Method for controlling the speed of cryogenic compressors arranged in series for cooling cryogenic helium

A method for controlling the speeds of compressors arranged in series that are formed to compress a fluid, comprising:

• Specifying a desired inlet pressure which the fluid should have at an entry of the compressor arranged the furthest upstream,
• Recording an actual inlet pressure of the fluid at said entry,
• Recording an actual discharge pressure of the fluid at an output of the compressor arranged the furthest downstream,
• Establishing an actual total pressure ratio, wherein the actual total pressure ratio corresponds to the quotient of the actual discharge pressure and the actual inlet pressure,
• Determining a proportional integral value based on the deviation of the actual inlet pressure from the desired inlet pressure,
• Determining the capacity factor of the proportional integral value and the actual total pressure ratio,
• Establishing a model total pressure ratio based on the actual total pressure ratio and the capacity factor,
• Determining a reduced desired speed for each compressor, wherein the respective reduced desired speed is determined as a function value of a control function associated with the respective compressor, which control function assigns a reduced desired speed to each value pair consisting of capacity factor and model total pressure ratio, and
• Converting the reduced desired speeds into target speeds and adjusting the speed of each compressor to the respectively assigned target speed.