See also

Carbon dioxide – Aust. Bushfires – & NASA



Original ⇒ HERE



19 May 2019 – by Alec Johnson


   R-744 Carbon Dioxide is one of the oldest refrigerants in the world. Its first usage can be traced all the way back to the nineteenth century. Before the popularity of CFC and HCFC refrigerants Carbon Dioxide was one of the most widely used refrigerants. Chances are if you went to a movie theater in the 1920’s then you were experiencing an R-744 air conditioning system.


When R-12, R-22, and other HCFC/CFC refrigerants began to rise to prominence we began to see a large decline in R-744 usage. This was due to its extremely high operating pressures which caused numerous part failures. The newer artificial refrigerants were much easier to maintain. In today’s world, as we progress through the twenty-first century we have begun to see resurgence in R-744. This is due to the detrimental effects that CFC, HCFC, and HFC refrigerants have had on the environment. Carbon Dioxide on the other hand is climate neutral with zero Ozone Depletion and a Global Warming Potential of only one.


If you haven’t run into an R-744 system yet you soon will as its popularity grows with each passing year. In this article we’re going to take a deep dive and take a look at everything there is to know on R-744. If I miss something please let me know!



The Facts

Name: R-744
Name – Scientific: Carbon Dioxide
Name (2): 744
Name (3): CO2
Name (4) R744
Classification: Natural Refrigerant
Chemistry: CO2
Status: Active & Growing
Future: Will Be Used All Over World in Various Applications
System Type: SubCritical (Cascade) & TransCritical
Application: Vehicle Air Conditioning & Transport Refrigeration
Application (2): Commercial Refrigerators & Freezers
Application (3): Commercial Vending Machines & Plug-Ins
Application (4): Industrial Refrigeration
Application (5): Ice Rinks
Replacement For: R-22, R-134a, R-404A, and other HFCs
Ozone Depletion Potential: 0
Global Warming Potential: 1
Global Warming Risk: Very Low
Toxicity Levels: A (No Toxicity Identified.)
Flammability Levels: Class 1 – No Flame Propagation
Lubricant Required: POE & PAG Oils
Boiling Point: −78 °C (-108.4 °F; 195.15 K)
Critical Temperature: 31.04 °C or 87.87 °F
Critical Pressure: 7,380 kpa
Triple Point: 4.2 bar (60.9 psi) and -56.6 °C (-69.8 °F)
Temperature Glide: None
Molar Mass: 44.009 g·mol−1
Density (2): 1101 kg/m3 (liquid at saturation −37°C)
Melting Point: −56.6 °C; −69.8 °F; 216.6 K
Vapor Pressure: 5.73 MPa (20 °C)
Heat Capacity: 37.135 J/K mol
Manufacturers: Various Including: Honeywell, Chemours, Arkema, Mexichem, Chinese, etc.
Manufacturing Facilities: All Over Including: USA, Mexico, EU, China, and others.
Form: Gas
Color: Colorless gas
Odor: Low concentrations: none.
Odor(2): High concentrations: sharp; acidic
EPA Certification Required: No
Require Certification to Purchase? No
Cylinder Color: Unknown
Purchasing: Buy R-744 in Bulk


The Facts R-744 Pressure Chart

Knowing the pressure and the temperatures associated to the machine you are working on is essential to being able to diagnose any possible issues. Without knowing the temperatures you are more or less walking blind. These pressure checks give you the facts so that you can move onto the next step of your diagnosis. Instead of pasting a large table of information here I will instead direct you to our specific R-744 refrigerant PT chart.


R-744 Applications

Let me first start out by saying that R-744 is a very unique refrigerant, more so then others.  R-744 is a natural refrigerant. But, unlike other natural refrigerants, there is not a safety concern. With hydrocarbons you have the flammability risk, with Ammonia you have the toxicity risk, but with CO2 the safety risk is minimal. Along with it being a safe natural refrigerant it also is very versatile. It is mainly used in a transcritical refrigeration system but it can also be used in subcritical systems when done through a cascade. On top of that R-744 can be used as a secondary fluid refrigeration system.

