Maximizing Centrifugal Vortex Separator Efficiency for Industrial Gas/Liquid Separation

Gas/Liquid Centrifugal Separators are designed to separate entrained liquid droplets and solids from a gas stream. They are sometimes referred to as vortex separators due to the underlying principles of conservation of angular momentum, tangential velocity, and resulting centrifugal inertia driving droplet separation. This article will explain the physics of why separation occurs and why it is maintained with an infinite turndown ratio, meaning separation efficiency is maintained at low flow (velocity) conditions.

Separation Efficiency Overview
Our range of centrifugal gas/liquid separators include two basic designs, single stage separation and dual stage separation. This article will focus on the single stage design for separation of entrained droplets and particles greater than 10 microns in diameter with 99% efficiency. The dual stage variations add a coalescing stage before or after the centrifugal stage to achieve finer separation levels, within the > 0.3-micron range.

A true technical understanding of the physics in-play requires understanding angular momentum and tangential velocity as relates to centrifugal gas/liquid separator designs. Tangential velocity (Vt) is the speed at which the droplet/particle is moving tangentially to the circle it is moving around. Angular momentum (L) describes the rotational motion of the gas vortex formed within the separator body; it is why the center of the vortex moves faster than its outer diameter.

Design of a Centrifugal Separator
The gas stream forms a vortex due to the cylindrical shaped body of the separator and the internal structure it passes through, and because angular momentum is conserved, the tangential velocity is highest near the center of rotation (vortex) and thus contributes to increased centrifugal inertia on particles and droplets greater than 10 microns in diameter, propelling them towards the outer edges of the vortex and ultimately impinging onto the vessel wall where agglomeration occurs (essentially turning small droplets into a liquid stream that will go to drain). The discharge nozzle extends into the center (low pressure area) section of the internal vortex, thus the gas with droplets less than 10 microns in diameter may navigate to the exit nozzle and leave the separator while the heavier droplets are kept away from this area of low pressure due to the centripetal force of the vortex.

More Than Centrifugal Inertia
Our centrifugal gas/liquid separators maintain their efficiency with an “infinite” turndown, meaning they maintain their separation efficiency even when the flow rate (velocity) is very low – how is that possible?

While it is true that centrifugal inertia is reduced at lower velocities, the gravitational force remains constant, and the residence time of the entrained droplets increases if the droplets are moving slower. Thus, droplet trajectory is downward instead of upward through the low-pressure center of the vortex to exit the vessel.

The “engineered” aspect of a centrifugal gas/liquid separator is related to the minimization of the diameter and length of the separator body that would otherwise be required. Technically if you fabricate a vessel large enough for a given flow rate you do not need any internals, the combination of expansion/low velocity and gravity will separate entrained droplets and particles. The internal geometry of a centrifugal gas/liquid separator enables miniaturization of the vessel, which is directly related to the cost. This is why a “knock-out drum” is not a centrifugal gas/liquid separator, although the terms are often interchanged, a “knock-out drum” implies a simplistic vessel to maximize the expansion and pressure drop aspect of droplet separation.

The formulas used to determine the minimum diameter of a centrifugal separator body are based upon the upper limits of acceptable internal velocity and thus if your process has a start/stop cycle or upset condition which reduces flow, efficiency will be maintained.
The most common separator applications involve separation of entrained water from steam, compressed air and industrial gasses; however, they are equally effective and commonly used for separating entrained droplets of other viscosities and buoyancies because the underlying principle is heavier components of the gas stream (droplets and particles) are affected by centrifugal inertia and gravitational forces more than the gas. This is why Gas/liquid separators will not remove vaporized liquid from a gas stream.

Comparison of Centrifugal Inertia: Water vs. Air
View of the vortex from its axis:
Don’t worry, we do not need to whip-out our calculators! Since the density of water is about 46 times greater than air and the underlying centripetal force driving centrifugal inertia is directly proportional to the density of the fluids, the water droplets in this example undergo 46x the centripetal force compared to air.

Internal Geometry
There are many configurations of centrifugal gas/liquid separators and they are mainly defined by:
A. the fineness of the droplet/particle to separate
B. the desired inlet/outlet nozzle orientation as relates to the gas flow (horizontal, vertical, or a combination thereof)

All centrifugal separators share common internal attributes: an impingement plate and a vortex containment plate.

The impingement plate opposite the inlet nozzle instigates the agglomeration process, forming a large mass that drops to a common drain point within the gas/liquid separator.

The vortex containment plate truncates the lower portion of the internal vortex to prevent the separated liquid from becoming re-entrained into the gas flow. The space between this containment plate and drain port can be enlarged to form a liquid hold-up volume to facilitate monitoring the liquid level or drainage via pumping. It can also be modified to improve drainage for high concentrations of particles or slurries.

Meaning of “Separator Size”
Centrifugal gas/liquid separators are often referred to by their inlet/outlet nozzle size, however this can be misleading. Our sizing calculations determine the minimum separator body diameter required to handle the specified gas flow at the lowest operational pressure and highest operational temperature. The “corresponding separator size” refers to the largest ID inlet/outlet nozzle that can be welded to that vessel diameter. In actuality the inlet/outlet nozzle ID will be sized to match your intended pipeline size even if the separator can accommodate a larger ID inlet/outlet nozzle.

It is quite common to “oversize” the separator body if your liquid load is high, thus we could attach 3” size inlet/outlet nozzles onto a 4” or larger size separator. Another reason for “oversizing” the separator body is to satisfy the maximum allowable pressure drop across the vessel, typical of low-pressure applications.

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