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Disc stack centrifuge capacity is often confusing to first-time centrifuge customers primarily due to the wide range of capacities mentioned by used equipment sellers, online literature, and general users.
So, in this article, we will explain the difference between swallowing capacity, rated capacity, and actual real-world capacity of disc stack centrifuges.
We will also explain why a disc centrifuge could potentially have different processing capacities mentioned for the same application.
In the first section, we explain the various terms that people use to describe the capacity of an industrial centrifuge with a disc stack.
The swallowing capacity, also known as the hydraulic capacity, is the centrifuge passages’ physical flow. This capacity is just a theoretical estimate often unrelated to the centrifuge’s actual (real-world) capacity.
The rated capacity of a centrifuge is the flow rate the centrifuge can process of a low viscosity fluid such as distillate or water. This capacity is the maximum separation capacity of the centrifuge under optimum conditions.
The actual capacity of the centrifuge is highly dependent on the physical properties of the process fluid. A specific Alfa Laval centrifuge model has a recommended capacity table of flow rates for liquids of different viscosities.
The capacity of the centrifuge decreases as the viscosity of the process liquid increases. The examples below explain this phenomenon.
The following table shows the various capacities of an Alfa Laval centrifuge for fluids with different viscosities.
|Centrifuge Model||Diesel||Turbine Lube||Heavy Fuel Oil|
|1.5 cSt||2 cSt||100 cSt||380 cSt||600 cSt|
|Alfa Laval WHPX-405||16||15||11||6||2|
|Alfa Laval WHPX-409||36||34||24||14||10|
|Alfa Laval WHPX-513||71||68||47||28||19|
The main factors that affect disc stack centrifuge capacity are the fluid viscosity, contamination level, and centrifuge settings.
The rated capacity is just theoretical 'swallowing capacity', and it rarely is the true throughput of a centrifuge. A disc stack centrifuge can process flow rates lower than the rated capacity and the separation is typically better.
A user may increase the capacity of a disc stack centrifuge by heating the process fluid and thus lowering the viscosity, which leads to higher throughput.
The capacities of disc stack centrifuges are typically rated in m3/hr, which means cubic meters of fluid per hour. Other measures of centrifuge capacity are GPM which is Gallons per Minute, or GPM, which is Gallons per Hour.
The original bowl size and design determine a disc stack centrifuge capacity. Therefore, the user cannot modify to increase the capacity of the centrifuge.
The physical properties of the centrifuge and those of the process fluid influence the processing capacity of the centrifuge.
The bowl volume of a centrifuge is proportional to the physical size of the centrifuge. Larger bowl volumes allow a higher residence time for the fluid under the high centrifugal force.
Therefore, a physically large centrifuge has a higher processing capacity than a small centrifuge for the same process fluid.
The process fluid properties have a significant effect on the processing capacity of the centrifuge. The following is a list of fluid properties and their effect on the centrifuge capacity.
The formula below predicts the velocity of particles in a fluid and is known as Stoke’s equation. The equation shows that the particle’s velocity (separation efficiency) is inversely proportional to the fluid viscosity.
V = gRp2(ρp–ρ)/4.5μ
V = Terminal Velocity of Particle
Rp = Radius of Particle
g = Gravitation or G-Force
ρp= Density of Particle
ρ = Density of Fluid
µ = Viscosity of Fluid
A reduced particle velocity implies a longer separation time which means viscous fluids require higher residence time for the particles to separate. Since the residence time is inversely proportional to the flow rate, viscous fluids reduce the centrifuge processing capacity.
In the case of an oil processing centrifuge, a thicker, more viscous oil will have a lower flow rate than a lighter, less viscous oil through the same oil centrifuge.
The fluid viscosity is a crucial factor in centrifuge processing. The processing temperature of the fluid often reduces the fluid viscosity, which improves the separation. Therefore, when processing thick, viscous fluids, the temperature of the fluid reduces the viscosity and increases throughput capacity.
In the case of a self-cleaning disc stack centrifuge, the sludge discharge cycle interrupts the centrifuge processing. The percentage of solids is directly proportional to the frequency of sludge discharge cycles. And therefore, a higher solids percentage leads to more process interruptions which lead to lower processing capacity.
The particle size in the process fluid affects the capacity of the centrifuge. All other factors being equal, the centrifuge flow rate is directly proportional to the particle size separated. In other words, a lesser flow rate leads to longer residence time which allows for smaller particle size separation.
Therefore, the same centrifuge can separate smaller particles at a lower flow rate.
Denser particles separate quickly from the fluid per Stoke's law. Therefore, a particular centrifuge can process a higher capacity for a fluid with denser particles than the same fluid with lighter particles.
For example, consider water with sand particles and water with organic particles. The disc stack centrifuge capacity on water with sand will be higher than the centrifuge capacity on the water with lighter, organic particles.
The process flow rate (capacity) is inversely proportional to the fluid separation efficiency. In other words, the lesser the flow rate, the better the separation.
Therefore, the operator can reduce the flow rate to achieve better results with the same centrifuge.
From the above example, it is clear that a disc stack centrifuge's capacity varies for the same application depending on the desired results.
Alfa Laval centrifuges and other disc stack centrifuges have design parameters specific to the original application. For example, a centrifuge to separate diesel from water has fluid pathways that allow a large proportion of the light phase (diesel) and small quantities of the heavy phase (water).
The processing capacity of this particular centrifuge degrades when the predominant proportion of the fluid is water with a small quantity of diesel. This degradation is because the water pathway does not allow the high volumes of water to exit the centrifuge fast enough, which degrades processing capacity.
Thus, the original design of the centrifuge plays a vital role in the centrifuge processing capacity for a particular application.
by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge