Vapor–liquid separator

A vapor–liquid separator is a device used in several industrial applications to separate a vapor–liquid mixture.

A vapor–liquid separator may also be referred to as a flash drum, breakpot, knock-out drum or knock-out pot, compressor suction drum or compressor inlet drum. When used to remove suspended water droplets from streams of air, it is often called a demister.

Method of operation

For the common variety, gravity is utilized in a vertical vessel to cause the liquid to settle to the bottom of the vessel, where it is withdrawn.[1][2][3][4]

In low gravity environments such as a space station, a common liquid separator will not function because gravity is not usable as a separation mechanism. In this case, centrifugal force needs to be utilised in a spinning centrifugal separator to drive liquid towards the outer edge of the chamber for removal. Gaseous components migrate towards the center.

For both varieties of separator, the gas outlet may itself be surrounded by a spinning mesh screen or grating, so that any liquid that does approach the outlet strikes the grating, is accelerated, and thrown away from the outlet.

The vapor travels through the gas outlet at a design velocity which minimises the entrainment of any liquid droplets in the vapor as it exits the vessel.

The feed to a vapor–liquid separator may also be a liquid that is being partially or totally flashed into a vapor and liquid as it enters the separator.

Liquid level monitoring

The separator is only effective as long as there is an vapor space inside the chamber. The separator can fail if either the mixed inlet is overwhelmed with supply material, or the liquid drain is unable to handle the volume of liquid being collected. The separator may therefore be combined with some other liquid level sensing mechanism such as a sight glass or float sensor. In this manner, both the supply and drain flow can be regulated to prevent the separator from becoming overloaded. (Agastya, Bayu)

Where vapor–liquid separators are used

Vapor–liquid separators are very widely used in a great many industries and applications, such as:

Preventing pump damage

In refrigeration systems, it is common for the system to contain a mixture of liquid and gas, but for the mechanical gas compressor to be intolerant of liquid.

Some compressor types such as the scroll compressor use a continuously shrinking compression volume. Once liquid completely fills this volume the pump may either stall and overload, or the pump chamber may be warped or otherwise damaged by the fluid that can not fit into a smaller space.

See also


  1. William D. Baasel (1990). Preliminary Chemical engineering Plant Design (2nd ed.). Van Nostrand Reinhold. ISBN 0-442-23440-6. OCLC 18907139.
  2. David H.F. Liu (1997). Environmental Engineers' Handbook (2nd ed.). CRC Press. ISBN 0-8493-9971-8. OCLC 35886123.
  3. Stanley S. Grossel (June 2004). "Design and sizing of knock-out drums/catchtanks for emergency relief systems". Plant/Operations Progress. 5 (3): 129–135. doi:10.1002/prsb.720050304. ISSN 0278-4513.
  4. Stanley M. Walas (1988). Chemical Process Equipment:Selection and Design. Butterworth-Heinemann. ISBN 0-409-90131-8. OCLC 16714037.

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