In food retail refrigeration applications, CO2 (R-744) has gained prominence as a refrigerant due to its environmental benefits and thermodynamic properties. We outlined the differences between CO2 and traditional HFC systems in an earlier article; however, the differences in system pressure considerations are substantial enough to warrant their own examination.
A typical transcritical CO2 booster refrigeration system operates with distinct pressure zones across its components. While variations exist between manufacturers and system designs, general ranges include:
These meticulously managed pressure differentials are fundamental to the efficient circulation of CO2 through the refrigeration cycle, which encompasses evaporators, compressors, gas coolers, and various control valves.
Power Interruptions
Power outages present a significant operational challenge for any type of refrigeration system, as they can lead to costly downtime and food spoilage. For CO2 systems, power interruptions can result in charge loss unless a charge loss mitigation solution is in place.
When a CO2 system loses power, a cascade of events occurs, starting with the closure of critical valves, including all expansion valves and the high-pressure valve. This prevents the system from actively circulating and cooling the refrigerant and separates the system into four quadrants, which enables the strategic management of CO2 charge.
Without active refrigeration, the temperature of the CO2 charge within the system begins to increase, influenced by ambient heat. This in turn increases CO2 charge pressure. For example, a temperature increase of approximately 20°F in liquid CO2 can lead to a pressure increase of about 180 psi. The liquid-filled sections, such as liquid receivers and liquid-side components, are particularly susceptible to rapid pressure increases due to CO2‘s significant thermal expansion in its liquid phase.
As pressures increases, safety relief valves that are engineered to open at predetermined thresholds (e.g., 45 bar for the MT suction side) open. When these valves activate, they vent CO2 refrigerant to the atmosphere. CO2 is a natural, nontoxic substance, so venting does not cause the same environmental implications as it would with HFC; however, it can lead to refrigerant loss and system downtime for recharging and restarting.
Preventing this loss of refrigerant charge is a critical concern for food retailers and system operators.
POWER OUTAGE: When a CO2 system loses power, a cascade of events occurs (Courtesy of Hillphoenix)
Mitigating Charge Loss
Historically, the industry has employed an external heat exchanger and condensing unit to manage high pressure during power outages. These exchangers condense CO2 by transferring heat to a secondary fluid. This can manage pressure but adds complexity to the system and introduces additional refrigerant and oil in the store to manage.
Another approach is to build the system with all high-pressure components, which is not economical in all applications. For example, 90-bar design on the low side requires high-pressure components such as store piping, evaporators, valves, LT compressors, flash tank, and filter driers, which makes the system’s total cost of ownership very high.
A more recent development in addressing this challenge is ChargeSecure, a charge preservation solution designed for Hillphoenix CO2 systems that utilizes the CO2 charge and oil already present within the system. ChargeSecure is designed to manage internal system pressures proactively, preventing the activation of relief valves and the associated loss of refrigerant.
ChargeSecure operates independently of the main power supply and can be powered by backup generators or an optional uninterruptible power supply (UPS) system with integrated battery packs. Upon detection of rising system pressures during a power interruption, ChargeSecure activates. It circulates a small amount of CO2 from the flash tank, compresses it to the gas cooler, and then expands it back to the flash tank. This action maintains the pressure within the threshold, keeping the refrigerant contained within the system’s closed loop.
The system is designed to safeguard the refrigerant charge, particularly the liquid section, which constitutes the majority of the system’s CO2 inventory and is most susceptible to pressure increases from heating. When equipped with the external three-battery system, ChargeSecure provides up to 39 hours of intermittent activation. The unit only engages when specific pressure thresholds are met, optimizing battery life and ensuring protection over prolonged outage periods.
By mitigating pressure increases during power loss, ChargeSecure eliminates the need for refrigerant replacement and the associated downtime. When power is restored, ChargeSecure ensures the system can restart quickly, translating to substantial operational savings.
Power interruptions pose a risk of CO2 charge loss for refrigeration systems. Next-generation solutions like ChargeSecure provide a proactive solution to help retailers maintain system integrity, minimize disruptions, and ensure continuous product protection during power outages.