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Associations
OPTIMISING CO2 REFRIGERATION SYSTEMS
By Eamonn Ryan
Reducing compressor work for greater efficiency.
At a FRIGAIR 2025 SAIRAC presentation, Jannie
Potgieter, a consulting engineer with Thermologika, laid out a clear imperative for anyone working with CO₂ refrigeration systems: reduce the work done by compressors. Every design decision, he argued, should be traced back to two core goals: lowering mass flow and reducing the pressure ratio across compressors.
Regardless of type – piston, scroll or screw – the compressor ' s job is to compress gas, consuming energy in the process. The less work a compressor performs, the more efficient the system. This principle is especially critical in CO₂ systems operating in transcritical mode, where energy demands can spike dramatically.
CO₂ VS R404A: UNDERSTANDING THE CHALLENGE Potgieter illustrated the unique challenge of CO₂ by comparing it to a conventional R404A system at ambient temperatures of 15 ° C and 32 ° C. With a combined cooling capacity of 100kW( 70kW MT, 30kW LT), the CO₂ system consumed 59.8kW – more than the 55.7kW required by the R404A system under the same conditions.
The core issue lies in CO₂’ s low critical temperature( 31.1 ° C). Above this threshold, the refrigerant enters transcritical operation and does not condense into a liquid, but becomes a dense fluid. When expanded to evaporator pressure, it creates significant flash gas – up to 80 % of the flow may be vapour – forcing the compressor to handle large volumes of non-refrigerating gas. This high mass flow severely hampers evaporator effectiveness and increases compressor energy use.
To mitigate flash gas, modern CO₂ systems use a flash gas tank( receiver), typically operating at 38 – 40 bar. This tank separates vapour from liquid after expansion from the gas cooler. The resulting liquid feeds the MT and LT evaporators, reducing the load on LT compressors.
In a system without parallel compression, flash gas is reduced in pressure and re-compressed by the main compressors – an inefficient process. A parallel compressor can instead draw flash gas directly from the receiver, compressing it at a lower pressure ratio. This adjustment improved system COP from 1.67 to 1.85 in Potgieter’ s example.
However, parallel compression isn ' t always necessary. In systems that operate mostly subcritically or have high evaporating temperatures, the benefit may not justify the complexity or cost. Each case requires evaluation.
SUBCRITICAL CO₂ OPERATIONS ARE THE IDEAL Subcritical operation, in which CO₂ condenses like a traditional refrigerant, is inherently more efficient. In these cases, systems avoid flash gas altogether, allowing COPs to reach 3.11 – better than R404A’ s 3.02 in the same scenario. System design should aim to maximise time spent in subcritical mode.
Internal heat exchangers are among the simplest and most effective enhancements. By subcooling the liquid before expansion and superheating suction gas, IHXs( Internal Heat Exchangers) reduce required mass flow, protect the compressor and improve efficiency across all modes.
Desuperheaters, positioned after LT compressors, lower gas discharge temperatures before the MT stage. This not only protects downstream compressors but improves volumetric efficiency slightly. Though modest in impact, desuperheaters offer continuous benefits and are especially valuable in systems with heavy LT loads.
Evaporative cooling, where ambient air is cooled by water evaporation before it reaches the gas cooler, dramatically reduces CO₂’ s transcritical pressure. A modest pre-cooling( for example, from 32 ° C to 26 ° C) can increase COP from 1.85 to 3.27 when combined with parallel compression.
However, Potgieter cautioned that the effectiveness of evaporative cooling depends on climate. Johannesburg, for example, has an average annual temperature of just 16.7 ° C and experiences relatively few hot hours annually. In such climates, the added cost and maintenance of evaporative cooling might not be justified. In contrast, consistently hot locations like Upington make a much stronger case.
Jannie Potgieter, a consulting engineer with Thermologika.
All images by © RACA Journal
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RACA Journal I November 2025 www. refrigerationandaircon. co. za