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 its 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, This is why 60 psig is known as the “triple point” or the point that could be either solid, liquid or vapor.

Now go to the top of the range with CO2, when you apply 1055 psig the saturation temperature is 87.8F but go up even 1 more degree and CO2 CANNOT be liquified, this is known as the critical point of the substance. Whenever a substance is forced beyond it’s 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 see more in this natural refrigerants PT chart.

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 it’s 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 are usually controlled to around 450 to 500psig.

It’s super critical that you understand all of this…

See what I did there.

— Bryan

 

 

Photo Courtesy of Emerson

What is Cascade refrigeration?

Cascade refrigeration is a term you will hear more and more over the coming years, and while some of the systems may be very complex, the concept is actually pretty simple.

Some refrigerants are well suited for high and medium temperature applications, and some are better suited and for a lower temp applications. In a cascade system the high/medium temp refrigerant circuit is used to cool the condenser of the low temp circuit by way of heat exchanger. In essence, the condenser for the low temp system is also the evaporator or part of the evaporator of the high/medium temp system.

In the diagram above the medium temp circuit is used in the medium temp cases and is ALSO used in the heat exchanger to condense the refrigerant in the low temp circuit.

There are many reasons for this type of system but one of the big reasons is it is a practical solution for using CO2 (R744) as a low temp refrigerant.

— Bryan

 

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