Capacity control on a helical-rotary (screw) compressor is obtained using:

Capacity control on a helical-rotary (screw) compressor is obtained using:

• A. Sliding valve
• B. Variable speed drive
• C. Capillary valve
• D. Thrust bearing

Answer: (A)

In a helical-rotary (screw) compressor, capacity control is achieved by directly altering the compressor’s displacement using a sliding valve. The sliding valve moves to cover or uncover portions of the compression chamber, effectively changing how many pockets participate in compression each revolution. When the valve shifts to unload, fewer pockets are active, so less refrigerant is moved per cycle and the system capacity drops while the motor still runs. As demand rises, the valve retracts to allow full displacement again, restoring capacity. Capillary valves are expansion devices, not capacity controls for the compressor. Thrust bearings are for rotor support and don’t affect how much gas the compressor moves. A variable speed drive can reduce capacity by lowering motor speed, but the explicit, direct method for capacity control in many screw compressor designs is the sliding valve.

In a helical-rotary (screw) compressor, capacity control is achieved by directly altering the compressor’s displacement using a sliding valve. The sliding valve moves to cover or uncover portions of the compression chamber, effectively changing how many pockets participate in compression each revolution. When the valve shifts to unload, fewer pockets are active, so less refrigerant is moved per cycle and the system capacity drops while the motor still runs. As demand rises, the valve retracts to allow full displacement again, restoring capacity.

Capillary valves are expansion devices, not capacity controls for the compressor. Thrust bearings are for rotor support and don’t affect how much gas the compressor moves. A variable speed drive can reduce capacity by lowering motor speed, but the explicit, direct method for capacity control in many screw compressor designs is the sliding valve.