Between these different types of refrigeration there are a wide a range of applications such as vending machines, supermarket refrigerators/freezers, industrial refrigeration, refrigerated transport, automotive air conditioning, heat pumps, and even in sports arenas for ice rinks. In this section we are going to take a look at each of these applications:


Vending Machines

One of the first targets in the global HFC phase down was R-404A. As you know, 404A was used in a variety of applications including vending machines. In the early 2010’s there was a push from a variety of companies, including Coca-Cola, to switch their vending machines away from 404A and over to R-744 Carbon Dioxide. Now, as I write this article CO2 vending machines are found all over the United States. One of the initial struggles of these systems was finding qualified technicians as these vending machines operate as a transcritical system rather then subcritical. The good news is that as the years go by and the amount of these transcritical machines grow then the technicians will become more seasoned and experienced with working on these kinds of systems.


Supermarket Refrigerators/Freezers

The grocery store refrigerators and freezer market didn’t switch over as fast as vending machines but there is significant progress being made. While R-744 isn’t necessarily the preferred refrigerant to use in these applications there are some companies moving forward. Depending on the application supermarkets will either use a stand alone plug-in unit that is very similar to a vending machine or they will use one system that connects to all of the various refrigerators and freezers.

When it comes to using R-744 the type of application will determine if the unit will be a subcritical cascade or a transcritical system. If we look at a stand alone refrigerator/freezer then we would be dealing with a standard transcritical system. This would operate very similar to how vending machines do. On the other hand, if we look at some of the larger systems that are all connected then we would be looking at a cascade system. A cascade system uses two or more refrigerants. In the example of R-744 we would find R-744 on the low temperature side of the cycle. By having R-744 isolated to the low end of the system we can prevent the refrigerant from going past the critical point and keep it subcritical. The other refrigerant used for the high side of the system can vary. It could be Ammonia, Propane/Isobutane, or even an HFC or HFO refrigerant.


Industrial Refrigeration

The term industrial refrigeration can be quite vague and can encompass a variety of applications from chillers, to chilled warehouses, to heat extraction, and so much more. In the past, before R-22 was phased out it was one of the top refrigerants used in these larger scale operations. When R-22 was phased out some companies switched over to the HFC R-404A/R-134a only to find that these refrigerants were going to be phased out soon as well.

In Europe, in Canada, and in other countries R-717 or Ammonia is one of the top picks when it comes to industrial refrigeration such as meat packing plants. Ammonia is chosen as it is highly regarded as the most energy efficient refrigerant out there. The downside, of course, is that Ammonia is toxic and can also be slightly flammable. Whenever you see a story in the new stating that a plant had to be evacuated due to a refrigerant leak the chances are that it is Ammonia is quite high. There are many instances of this occurring here in the United States and in most cases everyone is fine. We just have to ensure that the proper precautions are followed.

While R-744 may not be as efficient as Ammonia it has another thing going for it. It’s not toxic. That being said though it appears that the use of R-744 plants and chillers is still quite rare. I spent some time looking around online trying to find stories on R-744 plant usage but wasn’t able to find anything. It seems that Ammonia still has a strong hold on the industry but as technologies change and as HFCs become completely phased out we may begin to see more active R-744 industrial applications. If you know of some active R-744 plant applications please reach out to me and let me know.


Refrigerated Transport

When I hear the words refrigerated transport I instantly think of trucking. That’s most likely because I came from the trucking industry. I remember going through pallets of R-404A for our carrier refrigerated trucks. In the case of R-744 though the refrigerated transport we are discussing is naval transport or refrigerated shipping containers. It’s not just produce or meat being refrigerated on cargo ships though. No, in some cases cruise liners have installed CO2 systems to cool their larger refrigerators and freezers. Again, I looked around for any mention of R-744 being used in refrigerated cargo transport on trucks but saw no mention of it. This may still be down the road.


Automotive Air Conditioning

This one is definitely unique. If we rewind about ten years ago there were two refrigerants to choose from for automotive air conditioning. The first was the ever popular HFC R-134a. I am sure most of you are familiar with this refrigerant. You can buy cans of it at your local O’Reillys. At this time though a new refrigerant was introduced to the automotive sector. This refrigerant known as R-1234yf, was an HFO refrigerant invented between a partnership of DuPont and Honeywell. YF was to be the refrigerant of the future. It would replace R-134a and it would be used in every car from now on.

