www. refrigerationandaircon. co. za RACA Journal I November 2025 5
Associations
Ejectors, still emerging in the South African market, offer‘ free energy’ by using highpressure fluid to entrain and lift refrigerant from a lower-pressure zone. Beyond reducing compressor work, they allow for higher evaporating temperatures and improved evaporator flooding, especially in the MT loop.
By increasing MT evaporator temperatures, even if the LT circuit is unchanged, significant compressor power savings are achieved. However, high equipment costs and installation complexity remain major obstacles. Potgieter expects wider adoption over time, especially as European best practices migrate to southern Africa.
HIGHER EVAPORATING TEMPERATURES ARE ALWAYS A WIN Even without ejectors, designing systems to operate at higher evaporating temperatures is a highly effective strategy. Raising suction pressure decreases compressor pressure ratio, leading to year-round savings.
The caveat: all equipment in a loop must tolerate the higher temperature. If even one component requires lower temperatures, the entire loop is limited. This‘ weakest link’ effect can compromise potential gains.
Potgieter emphasised practical design elements, like proper gas cooler placement. Poor placement can lead to recirculated hot air entering the cooler, increasing inlet temperatures by 2 – 3 ° C. This small design flaw can have outsized impacts on lifetime energy consumption.
CO₂’ s properties make it exceptionally suited to heat recovery. Unlike traditional refrigerants that condense at a constant temperature, CO₂ undergoes a temperature glide. This makes it ideal for counter-current heat exchange, where water is also heating linearly – eliminating pinch points and improving thermal efficiency.
Moreover, CO₂ can produce hot water at 80 – 85 ° C, far above the 55 – 60 ° C limit of most conventional systems. In one example, Potgieter showed that an additional 10kW of compressor power in transcritical mode could yield up to 147kW of heating.
Yet, he cautioned against assuming transcritical operation is always beneficial. For small heating loads( for example, 20 – 30kW), subcritical waste heat might suffice. Forcing a system transcritical just for heat recovery could negate overall efficiency. Designers must evaluate each case based on heating needs,
ambient conditions and compressor power trade-offs. Properly sizing storage vessels for recovered heat is also vital. Larger vessels enable short bursts of heat recovery without forcing the system into continuous transcritical mode.
Part of the presentation at FRIGAIR 2025.
FINAL THOUGHTS AND RECOMMENDATIONS Potgieter summarised several key strategies:
• Parallel compressors: strongly recommended where transcritical flash gas loads are high
• Internal heat exchangers: universally beneficial and protective
• Evaporative cooling: powerful, but must be weighed against climate and maintenance costs
• Ejectors: promising, especially for MT evaporator optimisation, but currently costly
• Higher evaporating temperatures: a fundamental strategy that can drive lasting efficiency
• Gas cooler placement: often overlooked, but critical for performance
• Heat recovery integration: a major advantage of CO₂ systems, particularly for facilities needing both heating and cooling
In closing, Potgieter addressed a practical question: what happens when heat recovery must elevate water from 50 ° C to 70 ° C? He acknowledged that CO₂ systems are best suited to lower inlet temperatures, where thermal matching is easier. While heat recovery is still feasible at higher temperatures, some energy will inevitably be lost to the atmosphere, reducing recoverable efficiency. RACA
“ By increasing MT evaporator temperatures, even if the LT circuit is unchanged, significant compressor power savings are achieved.”
www. refrigerationandaircon. co. za RACA Journal I November 2025 5