Construction of the nuclotron, a superconducting accelerator for relativistic nuclei, was started at the Joint Institute for Nuclear Research (JINR) in 1993. The power equipment of the nuclotron cryogenic system includes a Kaskad-80125 screw compressor plant (SCP) as well as various models of piston compressors, which have a considerably lower output. The SCP is the principal machine for compressing helium and the piston compressors are used for stepped control of the amount of compressed helium and as a backup (305Np-20/30 and 2GM4-12/30) as well as for pumping helium to receivers (1VUV-45/150). The Kaskad-80/25 has now operated for 5200 h, has been started up more than I00 times, and has produced -2.5 million liters of liquid helium.The total compressor output is 12 180 m3/h, installed capacity 2588 kW, and cooling water flow rate 139.8 m3/h. The Kaskad-80/25, which was developed at the Scientific-Industrial Firm "Kazan'kompressormash,'" is one of the second-generation domestic screw oil-filled machines used in cryogenic helium-compressor plants. The main prototype of machines of this type was assembled at/INR. It passed comprehensive tests. Figure 1 shows the block diagram of the plant. It consists of four units: first-stage compressors, second-stage compressors, oil coolers, and control system.The first-stage compressors are two screw machines operating in parallel from one electric motor. A second-stage compressor consists of one screw machine with a slide-valve output governor, a vertical oil separator, and an electric motor of the same rating as in the first stage.The oil cooler contains four water-cooled parallel-flow heat exchangers. The control system ensures automatic start-up and shut-down of the plant, continually monitors the main parameters, provides safety protection and maintains the nominal discharge pressure of the first-stage compressors (intermediate pressure).The plant operates as follows. Gaseous helium from receivers 10 enters the first-stage compressors, where it is compressed to 0.6-0.7 MPa and at the same time is cooled by oil injected under pressure into the compression chamber, and then enters the second stage, where the oil-gas mixture is compressed to the final parameters and is sent to the oil separator 2. The separated gas passes through separator 3 and terminal cooler 4, enters the oil-removing system and then the KGU-1600/4.5. Oil from the oil separator is poured into an oil tank and passes through the secondary filter 5 into the oil cooler 9, where it is cooled by water.Oil enters the first-stage compressors after passing through the jet I3 and splitting into two lines; along one line oil is injected directly into the compression chamber and along the other, after passing through the primary filter 6, lubricates the bearings and creates an oil seal at the final packing. In the second stage, oil is delivered by the gear pump 8, where its pressure
