The imperative of reducing compressor work – this is the first instalment in a five-part series.
The primary challenge with CO2 (R744) lies in its critical temperature of 31.1°C. © RACA Journal
At a FRIGAIR 2025 SAIRAC presentation, Jannie Potgieter, a consulting engineer with Thermologica, delivered an address on optimising CO2 refrigeration systems. His core message underscored the critical importance of reducing compressor work to significantly enhance system efficiency – a vital consideration for HVAC&R professionals navigating the complexities of modern refrigeration.
Potgieter highlighted that the perceived complexity of a modern CO2 system, with its myriad components, is ultimately rooted in a singular objective: minimising the energy consumed by the compressor. To achieve this, he emphasised two fundamental principles: lowering mass flow and reducing the pressure ratio across the compressor. Any design alteration or added component in a CO2 system, he argued, can be traced back to one of these two goals.
The compressor: the heart of efficiency
Potgieter began his technical deep dive with the universal principles of refrigeration compressors. Regardless of technology (piston, scroll or screw), a compressor’s fundamental role is to compress gas, consuming electricity in the process. Therefore, reducing the work demanded of the compressor directly translates to lower electricity consumption and improved system efficiency. This foundational understanding, he asserted, is key to deciphering the rationale behind various CO2 system optimisation strategies.
CO2‘s transcritical challenge: a comparison with R404A
To provide context, Potgieter presented a comparative analysis of a basic R404A system against a CO2 system. All calculations were performed at ambient temperatures of 15°C (chosen as an average for Johannesburg) and 32°C, with constant evaporating capacities of 70kW for medium temperature (MT) and 30kW for low temperature (LT) applications, summing to a total of 100kW.
The primary challenge with CO2 (R744) lies in its critical temperature of 31.1°C. Operating above this temperature, in a transcritical state, the CO2 no longer condenses into a liquid but remains a high-density fluid. This transcritical operation leads to significant flash gas creation when the fluid is expanded down to evaporating temperatures. Potgieter illustrated that in a standard transcritical cycle, 70%–80% of the CO2 entering the evaporator could already be gas, severely reducing the effective liquid content and forcing the compressor to move a disproportionately high mass flow of non-refrigerating gas. This results in poor evaporator performance and higher compressor work.
Addressing flash gas tank/receiver
To mitigate the flash gas issue, a flash gas tank or receiver (typically operating at 38 to 40 bar) is essential in a basic CO2 system. This component allows the high-pressure transcritical fluid to flash into the tank, separating the vapor from the liquid. The liquid, now with a much better quality (can then be directed to the medium and low-temperature evaporators, significantly improving their efficiency and reducing the mass flow through the low-temperature compressors.
Initial efficiency comparison and the role of parallel compression
Potgieter presented the initial power consumption for the base CO2 system (with a flash gas tank but no further optimisation) at 59.8kW for 100kW of cooling. This was slightly higher than the 55.7kW required by the R404A system at 32°C ambient, demonstrating the inherent challenges of transcritical CO2 operation without advanced optimisation.
He then introduced the concept of parallel compression as the first major optimisation strategy. In a basic C system without parallel compression, all the flash gas in the receiver tank is expanded to a lower pressure and then re-compressed by the main compressors. This represents a significant inefficiency. By implementing a parallel compressor, this flash gas can be directly compressed from the receiver pressure, thereby reducing the pressure ratio over this specific compressor and significantly lowering the overall compressor work.
