CO2 Rack 3. |
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THE EVOLUTION OF REFRIGERANTS AND THE CASE FOR SUBCRITICAL CO2 CONDENSATION
Andre van der Merwe, managing director of
Evapco, emphasises the critical importance
of understanding refrigerant evolution
before delving into modern CO2 systems.
He referenced the insights shared by Dr
Andy Pearson of Star Refrigeration in last
year’ s Dreosti Memorial Lecture, which
highlighted the historical trajectory and
future challenges of refrigerants.
“ The Montreal Protocol of 1987 marked
a turning point by phasing out ozonedepleting
substances. The focus has since
shifted to refrigerants with low global
warming potential( GWP), such as CO2.”
Van der Merwe continued:“ However,
we ' re now facing a new, overarching
environmental challenge: global warming.
Our industry ' s focus has shifted. We now
need to develop and adopt refrigerants
and systems that are not only ozonefriendly
but also have a low Global
Warming Potential( GWP).”
On low-temperature
compressors, we
have desuperheating,
and on mediumtemperature
compressors, there ' s
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further compression. |
Refrigerants are generally divided into two categories: ' naturals ', which are inherently existing gases and fluids like ammonia( NH₃) and carbon dioxide( CO2); and ' non-naturals ', which are typically synthetic fluorocarbon blends, often historically called ' Freons’. Since its founding in 1976, Evapco has specialised in evaporative heat transfer, initially focusing on ammonia refrigeration condensers. Over the years, the company has expanded its product offerings to provide a comprehensive range of solutions. This evolution stems from the understanding that the optimal heat rejection technology is application-dependent, as each product possesses specific limitations and achieves peak efficiency under distinct operating conditions. Evapco’ s range includes:
• Traditional 100 % wet evaporative condensers
• Newer hybrid condensers that combine dry and wet coils, which can run in mixed or full dry mode to save water and energy( 50 % water saving)
• Adiabatic systems, which are dry coolers augmented with adiabatic pads for watersaving‘ wet trim’( 80 % water saving)
• Full dry cooler condensers that use no water at all
It ' s crucial to understand the fundamental difference between dry and wet cooling:
•“ Dry cooling performance is limited by the ambient dry bulb temperature. An efficient dry cooler typically condenses at 6 – 12 ° C above the ambient dry bulb, meaning condensation happens at a significantly higher temperature.
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•“ In contrast, evaporative cooling is limited by the ambient wet bulb temperature. These systems can achieve condensing temperatures within 8 – 10 ° C of the wet bulb, often resulting in condensing temperatures 2 – 3 ° C or even more, below the ambient dry bulb temperature.”
He illustrated the impact of this difference using a typical ammonia system example, showing that condensing at 32 ° C( evaporative) versus 40 ° C( dry) for a 1000kW heat load at-10 ° C suction could result in substantial energy savings, potentially 26 – 30 %, due to the lower condensing pressure enabling better compressor efficiency( higher Coefficient of Performance – COP). The principle is clear: lower condensing temperature directly translates to higher system energy efficiency.
Focusing specifically on CO2( R744), a refrigerant known since the mid-1900s with applications dating back to the 1930s, Van der Merwe outlined its key attributes. Its primary advantages are being a natural refrigerant, environmentally benign with a GWP of 1 and ODP of 0 and being relatively inexpensive due to its free availability compared to complex new synthetic blends being developed. Furthermore, CO2 is nontoxic, non-flammable and non-explosive, offering significant safety benefits compared to some other natural refrigerants like propane( which has flammability concerns) or ammonia( toxicity). CO2 also boasts a high heat transfer coefficient.
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However, CO2 does have its challenges, notably its critical point at 31 ° C. This property means that in many ambient conditions, particularly warmer ones, CO2 systems operate in a ' transcritical ' cycle, where the refrigerant does not condense but rather releases heat as a high-pressure gas in a ' gas cooler '. Van der Merwe reiterated that CO2 is not a universal solution, and like other refrigerants and system types, its viability depends on the specific application requirements.
He touched on current market trends, noting the prevalence of CO2 systems( observing around 80 % of stands at the recent Chillventa exhibition in Germany featured CO2) and the emergence of large heat pump systems in Europe driven by energy security concerns. Van der Merwe framed these challenges as exciting opportunities for engineers, emphasising that while the technology exists to create solutions, they must be cost-effective and sustainable to be truly viable and widely adopted.
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