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Rapport 2414173 - October 2007 __________________________________________________________________ properties also yield very efficient refrigeration systems. Because of its acute toxicity, stringent regulations apply for ammonia systems, which require close monitoring and highly skilled engineers and technicians. Ammonia is often used in large stores and in process refrigeration. With the development of small-charge equipment, ammonia may be used in smaller capacity units. 2. CARBON DIOXIDE Carbon dioxide (CO2) is not a new refrigerant. Rather, it was "rediscovered" in the early 90's at the Norwegian University of Science and Technology (NTNU). The use of carbon dioxide as a refrigerant lasted for well over a century. Its application has been progressively discontinued, until it was abandoned in the mid-50s, with the widespread use of the CFC refrigerants, which were more efficient, more stable and safer. This section will summarize its physical properties and examine its particularities when applied as a refrigerant. CO2 refrigerant will be referred to as "R744" in the following. 2.1.Properties R744 is a versatile material, being used in many processes and applications - each of which takes advantage of one or more these characteristics: reactivity, inertness and coldness (mostly when used in direct-contact refrigeration). Common uses include consumption as a raw material for production of various chemicals; a working material in fire extinguishing systems; carbonation of soft drinks; freezing of food products, chilling of meats prior to grinding; refrigeration and maintenance of ideal atmospheric conditions during transportation of food products to market, etc. R744 is present in the atmosphere at concentration3 levels of about 380 ppm. Exhaled air contains about 4% carbon dioxide. At normal conditions, it is gaseous. R744 is odorless at low concentrations and slightly toxic with a slightly pungent, acid taste. It is widely available as it occurs naturally and it is a by-product of fossil fuel combustion and other industrial processes. It combines with water in air to form carbonic acid which corrodes metals, limestone and marble. R744 will not burn or support combustion. Air with a carbon dioxide content of more than 10% will extinguish an open flame. Breathing air that contains more than 10% R744 can be life-threatening. This corresponds to spreading uniformly about 24 kg of R744 in a workshop that is 4 m wide, 10 m long and 3 m in height. 3 one ppm is equivalent to one cubic centimeter per cubic meter __________________________________________________________________________________________ SHERPHA PROJECT - Transcritical R744 Heat Pumps – Technician's Manual 9/63PDF Image | Transcritical R744 (CO2) Heat Pumps 2017
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