Turboexpander and method for using moveable inlet guide vanes at compressor inlet

A method of controlling a turboexpander-compressor system having an expander with an expander impeller connected via a shaft to a compressor impeller of a compressor that compresses a gas flow output by the expander, the method comprising:

• receiving first information on a pressure and a temperature of an incoming gas entering the expander;
• receiving second information on a pressure of a gas output by the expander;
• receiving third information on a rotating speed of the shaft attached to the compressor impeller and the expander impeller;
• determining when the turboexpander-compressor system functions in off-design conditions based on the received first, second and third information;
• comparing a ratio between the rotating speed of the shaft and a drop of an enthalpy across the expander with a predetermined value, when determined that the turboexpander-compressor system functions in the off-design conditions;
• and
• controlling a compressor set of moveable inlet guide vanes connected to a compressor inlet through which the gas output by the expander enters into the compressor, to adjust a pressure of the gas input into the compressor such as to bring the ratio closer to the predetermined value.

A controller configured to control a turboexpander-compressor system having an expander with an expander impeller, and a compressor with a compressor impeller, the expander impeller and the compressor impeller being rotated by a same shaft, and the compressor compressing a gas output by the expander, the controller comprising:

• an interface configured to,
• receive information on a pressure and a temperature of an incoming gas entering the expander, a pressure and a temperature of the gas output by the expander, and a rotating speed of the shaft, and
• output commands to a compressor set of inlet vanes configured to control a pressure of the gas input in the compressor;
• and
• a control unit connected to the interface and configured to,
• receive the first, second and third information from the interface,
• determine when the turboexpander-compressor system functions in off-design conditions,
• generate the commands for the compressor set of moveable inlet guide vanes to maintain a ratio between the rotating speed of the shaft and a drop of enthalpy across the expander within a predetermined range, in off-design conditions;
• and
• control the compressor set of moveable inlet guide vanes to adjust a pressure of the gas input into the compressor such as to bring the ratio closer to the predetermined range.