In a screw compressor the male and female rotors turn inside one another; the space between them shrinks from suction to discharge and the refrigerant is compressed in a smooth, continuous flow. There are no valves, few moving parts and little vibration. The capacity range runs from 50 kW to several megawatts; that is why it is standard in central refrigeration systems, ammonia plants and large chillers.
Capacity is controlled steplessly between 10 and 100% with a slide valve; it runs efficiently under varying load. Because oil seals and cools the rotors, the system has a large oil separator and oil cooler; this is also why the screw compressor is unsuitable for small systems.
Thanks to its economiser port it delivers high efficiency in a single stage at low evaporating temperatures; it suits blast freezing tunnels and large frozen stores. It is a machine with a high investment cost whose maintenance needs expertise; so for small and medium cold rooms, condensing units with reciprocating or scroll compressors are the better choice.