Method for controlling a compressor

1. A method for controlling a compressor that includes one or more compressor elements, each element having an inlet on an inlet side and an outlet on an outlet side, comprising:

during a transition from full load or partial load to zero load, at least a first of said compressor elements carries out a process A that comprises the steps:

at least temporarily reducing an inlet pressure of the first compressor element;

subsequently or simultaneously, reducing a drive torque of the first compressor element to a zero torque while a speed of the first compressor element is decreased to a minimum speed; and

increasing the drive torque from zero torque to a minimum torque to maintain the rotation of the first compressor element at the minimum speed at which the first compressor element is rotating, and

during a transition from zero load to partial load or full load, at least the first of said compressor elements carries out a process B that comprises the steps:

increasing the drive torque of the first compressor element from the minimum torque to a maximum torque in which the first compressor element is rotating from the minimum speed to a nominal speed or to within a nominal speed range; and

subsequently or simultaneously increasing the inlet pressure of the first compressor element.

2. The method according to claim 1 , wherein the process A comprises the step of, at least before reducing the inlet pressure of the first compressor element, reducing an outlet pressure of the first compressor element.

3. The method according to claim 2 , wherein the outlet pressure of the first compressor element is reduced by opening a blow-off valve on the outlet side of the first compressor element or on the outlet side of another compressor element located downstream from the first compressor element.

4. The method according to claim 1 , wherein the process B comprises the step of, at least after increasing the inlet pressure of the first compressor element, increasing an outlet pressure of the first compressor element.

5. The method according to claim 4 , wherein the outlet pressure of the first compressor element is raised by closing a blow-off valve on the outlet of the first compressor element or on an outlet side of another compressor element located downstream from the first compressor element.

6. The method according to claim 5 , wherein the another compressor element located downstream from the first compressor element is a screw compressor element.

7. The method according to claim 1 , wherein the process B comprises the step of increasing the inlet pressure of the first compressor element to an operating value.

8. The method according to claim 1 , wherein the inlet pressure of the first compressor element is changed by opening or closing an inlet valve on the inlet side of the first compressor element or on an inlet side of another compressor element located upstream from the first compressor element.

9. The method according to claim 1 , wherein the inlet pressure of the first compressor element is changed by opening or closing a blow-off valve located upstream from the first compressor element.

10. The method according to claim 1 , wherein the inlet pressure of the first compressor element is changed by adjusting an operating mode of another compressor element located upstream from the first compressor element.

11. The method according to claim 1 , wherein the inlet pressure of the first compressor element is changed by controlling another compressor element located upstream from the first compressor element, said another compressor element being constructed in the form of a screw compressor element.

12. The method according to claim 2 , wherein the outlet pressure of the first compressor element is changed by adjusting an operating mode of another compressor element, located downstream from the first compressor element.

13. The method according to claim 12 , wherein the another compressor element located downstream from the first compressor element is a screw compressor element.

14. The method according to claim 1 , wherein the one or more compressor elements comprises a directly driven compressor element or a compressor with at least one directly driven compressor element.

15. The method according to claim 7 , wherein an outlet pressure of the first compressor element is reduced by opening a blow-off valve on the outlet side of the first compressor element or on the outlet side of another compressor element located downstream from the first compressor.

16. The method according to claim 14 , wherein the inlet pressure of the first compressor element is changed by opening or closing an inlet valve on the inlet side of the first compressor element or on an inlet side of another compressor element located upstream from the first compressor element, wherein an outlet pressure of the first compressor element is reduced by opening a blow-off valve on the outlet side of the first compressor element or on the outlet side of another compressor element located downstream from the first compressor element, wherein said inlet valve and said blow-off valve are controlled together.

17. The method according to claim 1 , wherein the compressor comprises at least one compressor element that is constructed in the form of a directly driven turbo compressor element, and

at least another compressor element that is constructed in the form of a speed-controlled screw compressor element.

18. The method according to claim 17 , wherein all said compressor elements are incorporated in one housing.

19. The method according to claim 1 , wherein in process A the step of reducing the drive torque of the first compressor element to the zero torque takes place subsequently to the reduction of the inlet pressure of the first compressor element.

20. The method according to claim 1 , wherein the minimum speed is increased to the nominal speed or to within a nominal speed range with a minimal amount of torque.