Genius in Action—How gravity separators work [Part I]

May 14, 2016

While most engineers in the dry process industries know what a gravity separator does, precisely describing how it works can be difficult.

While most engineers in the dry process industry know what a gravity separator does, precisely describing how it works can be difficult. The statement "a separator classifies dry, free-flowing, granular mixtures by weight or bulk density or specific gravity" is accurate. But a more precise definition would add the qualification "if all of the particles in the mixture are the same size and shape." With equal accuracy the statement could be turned upside down "if all of the particles are the same shape and specific gravity, the separator will classify them according to size."

Unfortunately, all of the particles in any given mixture are never exactly alike in size or shape. One way to more clearly understand the specific gravity theory is to understand the particle’s terminal velocity. (That’s a mouthful!)

The Genius Behind Gravity Separators

You could go back to Galileo dropping two rocks (one heavy and one lighter) from a tower to Newton’s confirmation of the theory of gravity to get to the beginning of a particle’s terminal velocity.

Terminal velocity = √(2 x mass x gravity ÷
(density x frontal area x coefficient drag)

where "frontal area" is the size and "coefficient of drag" is shape and surface texture.

What Galileo had intended to demonstrate was that the attraction of gravity acts equally on all bodies, regardless of size or weight. This being true, they should, neglecting air resistance, accelerate at the same rate and thus travel equal distance in the same time.

Had he employed in his experiment smaller weights, say rock fragments a quarter and a sixteenth of an inch in diameter, he would have found air resistance hard to neglect. It is this resistance in any fluid (whether liquid or gas) to motion of a solid body, which makes gravity separation possible. Gravity alone, with equal intensity on all bodies regardless of size or weight, would be of no use unless there were some other resisting force, sensitive to size and weight, to balance it. And taken further to understand how a gravity separator works, the stratification upon which the separation largely depends, is made according to the terminal velocity, in air, of the particles composing the mixture. Particles with higher terminal velocities are "heavies," and those with lower values are "lights."

Part II of this series provides more detail on the operating principle on which gravity separators are based, as well as application guidelines for different types of gravity separators.

This content is sponsored by Triple/S Dynamics. Sponsored content is authorized by the client and does not necessarily reflect the views of the Process Flow Network editorial team.

To learn more about gravity separators, click here.

Sponsored Recommendations

Choosing The Right Partner for CHIPS Act Investments

As the U.S. looks to invest in the semiconductor research and production using CHIPS Act 2022 funding, it's important to choose the right partner.

EMWD Uses Technology to Meet Sustainability Goals

Eastern Municipal Water District pilots artificial intelligence-enabled control and machine learning to help save energy, reduce costs, and improve quality.

Protein Processing Solutions: Automation & Control

For protein processors looking to address industry challenges, improve efficiency, and stay ahead in a competitive market, Rockwell Automation offers tailored automation, control...

Automotive Manufacturing Innovation: Smart Solutions for a Connected Future

Rockwell Automation provides automation and control systems tailored for the automotive and tire industries, supporting electric vehicle production, tire production, battery production...