Method for controlling a rotary screw compressor

1. A method for controlling a rotary screw compressor with a first air-end and a second air-end, wherein the first air-end compresses a gaseous medium and leads to the second air-end, which further compresses the medium, the method comprising:

detecting a flow of the compressed gaseous medium at an outlet of the second air-end;

adjusting a rotation speed of the first air-end with a first speed-controlled direct drive when the detected flow fluctuates in a range between a maximum value and a predetermined minimum value, while maintaining a predetermined outlet pressure;

adjusting a rotational speed of the second air-end with a second speed-controlled direct drive when the detected flow fluctuates in the range between the maximum value and the predetermined minimum value, while maintaining the predetermined outlet pressure;

opening a pressure-relief valve when the detected flow falls below the predetermined minimum value to at least partially discharge compressed gaseous medium delivered by the second air-end via the pressure-relief valve;

reducing the rotational speed of the first air-end to a predetermined idling speed (V1L) via the first speed-controlled direct drive when the detected flow falls below the predetermined minimum value to reduce a flow delivered by the first air-end to the second air-end, wherein a speed ratio between the second air-end and the first air-end when the detected flow at the outlet of the second air-end fluctuates in a range between the maximum value and the predetermined minimum value is different than the speed ratio between the second air-end and the first air-end when the flow taken at the outlet of the second air-end falls below the predetermined minimum value.

2. A rotary screw compressor comprising a first air-end and a second air-end, wherein the first air-end is configured to compress a gaseous medium, wherein the second air-end is configured to further compress the gaseous medium, and wherein both air-ends are driven separately and speed controllable, characterized in that the compressor further comprises a control unit which is configured to carry out a method according to claim 1 .

3. The rotary screw compressor according to claim 2 , characterized in that the inlet of the first air-end lacks a flow limiting, controllable throttle element and is in direct communication with the atmosphere.

4. The rotary screw compressor according to claim 2 , characterized in that a pressure-relief valve is arranged at the outlet of the second air-end and configured to open in response to the flow decreasing below the predetermined minimum value.

5. The method of claim 1 , wherein reducing the rotational speed of the first air-end takes place simultaneously with the opening of the pressure-relief valve.

6. The method of claim 1 , further comprising reducing the rotational speed of the second air-end to a second predetermined idling speed (V2L) via the second speed-controlled direct drive when the detected flow falls below the predetermined minimum value.

7. The method of claim 6 , the ratio of the second predetermined idling speed (V2L) of the second air-end to the predetermined idling speed (V1L) of the first air-end is within a range from 2 to 3.

8. A method for controlling a rotary screw compressor with a first air-end and a second air-end, wherein the first air-end compresses a gaseous medium and leads to the second air-end, which further compresses the medium, and wherein both air-ends are driven separately and speed controlled, the method comprising:

detecting a flow of the compressed gaseous medium at an outlet of the second air-end;

adjusting a rotation speed of the first air-end with a first speed-controlled direct drive when the detected flow fluctuates in a range between a maximum value and a predetermined minimum value, while maintaining a predetermined outlet pressure;

adjusting a rotational speed of the second air-end with a second speed-controlled direct drive when the detected flow fluctuates in the range between the maximum value and the predetermined minimum value, while maintaining the predetermined outlet pressure;

opening a pressure-relief valve when the detected flow falls below the predetermined minimum value to at least partially discharge compressed gaseous medium delivered by the second air-end via the pressure-relief valve;

reducing the speed of the first air-end to a first predetermined idling speed when the detected flow falls below the predetermined minimum value; and

reducing the speed of the second air-end to a second predetermined idling speed when the detected flow falls below the predetermined minimum value,

wherein a speed ratio between the second air-end and the first air-end in response to the detected flow being above the predetermined minimum value differs from the ratio of the second predetermined idling speed to the first predetermined idling speed.

9. A rotary screw compressor comprising:

a first air-end configured to compress a gaseous medium;

a second air-end configured to further compress the gaseous medium;

a speed-controlled first direct drive configured to drive the first air-end; and

a speed-controlled second direct drive configured to drive the second air-end,

wherein the compressor further comprises a control unit configured to carry out a method according to claim 8 .

10. The method according to claim 8 , wherein the ratio of the second predetermined idling speed to the first predetermined idling speed is within a range from 2 to 3.