text: A system is said to be a two staged set-up if two gas compressors work together in serial to produce the compression. A normal booster installation is a two staged system that receives fluid to cool down the discharge of the first compressor, before it is input to the second compressor. The fluid that arrives at the interstage of both compressors comes from the liquid line and is normally controlled by expansion, pressure and solenoid valves.
   A standard two staged cycle of this kind has an expansion valve that expands and modulates the amount of refrigerant incoming at the interstage. As the fluid arriving at the interstage expands, it will tend to evaporate, producing a temperature drop and cooling the second compressor's suction when mixed with the fluid discharged by the first compressor. This kind of set-up may have a heat exchanger between the expansion and the interstage, which may be a second evaporator to produce an additional refrigeration effect, though not as cool as the main evaporator (for example to produce air conditioning or for keeping fresh products). A two staged system is said to be set-up as a booster with subcooling, if the refrigerant arriving at the interstage passes through a subcooling heat exchanger that subcools the main liquid line arriving at the main evaporator of the same system.