Over the past decade, a major shift has taken place in the refrigeration industry. Across food retailers and industrial applications, CO2 (R-744) has emerged as an ideal energy-efficient natural refrigerant because of its high thermal efficiency and low environmental impact. CO2 systems present advantages for retailers in terms of energy savings, but their streamlined piping and wiring also benefit contractors and technicians.
Future-focused retail owners and technicians should become familiar with CO2 refrigeration systems to keep up with the changing industry landscape.
Refrigeration Cycle Basics
Both HCFC/HFC and CO2 racks have basic components in common. For example, evaporators within the refrigeration case absorb heat into the refrigerant. Medium-temperature (MT) and low-temperature (LT) compressors create a pressure differential that drives the flow of the refrigerant. The condenser (for HFC systems) or gas cooler (for CO2 systems) then rejects the heat into the atmosphere, and the refrigerant is recirculated to continue the refrigeration cycle.
A closer look reveals key differences. HCFC/HFC systems operate in two or three different pressures — LT suction, MT suction, and discharge/liquid pressure — whereas CO2 systems operate in four different pressures — LT suction, MT suction/LT discharge, MT discharge, and liquid pressure.
Another key difference is, while HCFC/HFC leaves the condenser as a liquid, CO2 can leave the gas cooler as a liquid or, if the ambient temperature is high, as a supercritical fluid (SCF). SCF is a fluid at a temperature and pressure above the critical point, where distinct liquid and gas phases do not exist.
To manage this mixture of liquid and vapor, CO2 systems incorporate specific components that maximize the system coefficient of performance (COP).
REFRIGERANT COMPARISON: Refrigerant saturation domes diagram, comparing various refrigerants. (Courtesy of Hillphoenix)
CO2-Specific Components
In CO2 refrigeration systems, the high-pressure control valve (HPV) aids subcooling in the condenser when the system is in subcritical mode. It creates a pressure drop in the flash tank and maximizes the COP during transcritical operation. This HPV is like the hold-back valve in an HFC system but with advanced controls.
The flash gas bypass valve manages liquid quality and helps to maintain a constant pressure within the flash tanks by allowing CO2 vapor (or flash gas) to flow from the flash tank to the MT suction. The flash tank directs flash gas to the flash gas bypass valve, which maintains the tank’s pressure at a constant level. Here, the refrigerant is separated after expansion via the high-pressure control valve.
At the case level, HCFC/HFC cases have components that are not required for CO2 cases, such as the distributor, heat exchanger, hand valves, and separate defrost controls. Likewise, CO2 display cases contain unique components such as electronic expansion valves (EEVs), local case controls, and typically additional sensors on the coil outlet.
Maximizing Efficiency
EEVs can maximize efficiency for CO2 cases or HCFC/HFC cases and are available as either pulse or stepper valves. While they operate using different algorithms, both provide energy savings.
EEVs maintain the superheat at the case’s outlet, eliminating the need for manual superheat adjustments. If adjustments to the set point are required, they can be made remotely. The result is better case performance and a consistent temperature that protects product integrity. EEVs also eliminate floating suction pressure limits and set point drifting, which are common problems with evaporator pressure regulator (EPR) controls.
Case controllers are necessary for CO2 cases, but they can also improve efficiency in HCFC/HFC cases. Case controllers optimize evaporator performance by automatically regulating the injection of refrigerant. Controller settings are maintained locally, providing protection in case of network failure. In Hillphoenix cases, controllers are installed, wired, and tested at the factory for streamlined installation and startup.
Case controllers simplify wiring by providing a single point for electrical connection. Case lights, anti-sweat heaters, fans, electric defrost, and leak detection systems can be prewired to the controller. Some cases, including Hillphoenix CO2 MT cases, come with factory-installed connections between the control valve and case controller, as well as between the temperature sensor and controller, further simplifying startup. Case controllers also improve efficiency by facilitating loop piping rather than circuit piping.
| HFC | CO2 | Benefits of CO2 | |
| Refrigerant type | Synthetic | Natural | Sustainable alternative |
| Global warming potential (GWP) | Higher GWP (2,500+) | Low GWP (1) | Future-focused, compliant with emerging regulations |
| Cooling capacity | Lower | Higher | Smaller compressor and lines; increased efficiency |
| Piping configuration | Circuit | Loop piping | Saves cost; easier maintenance and servicing |
| Case controls and EEVs | Not mandatory | Required | Better control and increased efficiency |
TABLE 1: Summary of key differences between HFCs and CO2. (Courtesy of Hillphoenix)
Circuit piping is common in HFC systems, as each case requires its own set of lines to the system. In a typical supermarket, circuit piping uses an estimated five to 10 times more pipe than loop piping. More pipes increase the potential for leaks, complicate maintenance, and slow down startup.
Alternately, case controllers like those that come standard on Hillphoenix CO2 systems facilitate looped piping, where one pipe from the system can connect to multiple fixtures, streamlining operations. Fewer pipes mean fewer joints and opportunities for leaks. Other benefits include cost savings on materials and labor at installation plus easier maintenance at the fixture.
With these advantages — increased efficiency, streamlined operations, and more straightforward maintenance — CO2 adoption continues to rise. To stay prepared as the industry shifts toward sustainability, explore the Hillphoenix Learning Center, where experts share lessons learned from decades of natural refrigeration experience.