A few weeks ago I did an article on transcritical refrigeration. In that article I stated that I knew very little on the subject, but as I wrote the article I was able to teach myself a bit about it and understand the principles behind the transcritical system.
Well today we are going to be doing the same type of post except this time on cascade refrigeration systems. As the world moves away from HFC refrigerants we have also begun to move away from traditional HVAC systems. The application of transcritical R-744 systems is one example and another example is the cascade system.
What is Cascade Refigeration
As most of you know each refrigerant has its upsides and downsides. Along with climate friendliness, flammability, and toxicity, there are also refrigerants that are better for lower temperature applications and there are some better at higher and medium temperature applications. A cascade system takes advantage of this by using two different refrigerants, one for high side and one for low side. In some more complicated cases there can be more than two refrigerants used (source).
These two refrigerants operate independently and each have their own boiling points. The refrigerants run through their own cycles and then are joined together by the heat exchanger. At the heat exchanger the high/medium temperature refrigerant is used to cool the condenser of the lower temperature refrigerant. Another way to look at it is the condenser for the lower temperature system acts like the evaporator of the higher temperature refrigerant. In fact, the condenser in the lower temperature side is coupled to the evaporator on the higher side. So the evaporator on the higher side removes the heat released by the condenser in the lower cycle.
Cascade systems exist when we are working with two large temperature differences. Say for example, the ambient temperature is seventy-degrees but you need a desired temperature of negative fifty degrees. (Cascade systems are used a lot in laboratories and research institutes.) Now, with a normal system this extreme temperature difference would not be achievable. This is where the cascade system comes in handy as you are able to bridge the temperature difference when you have two different refrigerants. Using this method certain cascade systems can achieve temperatures as low as negative two-hundred degrees Fahrenheit.
The advantages of cascade systems are twofold. First, as we mentioned above, cascade systems can achieve drastically low temperatures that traditional refrigerant circuit systems just can’t reach. Along with the cold temperatures cascade systems can be more energy efficient than the standard systems. This is done by choosing the most efficient refrigerant for both high side and low side applications. This allows for maximum efficiency and a savings on operating costs per month.
The downsides to a cascade system are rather obvious. First, they can be more expensive to build then a traditional system. This may come into play if the business owner is looking to install a cascade system in their place of business. (An example of this will be in our next section.) The other downside is that while cascade systems still pretty much follow the traditional refrigerant circuit they can be a bit more complicated to diagnose then your standard system.
Future Use of Cascade Systems
While cascade systems were mainly used for lab work in the past we are beginning to see a growth of these applications in less traditional areas. An example of this is seeing a cascade system installed in a supermarket freezer/refrigerator section. Back in 2016 Whole Foods installed a cascade refrigeration system for their new Santa Clara, California store. This cascade system is comprised of propane (R-290) and Carbon Dioxide (R-744).
The system contains nearly three-hundred pounds of propane charge, BUT, the propane never leaves the roof of the building due to it being a cascade system. Inside the building the harmless and non-flammable CO2 is used (Seventeen-hundred pounds of CO2 used).
This system is a prime example of using a cascade system in place of a climate damaging HFC system such as R-404A or R-134a. With this cascade system, Whole Foods was able to use switch their entire store over to natural refrigerants while still remaining efficient and keeping their customers safe and secure. There are also instances where a split system of Ammonia (R-717) has been used.
This investment was a great way for Whole Foods to future proof their refrigeration system. Neither one of these refrigerants are going to be phased out as they have no impact on the climate rather it be through Ozone Depletion or Global Warming Potential. R-744 and R-290 will be around forever. While yes, the downside may have been a heavier investment then a traditional system, Whole Foods can sleep easy knowing that their system will be in compliance for decades to come.
Conclusion
As HFC refrigerants begin to see further decline across the country we will begin to see more and more cascade systems applied. The good news is that if you understand the basic refrigeration circuit then you’ll find that a diagnosing a cascade system isn’t much different.
To learn more about cascade systems I recommend you visit the site from RSES.org. While these articles are older they still provide a wealth of information on cascade systems and how they work.
Lastly, if you see anything that appears to be inaccurate or if I forgot something key on cascade refrigeration please reach out and let me know and I will correct as soon as I can.