As the evaporating temperature falls the suction pressure decreases and the pressure ratio the compressor has to overcome grows. When the ratio goes beyond 8–10 the volumetric efficiency of a single-stage compressor falls rapidly and the discharge temperature rises to a level the oil cannot withstand. In two-stage compression the low stage compresses the gas up to the intermediate pressure, the gas is cooled there, and the high stage completes the work. The difference from a cascade is that there is a single refrigerant and a single circuit. Each stage works at a reasonable ratio and with higher efficiency.
The intermediate pressure is approximately the square root of the product of the absolute suction and condensing pressures; in this way the two stages work at an equal ratio. For example, with R717 at −40 °C evaporating (0.72 bar) and +35 °C condensing (13.5 bar) the ratio in a single stage is 19; in two stages the intermediate pressure is around 3.1 bar and each stage works at a ratio of about 4.3. The discharge temperature also falls noticeably. You can read the values for your own refrigerant from our refrigerant pressure–temperature chart.
Intercooling is done in two ways. In an open intercooler (flash type) the hot gas of the low stage is passed through the liquid at intermediate pressure; in the same vessel the liquid going to the evaporator is also cooled. In the closed type the liquid is cooled inside a coil and keeps its pressure. In practice either two separate compressors (low stage and high stage) are used, or a single-body compound compressor in which some of the cylinders work as the low stage and some as the high stage.
Two-stage systems are suitable for blast freezing tunnels, spiral and plate freezers, ice cream production and stores below −30 °C. They use 15–30% less energy than a single stage for the same capacity; in return the initial investment and the control are more complex. At smaller capacities the simple form of the same idea is the screw compressor with an economiser or the vapour injection scroll compressor. Which solution is chosen is determined by capacity, annual running hours and the energy price; in large plants that run continuously two stages pay for themselves quickly.