the bowl by the operator 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.

Remanufactured Alfa Laval Disc Centrifuges in Stock at Dolphin

Alfa Laval WHPX405 Quench Oil CentrifugeIndustrial-Waste-Oil-Self-Cleaning-CentrifugesIndustrial Centrifuge for Used DieselAlfa-Laval-WHPX513-Reconditioned-Waste-Oil-Centrifuge
Alfa Laval WHPX 405
‘Self-Cleaning’ Disc Centrifuge
15 GPM on Diesel Fuel
Alfa Laval MOPX 207
‘Self-Cleaning’ Disc Centrifuge
28 GPM on Turbine Lube Oil
Alfa Laval WHPX 510
‘Self-Cleaning’ Disc Centrifuge
30 GPM on Waste Oil
Alfa Laval WHPX 513
‘Self-Cleaning’ Disc Centrifuge
75 GPM on Distillate