With carbon dioxide (R744) gaining momentum as a leading option for refrigeration systems, service technicians need to understand how R744 systems differ from their HFC counterparts. Accelerate America explores seven key strategies for service technicians working on CO2 transcritical systems to help them to understand the unique properties of this refrigerant.
1. Keep cool under pressure
R744 operates at a higher pressure than HFCs, but this is easy to manage if you are properly trained. Really high pressures will only occur on hot summer days between medium-temperature compressors and pressure-reducing valves, mostly on or near roofs. Pressures in this area can reach extremes of up to 1,400 psig at 115.6°C and typically require stainless steel piping. The remainder of the system, however, will be in 400–500 psig range — almost the same pressures as in 410A high-side systems.
2. Understand R744’s critical and triple points.
The critical point of R744 is only 31°C/1,055 psig, a relatively low temperature for a refrigerant. Above the critical point the pressures and temperatures are independent of each other. R744 systems operating above this point are said to be in “supercritical” mode.
Another important concept to understand is the “triple point”. The three phases of CO2 (solid, liquid and vapour) coexist at -56.6 °C. That may seem low, but the corresponding pressure is 60.4 psig. R744 at that point will turn to dry ice in an instant. For this reason, technicians cannot begin charging an R744 system with liquid, because the internal pressures will be well below 60.4 psig.
3. Optimise efficiency in warm ambient environments.
Transcritical R744 systems work more efficiently in colder environments. In Toronto, Canada, for example, a system might only operate 200 hours per year in supercritical mode. In Atlanta, Georgia, the same system could require 1,020 annual supercritical hours.
4. Proactively manage power outages and standstill pressures.
When an R744 system shuts down, pressures build faster than in HFC systems. It is critical to have a strategy for ‘power off’ conditions ahead of time.
One way to avoid losing R744 through pressure relief valves is to install a small standby condensing unit to cool liquid in the flash tank and to keep the pressure down. Another option used in some smaller systems is a ‘fade-out’ vessel that can increase system volume when the system is down.
5. Provide proper equipment for service technicians.
CO2 rack manufacturers are making life easier for service technicians by including multiple pressure transducers and temperature sensors in their systems. It is also important to have a good calibrated gauge set with proper high-pressure hoses in the machine room.
6. Use best practices for R744 cylinder storage, charging and maintenance
For an R744 system, it is important to have a refrigerant plan that accounts for three key issues:
- Local codes: Where can you store R744 cylinders? Do they have to be inside the building?
- Stock storage: If your system needs 2,000 pounds, that’s 4,600 pounds of cylinder and refrigerant. A mezzanine might not be the best place to put all that weight in a concentrated area.
- Transferring R744 to the machine room: If the machine room is on the mezzanine, it might be worth running permanent high-pressure hoses to a charging manifold on a ground-level storage area.
7. Know other unique properties of R744
Understanding R744’s peculiarities will significantly reduce maintenance costs and downtime.
This article is based on a longer version authored by André Patenaude, director of CO2 business development at Emerson Climate Technologies. For more tips on working smartly with CO2 systems, please click here.