Don starts the class with a Kahoot game to introduce the basics of carbon dioxide’s chemical properties. CO2 has a global warming potential (GWP) of 1, and it serves as the standard to which all other refrigerants are compared. Under atmospheric pressure conditions, solid CO2 sublimates and expands 845 times as it becomes a vapor. CO2 is approximately 1.52 times heavier than air. The critical temperature of CO2 is quite low, only 87.8 degrees Fahrenheit, meaning that the temperature-pressure relationship works up until that point. On the other hand, the triple point of CO2 occurs at 61 PSI; it may exist as a solid, liquid, or vapor at that pressure.
We used natural refrigerants, including CO2, before the invention of Freon. After that, natural refrigerants became less popular. However, the Montreal Protocol (and Kigali Amendment) have changed the regulatory landscape since then. As refrigerant regulations have targeted refrigerants with high ozone depletion potential (ODP) and GWP, natural refrigerants like CO2 have become commonplace in commercial refrigeration again.
At the moment, the United States’ CO2 adoption is behind quite a few European countries and Japan, which have already incentivized the shift to natural refrigerants. However, the EPA is updating its older policies to move in the same direction as Europe and Japan (SNAP 2021 falls into that category). The California Air Resources Board (CARB) and the U.S. Climate Alliance are also committed to moving in that direction by phasing down high-GWP HFCs, starting with air conditioning and then moving to commercial refrigeration.
As popular high-GWP HFCs are slated for a phasedown, manufacturers like Emerson have been expanding their testing facilities to test flammable refrigerant (A2L and A3) alternatives to those HFCs. A2Ls (like the new low-GWP blends) are non-toxic and mildly flammable, A3s (like propane) are non-toxic and highly flammable, B2Ls (like ammonia) are toxic and mildly flammable, and A1s are non-toxic and non-flammable. Apart from the current high-GWP HFCs, the only A1 refrigerant that is suitable for refrigeration is CO2.
Natural refrigerants like CO2 are gaining traction in the US. When we think about refrigeration system architecture, we need to consider square footage, application (restaurants, supermarkets, etc.), and the type of refrigeration (medium-temp, low-temp, etc.). You may end up using a full-CO2 transcritical booster system, a partial-CO2 cascade, or a secondary system, which treats CO2 as a secondary fluid like glycol.
CO2 isn’t toxic or flammable, but it displaces oxygen and may asphyxiate people if it fills a room due to its higher specific gravity; symptoms of excessive CO2 exposure range from drowsiness to shortness of breath to confusion to unconsciousness. Another danger comes from its high pressures. However, it can be exceptionally inexpensive and efficient, especially in low-ambient conditions. CO2 requires some special components (like pressure-relief valves), but it’s future-proof and can be vented, so it has its benefits over HFCs.
When CO2 exceeds the critical point, it becomes a transcritical fluid that no longer obeys the temperature-pressure relationship; the condenser becomes a gas cooler instead. On the other hand, when CO2 drops below the triple point, it could become solid dry ice in the system. Transcritical systems under high pressure result in warmer CO2, and some facilities use that additional heat for heat reclaim. Cascade systems use subcritical CO2, which always stays below the critical point and is used exclusively in the low stage.
High pressure is a quintessential element (and hazard) of CO2 systems. Pressure relief valves are distributed throughout the system, and those are modulated by electronic controls. Standstill pressures depend on ambient conditions but are still high. Pressure is also in play during sublimation. Ice plugs may even form while charging, and they could be hazardous due to the high pressure that accumulates during sublimation. Even in the liquid phase, we need to control pressures to avoid expansion (hydrostatic conditions).
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