How does a cylinder unloader operate on a reciprocating compressor?

• A. Keep the suction valve opened while the compressor is in the return stroke
• B. Close the suction valve during the suction stroke
• C. Open the discharge valve during the return stroke
• D. Keep both valves closed during the return stroke

Answer: (A)

The concept being tested is how cylinder unloading reduces compressor capacity by preventing compression in idle cylinders. By keeping the suction valve open during the return (compression) stroke, the cylinder cannot build pressure because the gas can flow back into the suction line instead of being trapped and compressed. This means that even though the piston moves, there is no effective compression and no discharge from that cylinder, so that cylinder is effectively unloaded and requires much less motor torque. The remaining loaded cylinders still operate normally, supplying the system as demand requires.

Opening the discharge valve during the return stroke would force gas out of the cylinder early but doesn’t prevent compression in the unloaded state. Closing both valves would trap gas and defeat unloading. Closing the suction valve during suction would prevent gas from entering at all, which isn’t how unloading is designed to work.