The transcritical has probably received the most press. It derives its name from the fact that the high side operates above the critical point while the low side operates below the critical point. The name indicates that the system operates on either side, or across, the critical point of the refrigerant. The critical point is the pressure and temperature at which the refrigerant can no longer condense to a liquid. So the “condenser” is really just a gas cooler. The refrigerant does not condense to a liquid until AFTER the pressure is dropped. The critical temperature of CO2 is 88°F, so any time the high side temperature rises above 88°F, the refrigerant will not condense. The pressures are bit higher than you might be used to – over 1200 psig, so not just any compressor and piping will do. However, in a way, the transcritical systems are a bit simpler than the other flavors.
Cascade systems use two complete refrigeration systems – one system’s job is really to cool the condenser of the other system. If you keep the CO2 cold enough, say 40°F, the pressures are not that high. However, to have a 40° condenser requires a system whose evaporator is in that same temperature range and whose condenser is at the normal operating temperature for an air cooled condenser. This system typically uses HFC refrigerant. So you have a heat exchanger and an HFC system to keep the CO2 system cool. This allows wider range of compressors because the CO2 compressors are not operating at 1200 psig. However, the system operating with the normal temperature condenser (80°F – 100°F) will have a refrigerant OTHER than CO2.
And then there are the booster systems. The booster systems incorporate both low and medium temperature racks into a single unit. The low temp CO2 compressors pump into the suction of the medium temp CO2 compressors, which operates at transcritical temperatures and pressures. This way the low temp compressors don’t have to be heavy duty transcritical compressors, just the medium temp compressors. The system uses only CO2, no HFC refrigerant is required, as in a cascade system.
There are also glycol systems, where the refrigeration system cools glycol, which is pumped through the store to the cases. This reduces the amount of refrigerant in the system and reduces leaks by reducing the amount of piping, fittings, and braze joints. What became apparent in my visit was that there are many ways to solve the same problem, and you can expect to see different solutions at different locations. It also is apparent that in commercial refrigeration, CO2 is here to stay and HFCs are on their way out.