Sub vs trans critical process

[newstart]

First time poster, so don't punish me too hard! I'm moving from the Auto Industry into refrig and need to get up to speed fast on CO2 technology for chillers in the hospitality industry. What is the the difference between a sub-critical and trans-critical process? I have read that one process is used above certain ambients and the other below. Is this different from cascaded systems (e.g. CO2/NH3)? If an explanation relative to a P-h or T-s diagram could be offered I'd be most appreciative.

This site has been a wonderful find! Thanks, Clark.

[Andy P]

Hi Clark, and welcome to the forum.

Every refrigerant has a "critical point" - the temperature and pressure above which the substance cannot exist as a liquid so stays as a gas. CO2 is unusual because the critical temperature is so low - only 31C compared with R-134a at 102C and ammonia at 133C. Transcritical means that the high pressure part of the fridge cycle is above this temperature level, so heat cannot be rejected by condensation, but must be by gas cooling. This may seem a disadvantage if aircooled heat rejection equipment is used (as is typical for small systems) but if you can use evaporative cooling it is possible to get really good CoPs in transcritical CO2 systems. Also transcritical operation makes some things, like high grade heat recovery, much more feasible than they woul be with traditional equipment. CO2 systems can operate sub-critical if the ambient is low enough (Glasgow?:p ) - usually only achieved if seawater or riverwater is used. Cascade systems have a sub-critical CO2 system rejecting its heat into another fridge circuit (sometimes ammonia, sometimes HFC, sometimes hydrocarbon) as a way of getting around the critical point issue.

Keep posting!

cheers


Andy P

[momo]

Not in for advertising... but the company below explains things clearly, also look for papers by the Norwegian inventor Prof. Gustav Lorentzen who put new life into C02...

http://www.shecco.com/technology/technology.html