Two-stage compression
Also known as: two-stage system, compound system, compound compressor
Short definitionThe answer for deep-freezing duties that splits the compressor’s work in two. The gas is first raised to an intermediate pressure, cooled there, and then brought up to condensing pressure in a second step.
A single compressor struggles when a room or tunnel needs evaporation at −35 °C or lower. Once the ratio between suction and discharge goes above 8–10, the machine pumps less gas and the discharge temperature climbs high enough to break down the oil. Splitting the job into two steps lets each step run at a comfortable ratio. There is one circuit and one refrigerant; that is where it differs from a cascade system.
This option comes up when the quotation includes a blast tunnel, a spiral or plate freezer, or a store colder than −30 °C. Electricity use falls by 15–30% against single stage, while investment and controls grow. Two separate compressors can be used, or a compound machine with its cylinders shared between the stages. On small jobs an economised screw or a vapour-injection scroll gives a similar benefit more simply. A large plant running many hours a year recovers the difference quickly.