You don't need to be concerned with critical refrigerant temp, discharge line temperature is a separate, unrelated and more important issue.

 

If you look at a Pressure-Enthalpy diagram for R404A you will see that at the very top of the saturation curve you have the critical point for the refrigerant, which for R404A is at 541.2 psia and 161.7°F. This represents the maximum pressure and temperature. Above this point the refrigerant can no longer condense.

View attachment Suva404A_P-H-Diagram-Eng.pdf

So to answer you question about a discharge line temperature of 190°F, it would depend on what the pressure is.

 

Critical temperature is easy to understand if you relate it to steam. Below the critical temperature with enough pressure you can convert the steam to water. Above the critical temperature you cannot convert steam to water NO MATTER HOW MUCH PRESSURE YOU PUT ON IT.
This explanation agrees with icemeister's post just a different substance reference.

 

Plenty of power generation steam plants operating well above critical point, when it comes to HVAC-R some supermarkets refrigeration systems use CO2 at supcritical and transcritical P/T, Walmart has a few same as Aldis and Wegmans. not something the average service tech will encounter, such systems are a specialty on their own.

 

What's a good source to learn about trans-critical C02? And what's a thermal siphon? I've heard that term before with C02 used as a refrigerant

 

Now go figure out triple point and how it may relate to critical temperature.....
There are a couple of videos of triple point online.

Properly operating hvac systems don't go there. Not familiar with operation in that area with co2

 

Triple point is at the opposite end. Around 70 psi co2 can exist as either a liquid, vapor, or a solid (dry ice). When you charge a co2 system you have to bring the entire system above the triple point by charging vapor. After you are well above the triple point you can dump the liquid to it.

 

Interestingly enough the first air cooling units used compressed air.

 

Thanks for the manual Icemeister, will be reading up! I was curious about the operating envelop. First system I've seen going critical or triple. Do you find diagnostics more difficult? Any unique or hazardous service issues.