During unloading, what valve condition is maintained?

• A. The suction valve is closed during the return stroke
• B. The discharge valve is opened during the return stroke
• C. The suction valve remains open during the return stroke
• D. All valves are closed during the return stroke

Answer: (C)

Unloading a reciprocating compressor aims to prevent compression in certain cylinders, so the intake path remains available and no gas is pushed out the discharge line during the intake phase. The return stroke is the intake stroke, and keeping the suction valve open then allows refrigerant to flow into the cylinder rather than being compressed. At the same time, the discharge valve stays closed to prevent any gas from entering the discharge line during this phase. This setup reduces the compressor’s effective capacity. Therefore, the suction valve remains open during the return stroke. Closing the suction valve, or opening the discharge valve during the return stroke, or shutting both valves would counteract unloading and disrupt proper operation.