I’ll be the first to admit that I know very little when it comes to a transcritical system. I have seen it mentioned numerous times and have also seen that it is starting to become a trend in certain newer environmentally friendly applications. In an effort to educate myself I’m going to take a look at transcritical systems and how they work in this article.
We are all familiar with subcritical refrigeration process. This is the same process that is used in most every air conditioner or refrigerator across the world. It consists of four specific processes known as evaporation, compression, condensation, and expansion. A subcritical system has ALL of it’s processes occur below the refrigerant’s critical temperature.
When parts of the cycle process take place at pressures above the critical point and other parts below the critical pressure the cycle process is referred to as transcritical cycle. Transcritical systems are found when using R-744 Carbon Dioxide refrigerant. This is due to R-744 having an extremely low critical temperature of thirty-one degrees Celsius. As a comparison, R-134a has a critical temperature of one-hundred and one degrees.
There are many cases where the ambient temperature could be between twenty-five to thirty degrees Celsius. If your critical point for R-744 is only at thirty degrees then how can you expect to remove the heat?
Differences between Subcritical & Transcritical
The key difference with transcritical systems is that the heat rejection process is different. There is in fact no condensation. This is due to the low critical temperature of certain refrigerants. In transcritical systems the heat rejection takes place at temperatures above the refrigerant’s critical temperature.
When a refrigerant reaches a temperature above it’s critical point it is no longer known as a gas or a liquid but instead known as a fluid. This fluid condition is also known as a gas condition or state. So, when rejecting heat with a transcritical system it is known as ‘gas cooling.’ Therefore the heat exchanger in an transcritical system is known as a ‘gas cooler.’
Besides the difference in heat rejection though the rest of the refrigerant cycle remains the same. We will go into the transcritical process in our next section:
The Process
A transcritical process begins with the compressor just like it does with a subcritical system. The difference here though is that as the compressor compresses the vapor refrigerant the temperature rises and rises until it reaches past the refrigerant’s critical temperature. This is where the state change differs. Instead of a liquid we get a state in between liquid and vapor known as fluid.
The next step in the process is the rejection of the heat gained from compressing the vapor. The heat exchanger, or gas cooler, expels the heat all the while having the temperature staying above the critical point. During this process you will also have the temperature vary between the point it left to the compressor to when it goes to the expansion valve.
Next, as you know, is the expansion process. At the time the refrigerant comes into the expansion vale it is above the critical temperature and in a fluid state. When leaving the expansion valve the refrigerant is no longer above the critical temperature and it is a mixture state of liquid and vapor.
Lastly, we are at the evaporator. In the evaporator the refrigerant comes in as a liquid at a constant pressure. Obviously, during the evaporation cycle we change states again to vapor that is slightly superheated. The vapor then makes it’s way to the compressor to start the process over again.
One thing to note that with a transcritical system superheat and subcooling temperature aren’t as important. While they can still be helpful, most folks only look at evaporating and condensing temperatures. In fact, with a transcritical system there is no condensation process and therefore no subcooling.
Frequently Asked Questions
Are There CO2 Systems That Aren’t Transcritical?
- Yes, most often these are found in what’s known as cascade systems. These systems contain two types of refrigerants. In these examples the CO2 refrigerant is used during the low temperature stage of the refrigerant cycle. This ensures that the refrigerant does not rise above the critical temperature.
Are there other popular transcritical refrigerants?
- From what I have researched CO2/R-744 is the only transcritical systems used today. I also went through a list of all refrigerants and their critical temperature and only found a few that were very low. R-744 was the only common one that I found that is used today. If you know otherwise, please reach out and let me know.
- One reader reached out to me and informed me that in some cases R-410A can be used in a transcritical system. That is because 410A’s critical temperature is only one-hundred and sixty-two degrees Fahrenheit. If you are in a warmer climate, in the summer, and the sun is beating down on a rooftop condenser then temperatures could very well come close tot hat one-hundred and sixty degrees mark.
How often do we use transcritical systems?
- With constantly improving technology and the push to move the worlds towards greener refrigerants we are seeing a substantial rise in transcritical systems across the globe. Most of these new systems are found in Europe and other countries but the United States is making inroads as well. We’re always just a bit behind Europe though…
Are transcritical systems more expensive?
- Yes, they are when compared to traditional HFC systems. This is especially true here in the United States as there aren’t as many technicians who are familiar with the technology and the parts aren’t as readily available. In the US these systems are nearly twice as expensive but in the EU they are only around thirty percent more.
- The good news here is that CO2 systems are slightly more efficient then HFC systems and the cost of R-744 refrigerant is significantly cheaper then HFC refrigerants.
Why Should I Choose a Transcritical CO2 System?
- Yes these R-744 systems are more expensive but you get peace of mind with a transcritical CO2 system. They are never going to be phased out like a R-404A application will be. CO2 has a negligible environmental impact and will be around for decades to come. It is a safe investment in the future of your business.
Conclusion
Well folks I learned quite a bit during writing this article. I had to dig through some articles to educate myself. All of my sources articles can be found below. If you have further interest in learning about transcritical systems then I highly recommend you look at ACHRNews. That article goes in-depth on the transcritical process as well as including diagrams and illustrations to help drive the points home.