Most of the world was on board except for Germany. The German automakers had tested with YF and found that it was flammable. In one instance during a simulated collision the lines ruptured and spilled the refrigerant onto the hot engine block. The refrigerant ignited and caused a fire. This one test scared the German automakers, especially Daimler, away from using YF. While the rest of the world pushed forward with YF Daimler set off on their own to create the first automotive R-744 application.

Years later they achieved their goals and we now have German made cars using CO2 as their refrigerant. There is now no risk of flammability with their cars and they are still being environmentally friendly. I love hearing this story again and again as it’s a prime example of forging your own way and still coming out on top.


Heat Pumps

Japan has put forth a lot of focus on R-744 heat pumps. CO2 heat pumps can produce a much higher temperature output then a traditional HFC heat pump system. This is thanks in part due to the transcritical process. These heat pumps can heat water all the way up to one-hundred and ninety-four degrees Fahrenheit. (Source) The adaption of CO2 based heat pumps is moving forward, but it has been slowed due to the extremely high operating pressures and the breakage of components. (The same story we have seen in other CO2 applications.) In the future we will most likely begin to see CO2 heat pumps in mini-split air conditioner systems. Perhaps, down the road, we may even see them in traditional split air conditioning systems.


Ice Rinks

From my experience a typical ice rink uses either Ammonia, R-22, or an HFC such as R-134a or R-404A. What refrigerant is used seems to depend on what country you are in. Outside of America the standard refrigerant has been Ammonia. As we discussed earlier in this article Ammonia is widely seen as the most efficient refrigerant. When dealing with such a large application like an ice rink efficiency is a must. The downside of course, is the toxicity. The toxicity is especially important when it comes to a public area like ice rinks. It’s not just technicians or employees who are at risk but you also have the general public.

Here in America we are always hesitant to use the more ‘dangerous’ refrigerants such as Ammonia or Hydrocarbons. Because of this hesitation we instead went the route of R-22 for our ice rinks and hockey arenas.

Now though, with R-22’s phase out coming to a close in 2020 ice rink owners are looking for alternative refrigerants. Sure, there are HFC and now even HFO alternatives that can be used in these applications but each of these alternatives still have a higher then neutral Global Warming Potential (GWP). The problem with these refrigerants is that they will not stand the test of time when it comes to climate impact and phase outs.

If I was an arena owner or manager I would only seriously be considering two options. The first is Ammonia like I discussed earlier. This comes with it’s own risks but you get the low cost and energy savings. The alternative is R-744 Carbon Dioxide. R-744 has it’s own Pros and Cons which I’ll get into in our next section, but the big selling point is that you get a climate neutral refrigerant that is safe to the public in case a leak occurs. While R-744 systems aren’t widely found in the world today, they are growing. An article I was reading from 2016 had this quote, ”

“Today the number of CO2 ice rinks is growing rapidly. There are now 25-30 CO2 ice rinks in the world,” he says. 20-25 of these CO2 ice rinks are in North America, 20 of which are in Canada (mostly in Quebec) and three in Alaska, according to EKA.”


R-744 Pros & Cons

As we all know, there is no perfect refrigerant. Each one has its own individual upsides and downsides. It could have a great efficiency but also end up being very flammable. Or, it could be non-flammable and non-toxic but have very high Global Warming Potential. The point I’m making here is that there isn’t a perfect one out there and there may never be. In this section we’re going to take a brief look at the various Pros and Cons of using R-744 as a refrigerant. I pulled this information from all over the web, but one site in particular stuck out to me. This article from Emerson has an entire page dedicated to R-744 Pros and Cons.

Let’s take a look at the Pros and Cons of R-744:



  • R-744 is seen as the ‘perfect’ natural refrigerant as it is climate neutral and there is not a flammability or toxicity risk.  It is rated as an A1 from ASHRAE. While it is non-toxic there is still risk if a leak occurs in an enclosed area as R-744 will displace the oxygen in the room and could cause asphyxiation. It is always best to have a leak detector with you so that you can detect the problem early before anything major occurs.

