Multi-stage compressor system

1. A multi-stage compressor system for compressing gas, comprising:

a plurality of compressor elements ( 2 - 5 - 28 ) discharging compressed and heated gas during operation located in series one after the other, said compressed gas circulating between said compressor elements;

one of said compressor elements ( 5 - 28 ) being driven by a motor ( 9 ), and at least another one of said compressor elements ( 2 ) being driven separately without a mechanical link with said motor ( 9 );

wherein the one compressor element ( 5 - 28 ) which is driven by the motor ( 9 ) is of the screw type, and the other compressor element ( 2 ) which is driven separately is of the centrifugal type;

an expander ( 18 ) included in a closed power cycle ( 12 ) using a circulating medium inside the power cycle and drivingly connected to said other compressor element;

a heater arranged to receive and heat said circulating medium;

said heater arranged to receive at least part of the compressed and heated gas discharged from a compressor element and to heat the circulating medium in said heater using said compressed and heated gas.

2. The compressor device according to claim 1 , wherein the other compressor element ( 2 ), which is driven separately by the expander ( 18 ) is located upstream of the one compressor element driven by the motor relative to the direction of circulation of compressed gas.

3. The compressor device according to claim 1 , wherein the motor ( 9 ) comprises a thermal motor.

4. The compressor device according to claim 1 , wherein the circulating medium in the closed power cycle ( 12 ) is circulated by means of a pump ( 15 ), successively through: said heater, which comprises at least one heat exchanger ( 17 - 27 - 25 ) through which the at least part of the compressed and heated gas discharged from a compressor element flows; said expander ( 18 ) which is connected with said other compressor element ( 2 ); and a condenser ( 19 ).

5. The compressor device according to claim 4 , wherein said at least one heat exchanger is included in a pressure line ( 10 ) of a last compressor element ( 5 ) of the multi-stage compressor device.

6. The compressor device according to claim 4 , wherein said at least one heat exchanger comprises an intercooler ( 25 ) located in a compressed gas line ( 8 ) which connects said one and other compressor elements ( 2 - 5 ) to each other and arranged to cool compressed and heated gas circulating in said line.

7. The compressor device according to claim 4 , including a drive comprising a thermal motor ( 9 ) with an outlet exhaust line ( 26 ) discharging the exhaust gases from the motor and wherein said heater in the closed power cycle ( 12 ) includes an additional heat exchanger ( 27 ) which is included in said outlet exhaust line ( 26 ).

8. The compressor device according to claim 4 , wherein the expander ( 18 ) is of the turbo type.

9. The compressor device according to claim 4 , wherein the expander ( 18 ) is of the screw type.

10. The compressor device according to claim 4 , wherein the expander ( 18 ) is of the spiral type.

11. The compressor device according to claim 4 , wherein the other compressor element ( 2 ) driven by the expander ( 18 ) has a compression ratio in the region of 1.8.

12. The compressor device according to claim 1 , wherein the medium in the power cycle ( 12 ) comprises a low boiling point medium.

13. The compressor device according to claim 1 , wherein at least one of said compressor elements ( 2 - 5 - 28 ) is of the oil-free type.

14. The compressor device according to claim 1 , wherein the one compressor element ( 5 ) driven by a motor has a compression ratio in the region of 4.5.

15. The compressor device according to claim 1 , wherein the compressor device is portable.