Multistage compressor and method for operating a multistage compressor

1. A multi-stage gas compressor comprising:

a rotor comprising a plurality of axially stacked impellers;

a tie rod extending through the plurality of axially stacked impellers and holding the plurality of axially stacked impellers together;

a gas compression path extending from a compressor inlet to a compressor outlet and through the plurality of axially stacked impellers;

a flow channel between the tie rod and the plurality of axially stacked impellers, the flow channel developing along at least a portion of the tie rod,

wherein the flow channel is in fluid communication with a first location along the gas compression path and a second location along the gas compression path, a pressure difference between the first location and the second location in the gas compression path generating a gas flow along the flow channel; and

at least a first passage fluidly connecting the first location with the flow channel, and at least a second passage fluidly connecting the second location with the flow channel,

wherein at least one passage of the first and second passages is provided between two toothed flanges meshing together or wherein at least one passage of the first and second passages is a duct provided through a hub of an impeller of the plurality of axially stacked impellers or through a terminal element at one end of the plurality of axially stacked impellers.

2. The multi-stage gas compressor according to claim 1 , wherein the first location is provided at the compressor inlet of a first compressor stage, and the second location is provided at the compressor outlet of a last compressor stage.

3. The multi-stage gas compressor according to claim 1 , wherein each impeller of the plurality of axially stacked impellers comprises two opposite contacting surfaces co-acting with respective surfaces of two adjacent impellers of the plurality of axially stacked impellers, or with a surface of an adjacent impeller and a surface of the terminal element at one end of the plurality of axially stacked impellers.

4. The multi-stage gas compressor according to claim 1 , wherein at least one passage of the first and second passages is defined between contacting surfaces of two adjacent impellers, or between the contacting surfaces of the terminal element and of an adjacent impeller.

5. The multi-stage gas compressor according claim 1 , wherein two adjacent impellers of the plurality of axially stacked impellers, or the impeller of the plurality of axially stacked impellers and the terminal element, contact each other by respective toothed flanges meshing together, and sealing members are arranged and configured for reducing or preventing gas leakage between at least some of the toothed flanges.

6. The multi-stage gas compressor according to claim 1 , further comprising a balancing drum comprising a first face facing a most downstream impeller, and a second opposite face facing a balancing zone fluidly connected with a most upstream compressor stage.

7. The multi-stage gas compressor according to claim 6 , further comprising a pathway fluidly connecting the most downstream impeller with the balancing zone of the balancing drum, wherein the pathway causes a pressure drop between the compressor outlet of the most downstream impeller and the balancing zone.

8. The multi-stage gas compressor according to claim 7 , wherein at least one passage fluidly connecting the flow channel and the balancing zone is provided through the balancing drum.

9. A multi-stage gas compressor comprising:

a plurality of axially stacked impellers;

a tie rod holding the plurality of axially stacked impellers together;

a gas compression path extending from a suction side to a delivery side of the multi-stage gas compressor and through the plurality of axially stacked impellers;

a return flow path, along which a fraction of a compressed process gas flowing along the gas compression path flows back from a downstream location to an upstream location of the gas compression path, the return flow path extending along the tie rod, so that heat generated by compression in the compressed processed gas is transferred to the tie rod by forced convection; and

at least a first passage fluidly connecting the upstream location with the return flow path, and at least a second passage fluidly connecting the downstream location with the return flow path,

wherein at least one passage of the first and second passages is provided between two toothed flanges meshing together or wherein at least one passage of the first and second passages is a duct provided through a hub of an impeller of the plurality of axially stacked impellers or through a terminal element at one end of the plurality of stacked impellers.

10. The multi-stage gas compressor according to claim 9 , wherein each impeller of the plurality of axially stacked impellers comprises two opposite contacting surfaces co-acting with respective surfaces of two adjacent impellers of the plurality of axially stacked impellers, or with a surface of an adjacent impeller and a surface of the terminal element at one end of the plurality of axially stacked impellers.

11. The multi-stage gas compressor according to claim 10 , wherein at least one passage of the first and second passages is defined between contacting surfaces of two adjacent impellers, or between the contacting surfaces of the terminal element and of an adjacent impeller.

12. The multi-stage gas compressor according to claim 9 , wherein at least one passage of the first and second passages is defined between contacting surfaces of two adjacent impellers of the plurality of axially stacked impellers, or between the contacting surfaces of the terminal element and of an adjacent impeller.

13. The multi-stage gas compressor according claim 9 , wherein two adjacent impellers of the plurality of axially stacked impellers, or the impeller of the plurality of stacked impellers and the terminal element, contact each other by respective toothed flanges meshing together, and sealing members are arranged and configured for reducing or preventing gas leakage between at least some of the toothed flanges.

14. The multi-stage gas compressor according to claim 9 , further comprising a balancing drum comprising a first face facing a most downstream impeller, and a second opposite face facing a balancing zone fluidly connected with a most upstream compressor stage.

15. The multi-stage gas compressor according to claim 14 , further comprising a pathway fluidly connecting the most downstream impeller with the balancing zone of the balancing drum, wherein the pathway causes a pressure drop between a compressor outlet of the most downstream impeller and the balancing zone.

16. The multi-stage gas compressor according to claim 15 , wherein at least one passage fluidly connecting the return flow path and the balancing zone is provided through the balancing drum.

17. A method for operating a multi-stage gas compressor, comprising a compressor rotor with a plurality of axially stacked impellers held together by a tie rod, and a flow channel extending along at least a portion of the tie rod, the method comprising:

heating the tie rod by flowing a hot gas along the flow channel and along the tie rod, wherein the hot gas flows from a most downstream compressor stage to a most upstream compressor stage.

18. The method according to claim 17 , further comprising:

diverting a portion of the hot gas processed by the multi-stage gas compressor from a high-pressure location along a compression path extending across the multi-stage gas compressor; and

flowing the portion of the hot gas along the flow channel towards a low-pressure location along the compression path.

19. The method according to claim 17 , further comprising:

flowing a portion of the hot gas from the most downstream compressor stage to a balancing zone defined on a balancing drum in a position opposite the most downstream compressor stage; and

flowing the portion of the hot gas from the balancing zone to an inlet of the most upstream compressor stage, passing on and along the tie rod, through the plurality of axially stacked impellers.

20. The multi-stage gas compressor according to claim 3 , wherein at least one passage of the first and second passages is defined between contacting surfaces of two adjacent impellers, or between the contacting surfaces of the terminal element and of an adjacent impeller.