Subcritical CO2 Systems
CO2, R-744 and Carbon Dioxide is gaining acceptance as a refrigerant. It is natural, does not have any ozone depletion effects, is cheap, colorless and has no odor.
It can be used directly as a refrigerant, either as high condensing temperature, (Transcritical,) with high discharge pressure, above 1100psi, or with lower condensing temperature (Subcritical.) Transcritical is mostly used with Bock semi hermetic compressors, avoiding high pressure shaft seals.
CO2 in larger systems are often used as a secondary refrigerant, (“volatile brine,”) being cooled by a primary refrigerant, for example, R-717 ammonia. The benefit is the ammonia charge can be very small and with no compressors on the CO2 side, there is only one system in part load, saving energy.
CO2 has excellent heat transfer qualities and suction line sizes can be less than half the diameter of ammonia. With a correctly designed system, no CO2 compressor is necessary to produce hot gas for defrost, allowing for oil free CO2 system.