CO{sub}2 belongs to the so-called "natural" refrigerants, together with ammonia, hydrocarbons such as propane and butane, and "water. All of these refrigerants have their respective disadvantages: ammonia is toxic, hydrocarbons are flammable, and water has limited application possibilities. In comparison, CO{sub}2 is non-toxic and non-flammable, but has a double role in the environment. CO{sub}2 is necessary for all life on earth, but is also a green house gas, which can change the environment if its concentration in the atmosphere changes. Traditionally, industrial refrigeration valves have been used for CO{sub}2 systems for more than 15 years the pressure has been limited to 25 bar. During the last few years, however, an increasing number of valves for several CO{sub}2 systems are being supplied at different pressure stages up to 50 ba.
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