I am trying to get a grasp on the basics of CO2 refrigeration systems - would someone be able to tell me in layman's terms the difference between subcritical cascade and transcritical refrigeration systems?
Any direction you would have in this matter would be much appreciated!
There is no really easy answer but the Danfoss little WAV file on CO2 behaviour is a good starting point.
The easiest way to explain the difference is to look at each system type individually by either construction or thermally on an enthalpy diagram.
The sub-critical system thermally stays inside the enthalpy hoop operating its condensing below the critical point, whereas the transcritical system discharge goes above the critical point of the enthalpy hoop, inother words above the critical point of the refrigerant; which is for CO2 being 31°C.
The sub-critical system operates as a cascade system meaning two seperate refrigeration systems are inter-connected thermally - in other words the condenser of the low stage is interwoven with the evaporator of the high stage. The high stage system simply provides the condensing medium for low stage system. The high stage system can have any refrigerant, such as a HC, HFC, ammonia etc; and run at about -15°C evaporating. The low stage is the CO2 system and run at about -10°C condensing and whatever evaporating the application desires to -50°C.
The transcritical system is a single system that looks basically like a normal refrigeration system but with some tricky valves fitted. The condenser is replaced by a gas cooler removing sensible heat only, as the discharge pressure and temperature is well above the critical values for CO2, and as a result they make wonderful water heaters or heat recovery systems. The values are in the order of 8 000 to 12 000kPa and 120°C. This cooled vapour passes through a control valve and it pressure drops below the critical value and liquid (and some flash gas) appears.
Otherwise I would advise you to just spend some time on our knowledge-papers section and look into some more general documents. If you have specific questions just use this forum, as for sure there are quite a few people out there struggling with the same questions.
![R744.com '.$SectionNames[($SectionId-1)].'](http://www.r744.com/images/banners/supermarket-survey-sm.jpg)