A system and method are provided for a variable speed compressor with a control module that controls an operating frequency of the variable speed compressor. The control module stores a prohibited frequency range that includes a resonant frequency of the variable speed compressor and an allowed upper frequency above the prohibited frequency range and an allowed lower frequency below the prohibited frequency range. The control module determines a requested frequency and operates the variable speed compressor at the allowed upper frequency for an upper frequency operating time and at the allowed lower frequency for a lower frequency operating time when the requested frequency is within the prohibited frequency range. The control module determines the upper frequency operating time and the lower frequency operating time such that a time-averaged frequency output over the upper frequency operating time and the lower frequency operating time corresponds to the requested frequency.

Scroll expander including a bypass passage and a bypass valve configured to open and close the bypass passage without significantly increasing the length in the shaft direction of the scroll expander. Scroll expander 23 includes suction port 513 which allows a high-pressure flow of a working fluid to enter and guides the entering flow of the working fluid into an expansion chamber. Discharge port 523 allows the working fluid expanded to a lower pressure in the expansion chamber to flow out. Bypass passage 27 communicates suction port 513 with the discharge port 523 while bypassing the expansion chamber. Bypass valve 28 is configured to open and close bypass passage 27. Suction port 513, bypass passage 27 and a bypass valve attaching portion 514 for attaching the bypass valve 28 are formed in a base plate of the fixed scroll 51.

Scroll expander including a bypass passage and a bypass valve configured to open and close the bypass passage without significantly increasing the length in the shaft direction of the scroll expander. Scroll expander 23 includes suction port 513 which allows a high-pressure flow of a working fluid to enter and guides the entering flow of the working fluid into an expansion chamber. Discharge port 523 allows the working fluid expanded to a lower pressure in the expansion chamber to flow out. Bypass passage 27 communicates suction port 513 with the discharge port 523 while bypassing the expansion chamber. Bypass valve 28 is configured to open and close bypass passage 27. Suction port 513, bypass passage 27 and a bypass valve attaching portion 514 for attaching the bypass valve 28 are formed in a base plate of the fixed scroll 51.

A method for expanding a gas flow between an inlet for the supply of the gas flow at certain inlet conditions of inlet pressure and inlet temperature and an outlet for the delivery of expanded gas at certain desired outlet conditions of outlet pressure and outlet temperature, whereby this method at least comprises the step of at least partly expanding the gas flow between the inlet and the outlet through a pressure reducing valve and at least partly expanding it through a pressure reducing unit with a rotor driven by the gas for converting the energy contained in the gas into mechanical energy on this shaft.

A method for expanding a gas flow between an inlet for the supply of the gas flow at certain inlet conditions of inlet pressure and inlet temperature and an outlet for the delivery of expanded gas at certain desired outlet conditions of outlet pressure and outlet temperature, whereby this method at least comprises the step of at least partly expanding the gas flow between the inlet and the outlet through a pressure reducing valve and at least partly expanding it through a pressure reducing unit with a rotor driven by the gas for converting the energy contained in the gas into mechanical energy on this shaft.