  • Overall, R-744 is more energy efficient and has better heat exchange then a standard HFC based system. While it may not be as efficient as Ammonia this gap between the two refrigerants is shrunk as the evaporator temperature drops. Carbon Dioxide also has a low compression pressure ratio which can improve volumetric efficiency. In some cases CO2’s volumetric efficiency is four to twelve times better then Ammonia. (Source – under Pressure & Temperature.)

  • I mentioned this earlier, but the biggest selling point of R-744 is that it is climate neutral. It has no Ozone Depletion Potential and it’s Global Warming Potential is one. In fact, R-744 is the zero basis for the whole GWP scale. This is a huge Pro as if there is one thing that business owners are looking for it is stability and consistency. R-744 is never going away due to it being so climate friendly.

  • One Pro to R-744 operating at such a high pressure and being such a dense gas is that the overall size of the parts and components is smaller and the overall charge required for a refrigerant cycle is lessened. In some cases the compressor can be up to ten times smaller than an ammonia compressor. As far as refrigerant charges, one example I read from stated that to cool a two-hundred thousand square foot warehouse you would need forty-thousand pounds of Ammonia but with CO2 you would need less than seven-thousand pounds.

  • Carbon Dioxide is readily available and the price for this refrigerant is much less then HFC refrigerants that we see today. This is a welcome relief from the instability of prices on HFCs and HCFC refrigerants that we all know about.

  • .


While R-744 is the ‘perfect’ natural refrigerant in theory there are a lot of downsides.

  • The biggest one is for Carbon Dioxide to be used as a refrigerant it has to run under extremely high pressure. As an example, R-744 operates at ten times higher pressure then R-134a. Because of this extremely high pressure everything has to be custom built for an R-744 system so that it can withstand the high operating pressure. This includes the pipes, components, and everything else that goes along with the machine. If lesser components are used then you pose risk of constant failure due to the pressure.

  • If you wish to use R-744 as a stand alone refrigerant not in a cascade system then you will have to be running it as what’s known as a transcritical system. This is because R-744’s critical temperature point is only eighty-eight degrees Fahrenheit. There are many cases where the ambient temperature could be between eighty to one-hundred degrees. If your critical point for R-744 is only at eighty-eight degrees then how can you expect to remove the heat?

  • Suffice to say, a transcritical system and a high operating pressure system means two things.

    • The first is that there is increased expense for these systems. Not only do you have to pay for high pressure rated materials and parts but you also have to pay for a transcritical system. This setup is different then your standard subcritical system. The good news here is that with each year that passes technology improves and the cost of these higher pressure parts goes down.

    • The second is the increased complexity. The higher the complexity means less available qualified technicians. It may be a struggle to find qualified R-744 technicians, at least here in the United States. Each year though this is getting better as more and more businesses are adopting R-744 systems.

  • I mentioned efficiency in our Pros section earlier. The reason I mention it again is that the efficiency of R-744 is highly dependent on the type of system it’s being used in and the surrounding climate. I mean, think about it for a moment. We could have a subcritical cascade system for a supermarket in Miami. Or, we could have a transcritical ice rink in British Columbia. In each example we’re using R-744 but we now have two entirely different systems as well as two entirely different climates. Because of these variety of systems and applications it is difficult to measure one single efficiency measurement.

  • .

R-744 Past/Future

I am a big fan of history and can never get enough of reading historical books or watching documentaries. If you don’t understand the past then how can you understand the present or even the future? While refrigerant history might not be as interesting as other historical topics it is still good to understand it. R-744 can be traced back all the way back to the nineteenth century. In fact it was one of the very first refrigerants to ever be developed and used across the world. Experiments in refrigeration began in the late seventeen-hundreds and began to pick up speed in the eighteen-hundreds.
It was in 1850 that Carbon Dioxide was first proposed as a refrigerant by Alexander Twinning.
In 1869 one of the very first ice machines invented used Carbon Dioxide. Then in 1897 the first Carbon Dioxide refrigerator was introduced. More and more inventions and innovations followed.

In the late 1800’s and the early 1900’s there were a few mainstream refrigerants that we saw. These were your natural and hydrocarbon refrigerants such as Ammonia, Propane, Isobutane, and Carbon Dioxide. At this time Carbon Dioxide was found in all kinds of applications ranging from display cabinets, cold storage areas, market places, home/commercial kitchens, movie theaters, hospitals, trains, and even on cargo transport ships. The other natural refrigerants weren’t used as widely as Carbon Dioxide due to their safety concerns.

It seemed that R-744 was going to reign supreme as the main refrigerant in the world. This held true until the 1930’s. It was then that a partnership was formed between General Motors and DuPont. This partnership was made with one goal in mind: To create a cheap, safety, and reliable refrigerant. While Carbon Dioxide was safe it had it’s own problems. Just like we mentioned in our Pros and Cons section R-744 systems had numerous failures due to the extreme pressure that they operated under. The technology just wasn’t there to prevent these failures either so these air conditioners and refrigerators would be very expensive to maintain.

After some time the General Motors & DuPont partnership came out with a new artificial class of refrigerants known as CFCs and HCFCs. Some of the refrigerants in this new classification were R-12 and R-22. These new refrigerants checked all of the boxes. They were safe. They were cheap. They were reliable. There was no more constant failure due to high operating pressures. At first, the adoption of these refrigerants was slow but that was only because of the manufacturing speed of the product.
It was in the 1950’s that an innovation was done that greatly increased the speed of manufacturing CFC and HCFC refrigerants.

Once the supply could be met the demand skyrocketed. It wasn’t long until CFC and HCFC refrigerants were found all over the world in various applications. They could be your home air conditioner, your automobile, your refrigerator, or your local grocery store. They were everywhere. In the 1960’s there were a few more CFC/HCFC refrigerants invented, including R-502, that led to even more explosive growth.

With the growth and dominance of these new refrigerants it seemed that R-744 had taken a backseat. It was cast aside when the newer refrigerants came to market due to the high pressure that it operated at. There was no reason to use this expensive refrigerant anymore due to the mass production and reliability of R-12, R-22, and R-502. At least for a while, R-744 had reached it’s peak. It was in the 1980’s that things began to change.


The Ozone

It was in the 1980’s that a problem was discovered. Two American scientists, Mario Molina and Shepwood Rowland, from a California university were the first to notice Chlorine’s effect on the atmosphere. (Remember now folks, all of these CFCs and HCFCs contain Chlorine.)

These two scientists found that when a CFC refrigerant was exposed to ultra-violet irradiation that the Chlorine atom would detach itself from the CFC molecules. The remaining residue is oxidized resulting in the creation of a Chlorine oxidized molecule and a new residue. The Chlorine atom and Chlorine oxidized molecule move their way up to the stratosphere. Within the stratosphere there is a layer called the Ozone layer. This Ozone layer protects the Earth from ultra-violet rays and irradiation. What these scientists found out is that all of this Chlorine from CFC and HCFC refrigerants was working it’s way to the stratosphere. When it reached the stratosphere the Chlorine began to attack and weaken the Ozone layer.

Over decades of using CFCs and HCFC refrierants Chlorine began to accumulate in the stratosphere and overtime a hole began to form in the Ozone layer. Now, I say hole but this wasn’t a hole per-say. Instead, there was a weakening of strength in the layer. So, while there was not a hole the thickness of the Ozone was decreasing and decreasing rapidly thanks to the CFC and HCFC refrigerants.

The Ozone prevents harmful UVB wavelengths of ultra-violet light from passing through the Earth’s atmosphere. Without it, or with a weakened version of it, a variety of bad things could happen. Some of these include a much higher increased chance of Skin Cancer, more severe sunburns, more chances of cataracts, and a whole host of other problems.

After discovering the weakening of the Ozone layer nations banded together in what is seen as one of the greatest and most effective treaty’s every made. In 1986-1987 the Montreal Protocol was created and signed by over one-hundred nations across the world. This Protocol was an international treaty designed to protect the Ozone layer and to completely phase out the chemicals responsible for the weakening of the Ozone. The treaty went into effect in 1989.

