Illustration Courtesy of Emerson

CO₂ is a pretty nice refrigerant.

It has zero ODP (ozone depletion potential) and a GWP (global warming potential) of 1. CO₂  has been used as a refrigerant almost from the very beginning of refrigerants, and it's been making a big comeback in market refrigeration (especially in colder climates).

CO₂ (R744) is naturally better suited for lower temperature refrigeration applications because of its low-temperature saturated state at atmospheric pressure (-109.3F). You will notice I said “saturated state” because CO₂ does not “boil” at atmospheric pressure. At any pressure below 60 PSIG, CO₂ goes straight from solid (dry ice) right to a vapor. That is why 60 PSIG is known as the “triple point,” or the point where CO₂ could be either solid, liquid, or vapor. (For more on saturated state and what that means for technicians, check out this article here.)

Now, go to the top of the range with CO₂. When you apply 1055 PSIG, the saturation temperature is 87.8°F, but go up even 1 more degree, and CO₂ CANNOT be liquified. This point is known as the critical point of the substance. Whenever a substance is forced beyond its critical point, it becomes what is known as a supercritical fluid and has properties that are unique to this state, but it is certainly not a liquid. You can learn more in this Emerson article about CO₂ as a refrigerant and in this Hussman CO₂ transcritical system training manual.

In a transcritical (trans means beyond or through, so transcritical means “beyond critical”) booster refrigeration system, the low-temp portion of the system operates using its own compressors that “boost” the refrigerant from the low temp side and discharge into the suction of the medium temp side. The high-stage compressors then pressurize the CO₂ (R744) above its critical pressure/temperature.

What is traditionally called a condenser becomes a gas cooler and decreases the temperature (rejects heat from) of the discharge without actually condensing it into liquid. The cooled supercritical fluid goes through a pressure-reducing valve, where some of it condenses into liquid, and the rest remains as gas. Liquid and gas are separated in a flash tank (receiver). Pressure in this tank is usually controlled to around 450 to 500 PSIG.

It's super critical that you understand all of this.

See what I did there?

—Bryan