Welded carbon dioxide piping, which is not the same as traditional brazingCarbon dioxide is here to stay and it’s going to become more widespread—at least that’s my prediction. In January 2020, the Government of Canada restricted the use of R507 in new installation for applications above -40C; we are just starting to see some of the significant implications of this change.
One of the industries having to deal quite rapidly with this change is industrial freezing applications. Most of these types of freezers operate in the -30C to -40C range, so don’t qualify for an exemption. They are also usually quite large and suffer significant performance degradation using direct expansion evaporator coils. This means there is a lot of motivation to use a pumped overfeed refrigeration system. While the majority of the large and very large systems use ammonia and will likely remain that way, a significant portion of the medium and smaller-sized industrial freezers are likely to use carbon dioxide. That’s partially because, at least at the moment, there really isn’t another choice. If you want to use liquid overfeed evaporator coils, you can’t use zeotropic blended refrigerants as they will separate in a saturated vessel. I am not currently aware of any azeotropic blends or pure refrigerants that are currently being used (effectively) to replace R507 in these applications. Carbon dioxide is also on the rise in small unitary equipment, automotive cooling and heating, and heat pumping applications.
There are some significant challenges associated with using carbon dioxide, including higher pressure, triple point, poor energy efficiency, and poor performance in hot weather. The industry has been making progress to tackle many of these challenges. I wanted to take some time to discuss a couple of the more common challenges.
Higher Pressure
Carbon dioxide requires more compression at a higher outdoor temperature, compared to lower outdoor temperatures. This means that multiple compressors is often neededIn transcritical carbon dioxide systems, the design pressure (i.e. the pressure that we design the piping to withstand) is usually around 1,750 psig. This raises several concerns. It can be difficult to find components, especially in Canada if CRN is required. It is also challenging to navigate the requirements in different provinces in Canada regarding the use of material that is sufficiently rated to a UL standard but does not meet the requirements of ASME B31.5.
The pressure tests result in a large amount of stored energy and require registration in some provinces, and large blast radii and fragment throw exclusion zones even for relatively small internal piping volumes. There is an increased hazard working with increased pressures. The codes and standards are not up to date with technology making it hard to ensure compliance.
While none of the challenges associated with high pressure are insurmountable, there is a learning curve for the industry to adapt to more rigorous safety and construction standards. Since supermarkets were early adopters of carbon dioxide refrigeration systems, the current expertise in carbon dioxide refrigeration at the contracting level is largely maintained with contractors who do supermarket work. It takes an effort for these contractors to implement quality control programs and more industrial-style construction, which is necessary and regulated in Canada.
Poor energy efficiency and performance
Transcritical carbon dioxide systems are simply not the most energy-efficient solution in hot weather if you compare them to other refrigerants. I often use Figure 1 to exaggerate this difference. Figure 1 shows the coefficient of performance for different refrigerants at 20F SST and 85F SCT in a simple refrigeration system. There are many ways to improve the carbon dioxide refrigeration cycle by modulating discharge pressures, using ejectors that take the high-pressure gas and use it to provide free interstage compression, and using more complex booster and two-stage compression cycles. With all of that said, it is a lot of work to make carbon dioxide reasonably efficient in hot weather. In cooler weather, however, the same carbon dioxide can be significantly more efficient. There are many particulars to examine but in several energy models I’ve seen for applications in Canada, carbon dioxide can be more efficient than most other refrigerants when the entire year is considered.
Figure 1: The ideal COP for different refrigerants at 2 0F SST and 85F SCT. This is not how you would operate a CO2 system, but it shows that at some conditions the energy efficiency is less than ideal.The poor performance and efficiency aspects of carbon dioxide are tied together. The fact that carbon dioxide requires much more compression at higher outdoor temperatures than it does at lower temperatures means that it is ideal to have multiple compressors or compressors with some method of capacity control.
Again, none of the challenges are insurmountable but they add cost and complexity to the system.
Advantages
On the other side of the spectrum, there are many advantages to carbon dioxide and reasons why it is gaining popularity. The first reason is that it is a low-toxicity natural refrigerant that will not be phased out or phased down. Certain sectors of the end-user market are frustrated by the never-ending change in halocarbons and what looks like another change that will lead to flammable refrigerants in commercial applications.
Carbon dioxide is seeing a rise in popularity in the use of heat pumps.The other major advantage to carbon dioxide over other refrigerants is its ability to be applied in heating applications. One of the applications I’ve found intriguing lately is the use of carbon dioxide in heat pumps to provide both cabin heating and air conditioning in electric cars. Since there is no engine to generate heat and resistance, electrical heaters are significantly less efficient than heat pumping and contribute to more rapid battery use. The reason that carbon dioxide has an advantage over other refrigerants is that it can achieve higher temperatures quite easily and it’s fairly easy to control the temperature heat is rejected. Carbon dioxide can do this because it is operating super critically. At this operating condition, there is no such thing as saturation and the refrigerant simply cools from a hot discharge vapour to a warm vapour. We can control both what temperature we obtain and how much energy we need by changing the discharge pressure.
It has been interesting to watch as carbon dioxide is being applied more often. As I mentioned previously, it is now becoming pretty common to propose carbon dioxide as the most reasonable solution for small and medium industrial freezing applications such as spiral, tunnels, and plate freezers. I think this trend is likely to continue. It has also been fun to see some commercial contractors adapt and grow in order to accommodate the necessary level of installation standards and code compliance. I look forward to future innovation and more progress adapting to some of the challenges associated with using carbon dioxide.