Cryogenics is the cooling of materials to extremely low temperatures using highly condensed gases. The ship shown below is used to transport liquefied gases used in cryogenic processes.

Liquefied Gases Transport Ship
Liquefied Gases Transport Ship
(Photo copyright

General Information

Cryogenics involves refrigeration at temperatures below 120 K (-153°C). These low temperatures, which are not natural on earth, make it possible to liquefy gases such as methane, oxygen, nitrogen, and hydrogen. The cryogenic plant below produces up to 100,000 gallons of liquefied natural gas per day

cryogenic plant
Cryogenic Plant
(Copyright Clean Energy, Seal Beach, CA)

Equipment Design

Cryogenic temperatures are generally achieved through a refrigeration cycle. First, the gaseous refrigerant (green) is compressed and heated in a compressor.

Next, the compressed refrigerant (red) is cooled. The heat from this cooling process is used in the compressor.

The refrigerant then travels through an expansion valve, cooling to the point of condensation. This condensed refrigerant now comes in contact with the material to be cooled, bringing the temperature of the material down to cryogenic temperatures.

As it cools the material, the liquid refrigerant is in turn reheated and undergoes the cycle again.

Cryogenic Cycle

Cryogenic systems can be modified to fit many situations, such as the storage of liquid oxygen, as shown below. The tank wall materials must be carefully chosen to avoid brittle failure at low temperatures. The piping used for cryogenic systems is usually insulated by a vacuum, which has poor conductivity.

Cryogenic systems
Storage Tank
(Copyright South Delta Branch Department of Water Resources, Sacramento, CA)

Usage Examples

A common use of cryogenic technology is the production of liquefied natural gas (LNG) for use as a vehicle fuel. LNG is fed from tanks to the engine through cryogenic pumps. LNG is very cost-efficient and is used by a few metropolitan transportation systems across the United States. The number of LNG fueling systems, known as peak shaving stations (shown below), rose from 3 in 1965 to approximately 100 in 1997.

roduction of liquefied natural gas (LNG) for use as a vehicle fuel.
Peak shaving Station
(Copyright Clean Energy, Seal Beach, CA)

Cryogenics are used widely in medical and biological processes, such as in blood transplantation. At cryogenic temperatures, air can be separated into pure oxygen, nitrogen, and argon. Liquid nitrogen is used in many processes, such as food preservation. Both units shown below use liquid oxygen to provide patients in the medical field with pure, breathable oxygen.

Portable Liquid Oxygen Unit
Portable Liquid Oxygen Unit
In-Home Liquid Oxygen Unit
In-Home Liquid Oxygen Unit

(Pictures copyright CAIRE Medical, Ball Ground, GA)

Cryogenics can also be used to preserve biological samples. In the picture below, liquid nitrogen is used to freeze DNA samples. This method of preservation significantly slows the decaying process. The liquid nitrogen evaporates and chills the samples to temperatures around -196°C.

liquid nitrogen
(Copyright Florida Museum of Natural History, Gainesville, FL)

As with standard refrigeration systems, cryogenic systems can also be installed in vehicles for the transportation of liquefied gases, as shown below.

cryogenic systems can also be installed in vehicles
(Copyright Kenworth Truck Company, Kirkland, WA)


  • Relatively simple systems.


  • Hazardous conditions due to extremely low temperatures.
  • High energy requirements to achieve low temperatures.



  • Agrawal, Rakesh, Rowles, Howard C., Kinard, Glenn E. “Cryogenics.” Encyclopedia of Chemical Technology. 4th ed. New York: John Wiley & Sons, Inc., 1993. Print
  • Barron, Randall F. Cryogenic Systems. 2nd ed. New York: Oxford University Press, 1985,  237-309. Print
  • Flynn, Thomas M. Cryogenic Engineering. New York: Marcel Dekker, Inc., 1997: 21-180. Print
  • Perry, Robert H. and Don W. Green. Perry’s Chemical Engineers’ Handbook. 7th ed. New York: McGraw-Hill, 1997: 11-96 – 11-107. Print


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  • Joseph Palazzolo
  • Steve Wesorick
  • Keith Minbiole