The present invention is provided with: a turbo compressor that compresses a refrigerant; a condenser that condenses the refrigerant compressed by the turbo compressor; an expansion valve that expands a liquid refrigerant introduced from the condenser; an evaporator that evaporates the refrigerant expanded by the expansion valve; an oil tank that stores a lubricating oil supplied to the turbo compressor; a pressure equalizing pipe that connects the oil tank and the evaporator; and a control unit that controls start-up. The control unit calculates the amount of a refrigerant eluded into the lubricating oil in the oil tank at the time of start-up, and reduces a pressure reduction speed in the oil tank by limiting the opening operation of an IGV when the refrigerant elution amount per prescribed time exceeds a prescribed value.

An expander generates cold by expanding a refrigerant gas in a cryogenic refrigerator. A compressor compresses the refrigerant gas returning from the expander. Pipes are connected to the expander and the compressor and circulate the refrigerant gas between the expander and the compressor. A determiner determines whether or not a change cycle of the pressure of the refrigerant gas flowing in the pipes is in a predetermined range. The determiner may determine whether or not the change cycle of the pressure of a low-pressure pipe in which a low-pressure refrigerant gas flows toward the compressor from the expander is in a predetermined range.

A vapor compression system includes a compressor configured to circulate a refrigerant through a refrigerant loop, a sump configured to receive a mixture of lubricant and the refrigerant from the compressor, and a controller having a memory and a processor. The processor is configured to receive a first signal indicative of a temperature of the mixture within the sump, receive a second signal indicative of a pressure of the mixture within the sump, determine a relative amount of the refrigerant in the mixture based on the first signal and the second signal, and output a control signal in response to the relative amount of the refrigerant in the mixture exceeding a threshold value.

An exemplary heat pump (10) includes: a compressor (16A, 16B) that discharges refrigerant; an oil separator (30) that separates oil from the refrigerant discharged from the compressor; an oil return channel (80) that returns the oil separated by the oil separator to the compressor; a pressure sensor (86A, 86B) that detects a pressure in the oil return channel; a first pressure loss member (84A, 84B) and a second pressure loss member (88A, 88B) disposed in portions of the oil return channel at an oil separator side and a compressor side relative to the pressure sensor; and a control device that increases an output of the compressor in a case where a pressure detected by the pressure sensor exceeds a suction pressure of the compressor and less than a discharge pressure of the compressor.

A method of operating a multiple-compressor refrigeration system is provided. This method includes the steps of supplying, via a common supply line, refrigerant gas and oil to a plurality of compressors coupled in series, and attaching an oil flow conduit between adjacent compressors of the plurality of compressors. The oil flow conduit is configured to move oil from a compressor with a relatively higher pressure to a compressor with a relatively lower pressure. The method further includes controlling the pressure for each of the plurality of compressors by regulating a speed at which each of the plurality of compressors operates in order to maintain a pressure differential between the adjacent compressors to facilitate the flow of oil from the compressor with the relatively higher pressure to the compressor with the relatively lower pressure.

An air compressor includes a tank unit that stores compressed air, a motor unit that generates compressed air to be stored in the tank unit, a pressure detection unit that detects a pressure value in the tank unit, and a control unit that drives the motor unit when a pressure value in the tank unit detected by the pressure detection unit is equal to or less than a motor start pressure value, and stops the motor unit when the pressure value in the tank unit detected by the pressure detection unit is equal to or greater than a motor stop pressure value. The control unit changes at least either of the motor start pressure value or the motor stop pressure value every time when a predetermined time passed.

A refrigerant circulation system which circulates refrigerant containing CO.sub.2 therethrough includes a compressor which compresses the refrigerant, a motor including a rotor, a stator, a rotation shaft coupled to the rotor, and slide bearings supporting the rotation shaft and lubricated using the refrigerant compressed by the compressor. The system is configured so that the refrigerant passing through the inside of the motor expands after having flowed out of the slide bearings and is used for cooling of one of the rotor and the stator, and a control device which controls at least the motor. The motor further includes a flow rate adjustment mechanism able to adjust a flow rate of the refrigerant. The control device controls the flow rate adjustment mechanism of the motor to adjust a degree of overheat in a refrigeration cycle of the refrigerant realized by the refrigerant circulation system.