Soon after that date marked the beginning of the end for CFC and HCFC refrigerants across the globe. The industrialized countries, like America, began to phase out the refrigerants first. R-12 was phased out in the early 1990’s along with all of the rest of the CFC refrigerants. The HCFC refrigerants such as R-22 or even R-502 were given a bit more time. Heck, R-22’s true phase out didn’t even begin until 2010.

Out with the old and in with the new, so they say. The refrigerants that were proposed to replace CFCs and HCFCs were known as HFCs, or Hydroflurocarbons. These refrigerants contained no Chlorine so there was no chance of them hurting the Ozone layer. Some of these refrigerants include popular refrigerants today known as R-134a, R-404A, and R-410A. But, now these refrigerants are under fire for their increase to Global Warming.


R-744 Present & Future

As I mentioned above HFCs were seen as the world’s savior from the Ozone depleting refrigerants. But, HFCs had their own problem. Instead of the Ozone this time it was Global Warming. These HFC refrigerants such as R-134a, R-404A, R-410A are known as ‘Super Pollutants,’ or ‘Greenhouse Gases.’ In order to measure their impact on the environment each of these refrigerants were given a Global Warming Potential number. The higher the number the more damage the refrigerant causes to the world. As a zero based scale for this measurement our old friend R-744 was used. Carbon Dioxide has a GWP of one. In comparison, R-404A has a GWP of three-thousand nine-hundred and twenty-two. Obviously, there is a large difference here.

I’m writing this article in 2019 and over the past ten years or so there has been a worldwide push to phase down and in some cases phase out HFC refrigerants completely. In order to phase out HFC refrigerants we have to have a replacement refrigerant. In some cases companies and countries have turned to a new classification of refrigerants known as HFOs. These HFO refrigerants are again synthetic products created by Honeywell & Chemours (Formerly DuPont). The problem with HFOs though is that they still have Global Warming Potential. Yes, not as high as HFCs… but the numbers are still there. Along with the GWP risk they also have a slight flammability risk. To me, I do not see HFOs being sustainable. I imagine the world will decide to phase them out in another ten or twenty years.

So, what is the solution you may ask? It’s R-744! Well, R-744 and other natural refrigerants. Technology has changed significantly since the last time R-744 was used widely. It’s been almost one-hundred years since we saw the mainstream use of Carbon Dioxide and now with nearly a century behind us the technology has significantly reduced the chance of component failures due to high operating pressures. We are now able to create efficient and stable R-744 systems without a large risk of failure.

While the cost of implementing R-744 systems is still quite higher then a traditional HFC system the costs have been coming down. This holds especially true in recent years as the push to innovate R-744 systems increases substantially with the phasing down of HFCs. While we are not there yet the costs are quickly shrinking the gap between HFCs and R-744.

There are many companies pushing forward with R-744 systems. Most of these are on smaller systems such as vending machines, but we all need to take baby steps. One company in particular, Coca-Cola, has installed hundreds of CO2 vending machines across the country. Along with Coca-Cola there are other grocery store chains out there using cascade R-744 systems mixed with other refrigerants such as Ammonia or lower GWP HFCs. We are even beginning to see R-744 uses in automobiles with the innovations that Daimler has made. As we completely phase down HFCs over the next ten years we will see more and more usage of R-744. It’s time has come again!



Well folks, after all of that I feel like we have covered our bases when it comes to R-744.  If you only take a few things away from this article let them be this. First, R-744 is a growing market across the world and you will begin to see more applications. It doesn’t matter what section of the industry you specialize in. R-744 is so adaptable that it’s only a matter of time before you come across it.

Secondly, since R-744 has no Ozone Depletion Potential and a Global Warming Potential of only one you can be safe in assuming that Carbon Dioxide will never be phased down our across the world. It is a natural refrigerant that has been used for over one-hundred years and it will continue to be used for another one-hundred years or more.

If you find that you’re still looking for more on R-744 then please check out our sources section below. This is where we pulled most of our data from and in some cases these websites can have a ton of information.


Relevant Links on R-744


News, Natural Refrigerants, R-744, Refrigerant Fact Sheets


Natural Refrigerants, news, R-744, Refrigerant Fact Sheet

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R-744 Carbon Dioxide Refrigerant PT Chart

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