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Rapport 2414173 - October 2007 __________________________________________________________________ pressure that is below the refrigerant critical pressure. In this cycle, condensing pressure and temperature are linked by a unique relationship that depends on the nature of the refrigerant. In a "transcritical" cycle, the liquefaction process (or gas cooling) occurs above the critical pressure, while evaporation occurs at a pressure below the critical pressure. With a few exceptions, all cycles use working fluids below their critical pressure. 1.2.A brief history of refrigerants Briefly stated, the history of refrigerants can be divided into three periods : - A first era which started roughly around 1830 and lasted up to about 1930, where any available substance that produced refrigeration was used. This era was qualified as the "whatever worked" period, since the main selection criterion for a refrigerant was its ability to be used in practical systems, regardless of its toxicity, flammability and compatibility with the environment. Although most of them were toxic and/or flammable, solvents and volatile fluids were then used. With the development of comfort air conditioning and refrigeration to a very large scale and to the public at large, new and safer refrigerants became necessary. - A second era, dating from 1931 to about 1994, marked by the development and promotion of the CFCs (Chloro- Fluro-Carbon refrugerants) and HCFCs (Hydoro-Chloro- Fluoro-Carbon refrigerant) as aerosol propellants, blowing agents and refrigerants. These refrigerants where named the "miracle substances" because they met all the criteria required to be used at the time: stability, safety, efficiency, etc. However, these substances (mostly chlorinated compounds) proved to be too stable and their long-term effects were totally ignored until their accumulation in the stratosphere resulted in the partial destruction of the ozone layer. The ozone hole in the Southern Hemisphere (Nasa, 2006) In order to restore the ozone layer, a scheduled worldwide phase-out of the CFCs and HCFCs was agreed upon by the Montreal Protocol and its subsequent amendments. As an example, HCFC refrigerant R 22 can no longer be used in new equipment in Europe since 2004. __________________________________________________________________________________________ SHERPHA PROJECT - Transcritical R744 Heat Pumps – Technician's Manual 5/63PDF Image | Transcritical R744 (CO2) Heat Pumps 2017
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