5 Ways to Increase Disc Stack Centrifuge Efficiency | Illustrated Guide

Disc Centrifuge Nomogram

Users of disc-stack centrifuges often ask about ways they can increase the capacity or quality of the effluent from their centrifuge equipment. There are a handful of considerations that have a big impact on centrifuge efficiency.

Disc Stack Centrifuge Efficiency

A disc stack centrifuge’s efficiency is the centrifuge’s effectiveness in separating the different phases in the process fluid. The centrifuge parameter settings and process fluid properties impact centrifuge efficiency. Optimization of these factors helps increase centrifuge efficiency and leads to better separation and higher throughput.

Rated Efficiency

The rated efficiency of a disc centrifuge is an indication of the maximum ability of the centrifuge to separate particles and fluids. For example, the efficiency of a typical disc-stack centrifuge is at 1-micron particle size, where the particle has a specific gravity of 3 or higher.

For liquid-liquid separation, a disc centrifuge can separate immiscible fluids whose specific gravity difference is less than 0.05. However, multiple factors affect efficiency, as discussed below.

Rated Efficiency Calculation

Stokes’ Law

Stokes law defines the velocity of a solid particle as it travels through a fluid medium. The application of Stoke’s law (shown below) helps us calculate centrifuge efficiency by calculating the terminal velocity of the particles.

V = gRp2(ρpρ)/4.5μ


Where:
V = Terminal Velocity of Particle
Rp = Radius of Particle
g = Gravitation or G-Force
ρ
p= Density of Particle
ρ
= Density of Fluid
µ  = Viscosity of Fluid

From the formula above, we can see that the particle’s velocity in the liquid is directly proportional to the g-force, the difference in specific gravity and particle size. It is inversely proportional to the viscosity of the fluid.

A disc stack centrifuge’s efficiency is the centrifuge’s effectiveness in separating the different phases in the process fluid. The centrifuge parameter settings and process fluid properties impact centrifuge efficiency. Optimization of these factors helps increase centrifuge efficiency and leads to better separation and higher throughput.

In practical terms, the goal of centrifuge separation is to remove the smallest particle possible, quickly! Which, in turn, means the higher the terminal velocity, the better the separation. The five critical factors listed below are directly related to the variables in Stoke’s law above.

Factors Affecting Centrifuge Efficiency

For a given centrifuge for a specific application, the primary physical attributes such as bowl speed (RPM), and therefore the RCF is fixed. The inherent fluid properties are also mostly predefined. The following factors affect centrifuge efficiency and some or all of them are adjustable by the centrifuge operator.

Flow Rate

The flow rate is not directly related to the Stokes’ equation above, but it is inversely proportional to the particles’ residence time in the bowl. Based on the liquid and solid particle properties, the velocity is fixed. Therefore, a higher residence time gives the particles more time to reach the bowl wall and separate.

In real terms, a lower flow rate (GPM) lends higher residence time, which means better separation. Conversely, a higher flow rate means lesser retention time that leads to worse separation results.

The following graph demonstrates the inverse relationship between the flow-rate and solids separation efficiency of a disc centrifuge.

Flow Rate Particle Separation Efficiency
Flow Rate vs Particle Separation Efficiency

The following table contains the actual data of this real-world test conducted by Dolphin Centrifuge

Centrifuge Type Alfa Laval ‘Self-Cleaning’ Disc-Stack Centrifuge WHPX-405
Process Fluid Wastewater with Suspended Solids Contamination
Original Sample Contamination (%v/v) 3.2%

Gravity Disc Selection

Alfa Laval Gravity Disc California
Disc Centrifuge Gravity Disc

Optimization of the centrifuge is nothing more than adjusting the centrifuge physically to allow the centrifuge to exert maximum G-force on the particles, thereby increasing separation efficiency.

For example, in the case of clarification (liquid/solid separation), installing a clarifier gravity disc in the bowl fills the bowl with the fluid. A full bowl forces the particles to travel longer through the disc-stack towards the bowl center. The extended path means more residence time for the same flow-rate leading to higher separation efficiency.

Disc Centrifuge Nomogram
Disc Centrifuge Nomogram

The process to select a gravity disc for a disc centrifuge involves the use of a nomogram (shown above). The typical process is to choose the curve that represents the density ratio (left) of the two liquids (915 kg/m3). Then follow the curve to the process temperature vertical grid (120 F). At this point draw a horizontal line towards the right into the gravity disc zones chart. The intersection of this horizontal line with the vertical grid representing the flow-rate (15 GPM) defines the gravity disc size (92 mm).

It worth noting that the gravity discs come in size increments. In other words, there is one gravity disc for a range of fluid densities. This range can be fine-tuned by the use of custom gravity discs closer to the actual fluid properties to dial in better centrifuge efficiency.

Back Pressure

Following the above mentioned limitations of gravity disc availability in size increment, centrifuge back-pressure (on the light-phase) comes into play. Increasing the back-pressure on the clean fluid outlet (refer the bowl cross section diagram below) tends to push the oil/water interface outwards. This can be seen as ‘fine-tuning’ the gravity disc size through back-pressure increase. The movement of the intetreface leads to higher g-forces acting on the oil which leads to cleaner oil or higher centrifuge efficiency.

It should be noted that increasing the back-pressure beyod a certin point can lead to the interface moving past the top-disc. This condition leads to the light-phase (oil) exiting the bowl through the water outlet and is called a ‘break-over’ condition and is not desireable.

Process Fluid Temperature

Oil Viscosity versus Temperature
Oil Viscosity versus Temperature Graph

Fluid temperature is inversely proportional to the fluid viscosity in the case of thicker or higher viscosity fluids. By increasing the liquid’s temperature, the viscosity reduces, which helps increase the velocity per Stokes’ law above. This factor goes back to the fluid properties discussed above.

In practice, higher process fluid temperature results in lower viscosity, which lead to better separation efficiency.

Operating Water Temperature

Disc Centrifuge Bowl Cross Section
Disc Centrifuge Bowl Cross Section

A ‘self-cleaning’ disc-centrifuge uses ‘operating water’ to operate the sludge discharge mechanism for automatic sludge ejection. The above bowl cross section diagram shows the operating water chamber inside the bowl bottom. The process fluid is in the chamber above the operating water chamber and is separated by the sliding piston.

When viscous fluids are processed, the temperature of the process fluid is critical for efficient separation as explained above in ‘Process Fluid Temperature’ section. These fluids are processed at maximum processing temperature to get maximum separation efficiency.

If the operating water is cold, it has a cooling effect on the incoming hot process fluid. This is because the sliding pistong (steel) losses heat to the cold operting water and cools down the process fluid. 

Therefore. the temperature of the operating water should be as close to the temperature of the process fluid as possible. This consideration higher separation efficiency.

Summary

A disc-centrifuge’s efficiency is optimized by tweaking some physical aspects of the centrifuge machine and also by controlling the process fluid parameters as described above. Under certain conditions, these centrifuge and process fluid adjustments can lead to dramatic increases in efficiency which leads to higher throughput and better quality product.