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Our Refrigerant Focus series delves into the history, properties, suitable applications, and pros and cons of some of today’s popular or otherwise noteworthy refrigerants. This installment will focus on R-744 (CO2).
Check out some of our refrigerant focus posts below.
CO2’s history as a refrigerant dates to the mid-19th century and increased steadily until the early 1930s, during which time it was used for applications like air conditioning and refrigerating shipments of food during intercontinental transit among others.
But with the emergence of synthetic refrigerants in the 1930s, CO2’s usage began to decline, as the newer HFC and CFC refrigerants became the norm.
In the years since, regulations – especially 1987’s Montreal Protocol – have taken aim at phasing out environmentally harmful substances, including HFC and CFC refrigerants, resulting in efforts to identify viable alternatives. One such alternative is CO2.
CO2 is naturally occurring compound consisting of two oxygen atoms and one carbon atom. When used as a refrigerant, it can be referred to as R-744 (ASHRAE designation) or simply CO2.
Normally, our Refrigerant Focus blogs include a performance comparison between the refrigerant in question and one that it’s used to replace. However, given that CO2 system componentry functions differently than more ‘typical’ refrigerants that change phase, we’ll focus on a comparison of properties instead of refrigerant performance.
|
R-744 (CO2) |
R-404A |
Difference (%) | Difference (abs.) | |
|
Molecular weight (g/mol) |
44 | 97.6 | 122% | 53.6 |
|
Boiling temperature ˚F (˚C) |
-109 (78.5) | -51 (-46.2) | 53.2% | 58 (14.4) |
|
Critical temperature ˚F (˚C) |
87.9 (31.1) | 161.7 (72) | 83.9% | 73.8 (23.2) |
|
Critical pressure, PSI (Bar) |
1073 (72.1) | 541 (37.7) | 49.6% | 532 (36.6) |
|
Pressure (condenser) PSI (Bar) |
1046 (72.1) | 206 (14.2) | 80.3% | 840 (57.9) |
|
Pressure (evaporator) PSI (Bar) |
196 (13.5) | 71 (4.9) | 63.7% | 125 (8.6) |
|
Mass flow per ton of refrigeration (lbs./min.) |
3.52 | 3.92 | 11.4% | 0.4 |
|
Compressor discharge temp. ˚F (˚C) |
196 (91.1) | 94 (34.4) | 52% | 102 (38.9) |
|
Global Warming Potential |
1 | 3922 | 392,100% | 3921 |
|
Ozone Depletion Potential |
0 | 0 | N/A | N/A |
|
ASHRAE safety group |
A1 | A1 | N/A | N/A |
The most notable benefit of CO2, as we touched on earlier, is its environmental friendliness. In fact, the reason it has a global warming potential (GWP) of 1 is because that metric was created to estimate a refrigerant’s environmental impact relative to that of carbon dioxide. The United States EPA defines GWP as “a measure of how much energy the emissions of 1 ton of a gas will absorb over a given period of time, relative to the emissions of 1 ton of carbon dioxide (CO2)[1].”
As explained by Wynand Groenewald on an SRC podcast, “with CO2, we are basically borrowing CO2 from the atmosphere and putting it in a system for a while. If we release it, we’re basically just back to where we were at square one.”
Another benefit of CO2 is that it’s inexpensive, typically trading between $1 and $4 per pound, whereas some synthetic refrigerants regularly exceed $15-$20/lb.
Also, as you can see in the property table, CO2’s discharge pressure as it exits the compressor is nearly three times that of R-404A. Because of this, CO2 affords users some excellent heat reclaim opportunities. For example, one of our CO2 customers – a dairy – uses that excess heat to heat water that they use for washdown after production shifts.
One of the main causes of hesitation to adopt CO2 is its operating pressure. As you can see from the properties table above, the operating pressures, both in evaporation and condensation conditions are more than twice that of R-404.
There’s no doubt these pressures are higher than those found in most HFC/CFC systems, but that’s not necessarily the drawback. Rather, the higher pressures inherent to CO2 systems, mean retrofitting is rarely feasible, if ever. For new installations, that’s obviously not much of a problem, but for those wanting to transition an HFC system to CO2, there will be upfront cost.
These costs include things like upgraded compressor, possible changes to materials, etc. For more information on the impact of CO2’s properties on system componentry, the state of that supply chain in the U.S., and more topics, check out our podcast on the subject.
Don’t get left out in the cold when it comes to heat transfer information. To stay up to date on a variety of topics on the subject, subscribe to The Super Blog, our technical blog, Doctor's Orders, and follow us on LinkedIn, Twitter, and YouTube.
[1] https://www.epa.gov/ghgemissions/understanding-global-warming-potentials
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