# Disc-stack centrifuges

How centrifugal sedimentation extends Stokes' law into gaps of fractions of a millimetre, why equivalent settling area — not power — sets capacity, and where continuous solids discharge becomes the real engineering limit.

Gravity settling cannot separate microbes on a working-day timescale; a disc stack buys a few thousand gravities and then pays for them in shear, floc damage and lost seal liquid.

Source: https://en.bioecon.ru/docs/bioproduction-equipment/downstream-purification/disc-stack-centrifuges/
Updated: 2026-09-07



A particle settling under gravity moves at the Stokes velocity v = Δρ·d²·g/18μ — proportional to the square of its diameter and to the density difference with the liquid. A microbial cell is about a micron across and a few percent denser than its broth; at one gravity it needs days, which is not a process but a spill. The centrifuge replaces g with the centrifugal acceleration ω²r, and thousands of gravities turn days into seconds. Everything else about the machine is engineering consequences of that one substitution.

## Thin gaps, not big bowls

The disc stack is the geometry that converts field into capacity efficiently. A stack of conical discs splits the flowing liquid into dozens of thin layers: a particle must travel only the fraction of a millimetre between two discs before it lands on a surface, after which the inclined disc slides it outward to the bowl wall. Settling distance, not bowl radius, is what was shrunk. Capacity follows the classical sigma relation — throughput equals the gravitational settling velocity times the equivalent clarifying area, a quantity fixed by disc number, disc angle, diameter and rotational speed alone. That theorem is why scale-up is honest: two machines with the same equivalent area make the same separation at the same flow, whatever their maker says. The hard limits on adding area are materials ones — the hoop stress in a spinning bowl grows with the square of speed and radius, which is where the ceiling on separation factor comes from.

## Paying for continuity: discharge

Continuous feed demands a way out for the solids. Nozzle machines leak a steady slurry through small rim nozzles — good for yeast-scale solids loads, at the price of a diluted, wetted discharge. The self-cleaning design instead opens an annular slot at the bowl rim for a fraction of a second at intervals, hydraulic ejection without stopping the flow; the discharged solid must be flowable, and slimy or pasty solids paste the discs and defeat the machine. The moment of highest violence is not the ejection, though — it is the feed. Accelerating broth from rest to rim speed at the distributor is the most shearing event in bioprocessing: it breaks flocs that would have settled happily, ruptures mammalian cells and releases the very debris the machine is meant to remove, which is why centrifuges are followed by depth filtration and why hermetic, bottom-fed low-shear feed zones exist as a product category.

## The seal and its losses

Taking clarified liquid out under pressure requires closing a rotating bowl against a stationary pipe. The classical answer is a water seal: a ring of separate seal liquid holds the product away from the discharge path. The seal is continuously consumed and replenished — a small, permanent dilution of the product stream and a water balance the plant must manage — and when it breaks, feed floods straight to the sewer outlet. Air ingress, aerosol generation and oxygen pickup are the quieter costs: they are why hermetic designs fill the bowl from below through a hollow spindle, and why oxygen-sensitive broths and shear-sensitive proteins pay a premium for the machine that treats them gently. The disc stack, in short, is not limited by how fast it can spin; it is limited by how much violence the product survives on the way through.