Cascade system
Also known as: cascade refrigeration, two-circuit system
Short definitionAn arrangement of two separate refrigeration circuits with different refrigerants working in series: the upper circuit cools the condenser of the lower one. It is used for duties between −50 and −80 °C and in industrial CO₂ plants.
A single circuit struggles at very low temperatures: the compression ratio grows, the discharge temperature rises and the suction falls into vacuum. In a cascade the low-temperature circuit cools the room and leaves its heat in the heat exchanger between the two. That heat exchanger is the evaporator of the high-temperature circuit, which rejects the heat outdoors. The two refrigerants never mix. A common pair is CO₂ on the low side with ammonia or propane on the high side.
It is preferred between −30 and −50 °C in frozen storage and freezing plants; the ammonia stays in the machine room and only CO₂ goes to the rooms. A difference of 3–6 K is left across the cascade heat exchanger; the smaller it is, the better the efficiency and the larger the exchanger. The high circuit must start first. If it stops, the low circuit pressure rises quickly; standing CO₂ can reach 60–70 bar at ambient temperature, so standstill protection is part of the design.