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An apparatus and method for compressing gas refrigerant using two different compressors operated alternatively in a series or parallel mode for obtaining two different compression ratios and thereby provide efficient operation for both a relatively lower suction pressure and a relatively higher suct

An apparatus and method for compressing gas refrigerant using two different compressors operated alternatively in a series or parallel mode for obtaining two different compression ratios and thereby provide efficient operation for both a relatively lower suction pressure and a relatively higher suction pressure. This system avoids an unbalanced mass flow rate when the compressors are operated in series by unloading a downstream one of the compressors. When operated in the series mode, refrigerant is bypassed back to a suction inlet of the downstream compressor during a portion of a compression stroke of the downstream compressor for equalizing the mass flow rate through the two compressors.

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The invention claimed is: 1. A dual-mode compressor apparatus for receiving and compressing a gas from a gas supplying enclosure and discharging compressed gas into a gas receiving enclosure, the apparatus comprising: (a) a first compressor having a piston reciprocating in a cylinder between limits

The invention claimed is: 1. A dual-mode compressor apparatus for receiving and compressing a gas from a gas supplying enclosure and discharging compressed gas into a gas receiving enclosure, the apparatus comprising: (a) a first compressor having a piston reciprocating in a cylinder between limits, the first compressor having a first suction port connected to receive gas from the gas supplying enclosure and having a first discharge port; (b) a second compressor having a piston reciprocating in a cylinder between reciprocation limits, the second compressor having a second suction port and having a second discharge port connected to an inlet of the gas receiving enclosure; (c) a 3/2 way valve having an inlet connected to the first discharge port, a first outlet connected to an inlet of the gas receiving enclosure and a second outlet connected to the second suction port; (d) a one way valve connected between the gas supplying enclosure and the second suction port for permitting passage of the gas from the gas supplying enclosure into the second suction port except when pressure at the second suction port exceeds pressure at the gas supplying enclosure; (e) an unloading valve having an inlet connected in communication with an unloading port through the cylinder of the second compressor between the reciprocation limits and having an outlet connected to the second suction port, the unloading valve being adapted to be closed except when pressure at the second suction port exceeds the pressure at the first suction port, the unloading valve, when opened, reducing the volume displacement of the second compressor in order to equalize gas mass flow rate through the two compressors; and (f) a first refrigeration evaporator that is said gas supplying enclosure; a second evaporator, the evaporators operating at different temperatures, a cooling control valve for controlling the flow of refrigerant into each evaporator and a temperature control for controlling the control valves and directing refrigerant into the evaporators, the temperature control being connected to an actuator that actuates the 3/2 way valve for switching the 3/2 way valve between its two states wherein the unloading valve is linked to a differential pressure actuator having two chambers, one chamber being connected in fluid communication with the first suction port and the other chamber being connected in fluid communication with the second suction port, the differential actuator being biased to close the unloading valve except when pressure at the second suction port exceeds the pressure at the first suction port; and wherein the gas is a refrigerant, and the gas receiving enclosure is a refrigeration condenser. 2. An apparatus in accordance with claim 1 wherein a heat exchanger is interposed in connection between the first discharge port and the second suction port for rejecting heat from refrigerant compressed by the first compressor before being compressed by second compressor. 3. An apparatus in accordance with claim 1 wherein each compressor comprises a free piston compressor having its piston driven by a linear motor and enclosed in a housing, the suction port of each compressor being in communication with the interior of its housing, the compressors being mounted coaxially and having a passage between their housings, the one way valve being mounted at said passage. 4. An apparatus in accordance with claim 3 wherein the unloading valve is linked to a differential pressure actuator having two chambers, one chamber being connected in fluid communication with the first suction port and the other chamber being connected in fluid communication with the second suction port, the differential actuator being biased to close the unloading valve except when pressure at the second suction port exceeds the pressure at the first suction port. 5. An apparatus in accordance with claim 4 wherein the gas is a refrigerant, the gas supplying enclosure is a refrigeration evaporator and the gas receiving enclosure is a refrigeration condenser. 6. An apparatus in accordance with claim 5 wherein the 3/2 way valve is manually actuable. 