The imperative of reducing compressor work – this is the third instalment in a five-part series.
Evaporative cooling comes with its own set of trade-offs, including increased complexity. © RACA Journal
At a FRIGAIR 2025 SAIRAC presentation, Jannie Potgieter, a consulting engineer with Thermologica, said that one of the most contentious, yet impactful, optimisation strategies is evaporative cooling. This process, regardless of its implementation method, leverages the cooling effect of water evaporation. By pre-cooling the ambient air entering the gas cooler, it effectively reduces the dry bulb temperature from, for instance, 32°C to 26°C (with 60% effectiveness).
This pre-cooled air drastically lowers the optimum pressure in the gas cooler, even if the system still operates in a transcritical transition zone. Potgieter demonstrated a substantial jump in COP, from 1.85 (with parallel compressors) to 3.27, purely by incorporating evaporative cooling. This illustrates its profound potential to enhance efficiency.
However, evaporative cooling comes with its own set of trade-offs, including increased complexity, higher maintenance costs and potential for added pressure drop across the fan (leading to higher fan power consumption). The decision to implement evaporative cooling requires a thorough cost-benefit analysis, especially considering regional average temperatures. For Johannesburg, with an annual average temperature of 16.3°C (ASHRAE data), the benefits of evaporative cooling might need to be weighed against the operating conditions where it’s truly advantageous.
This data underscores a critical point: if a location experiences high ambient temperatures for only a limited number of hours annually, the substantial capital and maintenance costs associated with evaporative cooling might not always be justified by the energy savings. Conversely, in consistently hot environments like Upington in South Africa, evaporative cooling becomes a highly compelling solution. The decision to implement evaporative cooling must involve a detailed month-to-month analysis of expected operating temperatures, predicting time spent in transcritical versus subcritical modes, to determine the true return on investment.
Ejectors: a promising but costly frontier
Ejectors represent a relatively new and exciting technology in CO2 refrigeration. These static components, essentially Venturi nozzles, leverage the high-pressure fluid from the gas cooler to create a low-pressure region. This low-pressure zone then ‘sucks in’ refrigerant (liquid or gas) from the medium-temperature loop, effectively ‘free energy’ that moves refrigerant from a lower pressure to the receiver pressure. This reduces the mass flow that the main compressors would otherwise have to handle, potentially allowing for the elimination of a medium-temperature compressor in some designs.
However, the primary advantage of ejectors, according to Potgieter, extends beyond this free energy. Ejectors facilitate the ability to flood evaporator coils (especially on the medium-temperature side) and increase evaporator temperatures. By ensuring a higher liquid content in the evaporators and returning any unevaporated droplets to a dedicated receiver (from which the ejector draws), the system can operate with higher evaporating temperatures. While the low-temperature system remains unaffected, increasing the medium-temperature evaporating temperature leads to significant power savings due to a smaller pressure ratio across the compressor.
Currently, ejectors are noted for their high cost and complexity in implementation, which has limited their widespread adoption in South Africa. Nevertheless, Potgieter anticipates their increasing relevance, driven by strong implementation in Europe and the typical trickle-down of European best practices into the South African industry.
