Making Sense of Servicing CO2 Transcritical Booster Systems
Amidst increasing regulations to phase-down hydrofluorocarbon-based (HFC) refrigerants, global adoption of CO2 (R744) as a natural alternative is on the rise. But to successfully perform service on a CO2 system, mechanics will need proper training and a deep understanding of the refrigerant’s unique properties.
Our 13th Making Sense webinar, Seven Keys to Servicing CO2 Systems, offered a basic primer on CO2-based refrigeration and discussed the many implications of servicing a CO2 transcritical booster system. The webinar was presented by Andre Patenaude, director of CO2 business development for Emerson Climate Technologies.
Andre introduced common CO2 system architectures — secondary, hybrid cascade and transcritical booster; the latter is the only system that relies completely on CO2 as its only refrigerant. He explained that CO2’s global adoption has largely been driven by its effectiveness in cooler climates due to its low critical point of 87.8 °F. This is primarily the reason that subcritical systems (where the ambient temperature is typically below 87.8 °F) have been successful to date. It’s also the reason that CO2 transcritical booster system technology (above 87.8 °F) is evolving to enable operation in warmer climates.
In addition to low critical point, Andre explained two other primary differences between CO2– and HFC-based systems:
- High triple point (where three phases of refrigerant co-exist) — the triple point temperature for CO2 is -69.8 °F, but the pressure is relatively high at 60.4 psig. Technicians need to avoid approaching that point, or the CO2 refrigerant will turn to dry ice in the system. This is why systems need to be charged with vapor CO2 first in order to reach sufficient pressure (145 psig) before switching to its liquid form to complete the charge.
- High pressure — historically, CO2’s high pressure has been the primary concern for adoption. At supercritical mode, on a hot day above 87.8 °F, the pressures on the roof (condenser) could be as high as 1,400 psig, with temperatures approaching 240 °F. To handle those conditions, stainless steel piping is required. A pressure-reducing valve is also needed to lower the pressures to a point where it’s similar to working with an HFC system (400–500 psig).
Because CO2 transcritical booster systems lose efficiency in warmer climates, several techniques are used to offset the impacts of high ambient air temperatures.
- Spray nozzles — condenser that mists water to cool air across condenser coils
- Adiabatic gas cooler — wet pads lining the outside of condenser are used to cool air and keep the system from going into transcritical mode
- Parallel compression — flash tank feeds an independent compressor with increased suction pressure and smaller motor
- Subcooling — cools the gas to increase efficiency
- Ejectors — a means of using high-pressure gas energy to reduce evaporator superheat and increase suction temperature
Andre stressed the importance of having a plan to deal with power outages, including designing systems with generators and standby condensing units to keep CO2 pressures from building up during system shutdown. In fact, retailers should test the effectiveness of their response by performing a trial power outage.
Most importantly, service technicians should be well-trained to understand the intricacies of dealing with CO2-based refrigeration systems.
To hear this webinar in its entirety, please visit the webinar archives on our MAKING SENSE website.