7. An apparatus in accordance with claim 1 wherein a pressure or temperature sensor is mounted to detect pressure or temperature at the first suction port, the sensor being connected to a control and actuator for switching the 3/2 way valve, the control switching the 3/2 way valve to a state connecting its inlet to the condenser inlet in response to a sensed pressure or temperature above a selected value and to connect its inlet to the second suction port in response to a sensed pressure or temperature below a selected value. 8. An apparatus in accordance with claim 1 wherein the 3/2 way valve is manually actuable. 9. A dual-mode compressor apparatus for receiving and compressing a gas from a gas supplying enclosure and discharging compressed gas into a gas receiving enclosure, the apparatus comprising: (a) a first compressor having a piston reciprocating in a cylinder between limits, the first compressor having a first suction port connected to receive gas from the gas supplying enclosure and having a first discharge port; (b) a second compressor having a piston reciprocating in a cylinder between reciprocation limits, the second compressor having a second suction port and having a second discharge port connected to an inlet of the gas receiving enclosure; (c) a 3/2 way valve having an inlet connected to the first discharge port, a first outlet connected to an inlet of the gas receiving enclosure and a second outlet connected to the second suction port; d) a one way valve connected between the gas supplying enclosure and the second suction port for permitting passage of the gas from the gas supplying enclosure into the second suction port except when pressure at the second suction port exceeds pressure at the gas supplying enclosure; and (e) an unloading valve having an inlet connected in communication with an unloading port through the cylinder of the second compressor between the reciprocation limits and having an outlet connected to the second suction port, the unloading valve being adapted to be closed except when pressure at the second suction port exceeds the pressure at the first suction port, the unloading valve, when opened, reducing the volume displacement of the second compressor in order to equalize gas mass flow rate through the two compressors, wherein the unloading valve comprises; (a) an annular sleeve sealingly surrounding the cylinder of the second compressor and slidable to alternatively cover and uncover the unloading port; (b) an annular bellows sealingly attached at one end to the annular sleeve and sealingly attached at its opposite end to the cylinder of the second compressor to form an interior chamber, the bellows having an exterior surface in fluid communication with second suction port, the attached bellows and sleeve being mounted with the sleeve covering the unloading port when the pressure differential between the interior chamber and the exterior surface of the bellows is less than a selected pressure differential; and (c) a conduit connecting the interior chamber of the bellows in fluid communication with the first suction port for permitting the bellows to be collapsed and move the sleeve away from the unloading port when the pressure on the exterior surface of the bellows exceeds the pressure of the first suction port. 10. An apparatus in accordance with claim 9 wherein the gas is a refrigerant, the gas supplying enclosure is a refrigeration evaporator and the gas receiving enclosure is a refrigeration condenser. 11. An apparatus in accordance with claim 9 wherein a heat exchanger is interposed in connection between the first discharge port and the second suction port for rejecting heat from refrigerant compressed by the first compressor before being compressed by second compressor. 12. An apparatus in accordance with claim 9 wherein the 3/2 way valve is manually actuable. 13. An apparatus in accordance with claim 9 wherein a pressure or temperature sensor is mounted to detect pressure or temperature at the first suction port, the sensor being connected to a control and actuator for switching the 3/2 way valve, the control switching the 3/2 way valve to a state connecting its inlet to the condenser inlet in response to a sensed pressure or temperature above a selected value and to connect its inlet to the second suction port in response to a sensed pressure or temperature below a selected value. 14. An apparatus in accordance with claim 9 wherein each compressor comprises a free piston compressor having its piston driven by a linear motor and enclosed in a housing, the suction port of each compressor being in communication with the interior of its housing, the compressors being mounted coaxially and having a passage between their housings, the one way valve being mounted at said passage. 15. An apparatus in accordance with claim 14 wherein the unloading valve is linked to a differential pressure actuator having two chambers, one chamber being connected in fluid communication with the first suction port and the other chamber being connected in fluid communication with the second suction port, the differential actuator being biased to close the unloading valve except when pressure at the second suction port exceeds the pressure at the first suction port. 16. An apparatus in accordance with claim 15 wherein the gas is a refrigerant, the gas supplying enclosure is a refrigeration evaporator and the gas receiving enclosure is a refrigeration condenser. 17. An apparatus in accordance with claim 16 wherein the 3/2 way valve is manually actuable.