## Synopsis

RCF and RPM are standard terms in the context of centrifuges. This article explains the difference between the two terms and why RCF is more relevant to centrifuges. Though the two terms are related to each other mathematically, they are vastly different from the application’s perspective.

## What is RCF?

RCF stands for **R**elative **C**entrifugal **F**orce. The force experienced by any mass (solid, liquid, or gaseous) rotates around a rotation’s fixed axis. This force is referred to as the centrifugal force. RCF is this centrifugal force expressed in terms of gravity. In other words, RCF is the ratio of the centrifugal force to gravitational force. Therefore it is also referred to as g-force.

## What is RPM?

RPM is the abbreviation for Rotations Per Minute. It is a measure of the speed of rotation of any object. In a centrifuge, the RPM indicates the number of rotations the centrifuge bowl completes in one minute. RPM is a unitless number that is independent of any properties of the object.

## Difference Between RCF and RPM

Though RCF and RPM are units of measurement related to centrifuges, they are fundamentally different. The RCF of a centrifuge is the actual centrifugal force (or g-force) generated by the centrifuge bowl rotation. Whereas the RPM is the speed of rotation of the bowl, the RCF is a function of the RPM and the rotating object’s radius.

## Why use RCF and not RPM?

As explained above, the RCF is a measure of the centrifugal force generated by the centrifuge. This force (g-force) is directly related to the efficiency of the centrifuge. This is because the RCF indicates the centrifuge force exerted, which defines the centrifuge’s separation efficiency.

The centrifuge RPM is merely an indication of the bowl speed but does not define the centrifugal force. In conjunction with the RPM, the rotation radius determines the centrifugal force shown by the RCF formula below.

For example, consider two centrifuge bowls rotating at the same speed, say 5,000 RPM. One bowl has a radius of 5 cm while the other has a radius of 25 cm. Based on the formula, the RCF for the first bowl is 1,400 g-force. However, for the same RPM, the RCF generated by the second (larger) bowl is 7,000 g-force!

The centrifuge’s separation efficiency with the 7,000g RCF will be much higher than that of the centrifuge with the 1,400g RCF. Therefore, if the user were to select the centrifuge solely based on RPM, they would make the wrong choice based on centrifuge effectiveness.

## Formula to Calculate RPM from RCF

The following formula calculates the Relative Centrifugal Force (RCF) of a centrifuge based on the RPM and Centrifuge Bowl Radius. The diagram also illustrates the two terms, namely RPM and bowl radius (r), in the context of the centrifuge bowl.

$RCF=1.18\times R\times 1000/RPM$## RFC in Disc Stack Centrifuge

The Relative Centrifugal Force (RCF) is the crucial factor that differentiates a disc-stack centrifuge from other types of centrifuges. An Alfa Laval disc centrifuge generates a high centrifugal force (g-force) compared to simple open-bowl or decanter centrifuges. This higher force allows the disc-stack centrifuge to have higher separation efficiency.

Therefore, these centrifugal separators can separate much smaller particles than other centrifuges. They are also able to separate immiscible liquids with small specific gravity differentials due to their high g-force.

## Summary

RCF and RPM are commonly confused and interchanged terms in the centrifuge world. These two terms are significantly different as explained above. For the end-user, the RCF (aka g-force) is the crucial term because it defines the effectiveness of the centrifuge. The RPM is the bowl speed indicator but the RPM by itself does not convey any useful information for the application, or the process.