Understanding Cryogenic Applications
The word “cryogenics” comes from two Greek words, “kryos” meaning icy cold, and “genes” meaning to form. The term cryogenics was ﬁrst used in 1875 and has come into general usage since 1955. Cryogenic temperatures are normally considered as below (-)100ºC.
One of the major beneﬁts of handling gases in liqueﬁed form is the enormous reduction in volume resulting from the liquefaction of gas. Savings are achieved because one cubic meter of liqueﬁed gas is equivalent to many hundreds of cubic meter of gas volume at normal pressure and temperature. Thus the handling of cryogenic ﬂuids requires less container space. Cryogenic applications are found in the steel, space, reﬁning, welding, chemical, glass, cement, food, electronics and medical industries. The principle gases include propane, ammonia, CO2, argon, oxygen, helium, hydrogen, nitrogen, ethane, ethylene, methane and chlorine.
There is a variety of equipment associated with separation and liquefaction of gases. These include heat exchangers, cold box equipment, distillation columns, storage vessels and compressors. In addition the storage, transportation, distribution and ultimate consumption of cryogenic ﬂuids also require an array of mechanical equipment, piping, valves and instrumentation. Due to the brittle nature of carbon steel at low temperature, this material is not suitable for cryogenic applications. Carbon steels can be utilized in low temperature service down to only about (-) 45ºC, with the right testing and precautions.
Most nonferrous metals are suitable for low temperature service. Essentially, all copper, aluminium, and high nickel based alloys remain tough and ductile in the cryogenic range. Low temperature applications also utilize low nickel alloys, such as 2-1/4 Ni and 3-1/2 Ni. Cryogenic applications utilize 9% Ni, stainless steel and aluminium. Austenitic stainless steels are capable of exposure to temperatures to absolute zero, (-) 275ºC.
Typical containers for cryogenic liquids are pressure vessels, spheres or tanks. In any case the normal storage device consists of a double wall, much like a thermos. Between the two walls is an insulation barrier to keep the cold within the inner vessel. The maintenance of this vacuum becomes one of the long term service problems. Many of the developments in the cryogenic ﬁeld have only been possible because of the development of high efﬁciency insulation. Unlike most of our processes which are insulated to keep the heat in, cryogenic insulation is designed to keep the heat out.
Source: Pressure Vessel Design Manual by Dennis Moss