CASESηmax CO2/CO2 Cascade Refrigeration System

ηmax CO2/CO2 Cascade Refrigeration System

Those who know ηmax knows what we believe: Efficiency through Simplicity.

Back in 2019, demand for industrial carbon dioxide (CO2) refrigeration solutions was starting to grow in Japan. However, there was really only one offering in the market for end users: CO2 booster systems.

A long time customer of ours was keen to explore alternative CO2 solutions and during one of our business discussions, a challenge was raised: are you guys able to come up with something different?

Observing the lack of choices for customers who were keen to explore CO2 solutions, we started the project by studying existing CO2 booster technology.

Basic theory of CO2 transcritical system 

CO2 was actually one of the first cooling medium to be used in refrigeration systems, and dates back to the 1850s.

However, CO2 gave the perception of being a difficult refrigerant to work with.

  1. High operating pressure level was seen as a potential safety hazard
  2. Low critical point seemed to imply low theoretical cycle efficiency
  3. Lack of development in components and systems meant many limitations during system design

As technology advanced and synthetic refrigerants were developed over the years, many refrigeration companies chose to work with these newer refrigerants instead.

It was not until Dr. Lorentz’s development of the modern CO2 transcritical refrigeration system, coupled with the global push for cleaner and more environmentally friendly refrigerants, that CO2 experienced a renaissance in the refrigeration industry.

While much resource have been poured into improving the efficiency of the booster system over the past 30 years, we realized that these advancements have also made the booster system more and more complicated.

On the other hand, advancements in component development and CO2 know-how provided us a platform to realize that the ηmax CO2/CO2 cascade refrigeration system can become a viable alternative to the booster system.

Why a CO2 cascade system?

Many equate cascade systems with poor efficiency, which is partly true. Typical cascade refrigeration systems in the market would employ plate heat exchangers in their systems, but due to uneven refrigerant flow between the boiling plates (both horizontally and vertically) and the short distance between the boiling surface and the suction tube, the refrigerant flow rate is forced to be limited and the superheat increased, resulting in inefficient operation that does not provide the expected cooling capacity at the designed temperature difference.

However, we believe that safe and efficient cascade systems are exactly what the market needs today.

By identifying the main problem with existing cascade refrigeration systems, i.e. poor heat exchange efficiency, Nakayama Engineering has succeeded in developing a much more efficient heat exchanger that would make the cascade refrigeration system the optimal and simplest solution for all climates and regions. This means that even in environments where the outdoor air temperature exceeds 40°C, which is supercritical operation, no water sprinkling or other assistance is required at all. The reverse is also true: when the outdoor air temperature goes below 7°C, the high side circuit shuts down, resulting in a much higher COP. As the temperature drops further, at -20°C (to = -32°C), COP=7 is exceeded.

Furthermore, the ability to reduce power consumption in low ambient temperature conditions by running only the low side circuit allows the ηmax system to omit VFDs from our design without compromising on performance and efficiency.

Fig.1 Cross sections of Shell and Fin Coil Heat Exchanger (SFCHX)

Fig 2. Actual SFCHX

With our extensive understanding and experience of how the fin coil heat exchanger works in our unit cooler design, of which we have delivered more than 3,000 units over the years, we believed the heat exchange efficiency can be replicated by placing the fin coil assembly within a sealed shell. Not only does the heat exchange take place through the tubes/fins that are coming into contact with the low side discharge gas, the heat exchange efficiency is improved through the subcooling effect as the fins are partially immersed in the condensed refrigerant that remains in the shell. Finally, direct contact condensation is promoted when the low side discharge gas comes into contact with the wavy liquid surface of the subcooled liquid refrigerant collected at the bottom of the shell created through the diffusion of the discharge gas.

In addition, by arranging the fin coils in a multi-circuit configuration inside the shell, the heat transfer surface becomes much larger than that of a plate heat exchanger, a safe and efficient operation is ensured with a very small difference between the high side evaporation temperature and low side condensation temperature, with completely superheated gas exiting at the coil outlet.

We are convinced that this technology has the potential to become the mainstream heat exchanger of the future with its revolutionary performance. Detailed technical information can be downloaded here free of charge for IIR members.


First installation brings 50% energy savings to end user!

Model:                          EM-R2000

Room Temp:                  -25°C

Application:                   Cold Storage

Location:                       Northern Japan

Installation Date:            October 2021

Based on data collected over the past year, the ηmax CO2/CO2 Cascade Refrigeration System is providing the end user more than 50% energy savings from their old R22 refrigeration system.

With four distinct seasons like Japan, the ability to have low side only operation during the winter and transition periods before and after it allows for tremendous energy savings potential. Furthermore, the ηmax unit cooler can be operated with a temperature difference(Td) of 2 to 3°C, enabling operation for room temperatures as low as -50°C a possibility. This unique and highly advanced technology has made the cascade system an optimal and simple solution.

The successful operation of the system also proves that efficiency can be achieved through simplicity and with a couple more projects lined up before the end 2022, it is a big step forward for the natural refrigeration industry!

*The cascaded system, in this case, is implemented with CO2/CO2. However, different combination of refrigerants can be used, and the material of the tubes and shell can be changed accordingly.