Capacity control on a helical-rotary (screw) compressor is obtained using which method?

Capacity control on a helical-rotary (screw) compressor is obtained using which method?

• A. variable speed motors
• B. inlet vanes
• C. a sliding valve
• D. hot gas bypass

Answer: (C)

Capacity control in a helical-rotary screw compressor comes from changing how much gas the compressor moves in one revolution, its displacement. This is done with a sliding valve that moves axially inside the compressor to reduce the effective swept volume. As the valve shifts, part of the suction path is blocked or the active compression chamber is shortened, so less gas is drawn and compressed each cycle without altering the rotor speed. The result is lower flow at the same pressure ratio, giving smooth, efficient part-load operation. Other methods are not the built-in mechanism for this type. