Reaching very low temperatures with a single refrigeration circuit is difficult: the compression ratio grows, the discharge temperature rises and the suction pressure falls into vacuum. In a cascade system the work is divided in two. The low-temperature circuit cools the room or the product and rejects its heat not to the outside air but to the cascade heat exchanger. This heat exchanger is at the same time the evaporator of the high-temperature circuit, which takes the heat and rejects it to the outside through its own condenser. The refrigerants of the two circuits do not mix.
For each circuit the refrigerant that works best in its own temperature range is chosen. In industrial refrigeration the most common pair is CO₂ (R744) on the low side and ammonia (R717) or propane (R290) on the high side; it is used in freezing and frozen storage plants between −30 and −50 °C. In laboratory cabinets and special applications between −60 and −80 °C, refrigerants such as R23, R170 (ethane) or R508B are found on the low side.
In the cascade heat exchanger a temperature difference of 3–6 K is left between the two refrigerants; if the low circuit condenses at −10 °C, the high circuit evaporates at around −15 °C. This difference affects the efficiency of the system directly: the smaller it is, the higher the efficiency and the larger the heat exchanger. The attraction of a CO₂ cascade is that the ammonia stays in the machinery room only, that only CO₂ goes to the areas where the food is, and that CO₂ pipes are of small diameter. Plate or shell-and-tube types are used as the heat exchanger.
In operation the sequence matters: the high circuit runs first, then the low circuit. If the high circuit stops, the low circuit has nowhere left to condense and its pressure rises quickly. Because the standstill pressure of CO₂ can reach 60–70 bar at ambient temperature, a small auxiliary condensing unit, a vessel rated for high pressure or controlled venting through the safety valve is provided for long standstills. The design is done project by project. The controls of the two circuits are interlocked; when one goes into fault the other behaves accordingly.