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REAL Alternatives Module 2 Design Differences for Alternative Refrigerants | 7 Transcritical systems: These systems operate above the critical temperature on the high side for all or part of the time. In these are systems the heat is rejected from the R744 to ambient air and is therefore transcritical at high ambient conditions, typically when the ambient temperature exceeds 21 to 25OC. Small transcritical systems such as beverage coolers, located inside a building, will normally operate transcritically all of the time. Cascade systems: These systems which are always subcritical. In these systems R744 is the low stage refrigerant in a cascade system, and the heat rejected by the condensing R744 is absorbed by the evaporating high stage refrigerant. The high stage system is usually a conventional system using HFC or HC or R717. On some systems R744 is used in the high stage as well as the low stage. The R744 in the low stage is always subcritical, but in the high stage will be transcritical at high ambient conditions. Secondary systems: The R744 is used as a secondary fluid and is pumped around the heat exchangers. Partial evaporation can occur because of the volatility of R744, but saturated refrigerant will exit the evaporator (i.e. it will not be superheated as in the systems above). The R744 is cooled by a chiller. Two stage compression is used for transcritical low temperature (LT) systems due to the high discharge temperatures that are possible. Mechanical subcooling such as the use of suction to liquid line heat exchangers is used in many systems where normally the liquid temperature is below ambient temperature because of the system configuration. Natural liquid subcooling will therefore not occur. Many R744 systems combine two or more of the system types above, for example a cascade system can include a pumped secondary circuit and / or can be cooled by a transcritical R744 system.PDF Image | Alternative Refrigerants
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