RCF is a calculated acceleration, not a performance certificate. It tells you the acceleration available at a stated radius and speed; it does not tell you whether the feed can settle in the available residence time, whether the conveyor can carry the solids load, or whether the machine is producing the separation objective you actually need.
Quick answer: In centrifuge language, RCF and G-force normally describe the same dimensionless value: centrifugal acceleration divided by standard gravity. RPM is not the same thing. RPM becomes RCF only after the bowl radius or diameter is included.
RCF is acceleration relative to gravity
Relative centrifugal force (RCF), commonly called G-force, is the radial acceleration at a selected point in the rotating bowl divided by standard gravitational acceleration. For a particle at radius r, the acceleration is a = ω²r. Dividing by g = 9.80665 m/s² gives a dimensionless multiple of gravity.
RCF = ω²r / g
Using rotational speed in revolutions per minute:
RCF = 1.118 × 10−5 × r(cm) × RPM²
or RCF = 5.59 × 10−6 × D(cm) × RPM² when bowl diameter D is used.
The radius must match the location where the acceleration is being stated. Manufacturers commonly quote maximum G at or near the bowl wall. A particle closer to the liquid surface experiences less acceleration. This is why two apparently identical “2,000 G” claims may not describe identical clarification geometry or working volume.
RPM to RCF converter examples
The table below uses the standard radius formula and shows why RPM cannot be compared without bowl size. Values are rounded for field discussion, not equipment certification.
| Bowl radius | RPM | Calculated RCF |
|---|---|---|
| 15 cm | 2,700 | ~1,222 G |
| 22.86 cm | 2,700 | ~1,863 G |
| 22.86 cm | 3,000 | ~2,300 G |
| 30 cm | 2,700 | ~2,445 G |
A worked field calculation
Consider an 18 in bowl. The diameter is 45.72 cm and the radius is 22.86 cm. At 2,700 rpm:
If the same bowl is increased to 3,000 rpm, RCF becomes approximately 2,300 G. A speed increase of 11.1% therefore produces about 23.5% more RCF because RCF varies with RPM squared.
That square relationship is useful but dangerous when misunderstood. Higher speed raises the rotational stress component and can increase bed compaction and wear exposure. Actual absorbed power depends on drive architecture, energy recovery and process load; RCF alone does not prescribe it. The rated maximum speed remains a hard equipment limit; it is not an operating target.
Why RPM cannot compare two machines
RPM contains no bowl-radius information. A small bowl must rotate faster than a large bowl to create the same wall acceleration. Therefore “we ran the last centrifuge at 3,000 rpm” is not a transferable setting. Compare RCF, bowl geometry, clarification length, pond volume, feed rate, solids load, differential speed and torque capacity.
| Input | What it changes | What it does not prove |
|---|---|---|
| RPM | Rotational speed | Acceleration without radius |
| RCF | Available radial acceleration | Actual cut point or recovery |
| Feed rate | Hydraulic residence time | Solids mass load without feed concentration |
| Differential speed | Solids residence and transport rate | Separation result without torque and cake data |
RCF enters settling physics, but it is not the whole model
In the Stokes settling region, terminal settling velocity increases with acceleration, particle-density contrast and particle diameter squared, and decreases with continuous-phase viscosity. Replacing gravitational acceleration with centrifugal acceleration shows why a higher RCF can move smaller particles toward the wall faster. Real drilling fluids, however, are often non-Newtonian, polydisperse and concentrated. Hindered settling, particle interaction, irregular particle shape, emulsified phases and changing viscosity make a single theoretical cut point uncertain.
Actual separation is a race between radial migration and axial escape. A particle must reach the solids bed before the liquid carries it out of the bowl. Higher RCF helps the radial side of that race; lower feed rate and adequate pond/clarification volume help the time side. Neither can substitute indefinitely for the other.
How RCF should be used in the field
- Confirm bowl diameter and the radius used for the displayed G-force.
- Record actual bowl speed, not only the VFD percentage.
- Calculate RCF and verify it against the controller or manual.
- Record flow rate and feed density or solids concentration at the same time.
- Track differential speed, conveyor torque, vibration and motor load.
- Sample feed, centrate and cake using a defined time window.
- Judge the setting against the operating objective: LGS rejection, barite recovery, clarification or dewatering.
Use RCF to normalize speed for bowl size and to understand available acceleration. Never use it alone to declare a fine cut, good recovery or correct optimization. Performance must be closed with mass balance, outlet quality and the condition of the machine.
Measurement uncertainty and reporting discipline
RCF is often reported with more precision than the inputs justify. Bowl speed may be taken from a setpoint rather than an independently verified actual speed; bowl “diameter” may refer to nominal model size; and the selected radius may not be stated. A defensible report records model, nominal and working radius where available, actual RPM source, calculation equation and rounding. If the controller reports G, compare it with the independent calculation and investigate a material difference rather than choosing the preferred number.
RCF and effective cut point
Under simplified Stokes behavior, the smallest particle that can migrate a required radial distance during the available residence time decreases as acceleration and residence time increase. But the relationship is not a universal square-root shortcut because the path length, flow field, non-Newtonian viscosity and hindered settling are not constant. For field decisions, use theoretical RCF to design a test, then use matched PSD or mass balance to determine what the installed machine actually separated.
Reporting template
- Bowl model, diameter/radius and actual RPM.
- Calculated wall RCF and controller-displayed RCF.
- Feed flow, temperature, density, rheology and solids fractions.
- Differential speed, torque, pond configuration, vibration and motor load.
- Feed, centrate and cake sampling method and time.
- Separation objective and acceptance criteria.
Yield-stress limit. In a structured or yield-stress fluid, a simple terminal velocity may not exist below the local stress threshold. Stokes relationships explain direction; they do not predict the field cut point.
Common questions
Is RCF the same as G-force?
In centrifuge practice they are generally used for the same dimensionless ratio: centrifugal acceleration divided by standard gravitational acceleration.
Why does a small centrifuge need more RPM?
Because centrifugal acceleration depends on both radius and rotational speed squared. A smaller radius requires a higher speed to create the same RCF.
Does higher RCF always produce a finer cut?
No. It increases the available settling acceleration, but actual separation also depends on feed rate, viscosity, particle properties, bowl geometry, solids loading, differential speed and torque capacity.
How do you convert RPM to G-force?
Use RCF = 1.118 × 10−5 × radius(cm) × RPM². If you only know bowl diameter, use half the diameter as the radius.
What are the units of RCF?
RCF has no physical unit. It is a ratio to standard gravity, so it is normally reported as “G” or “× g”.
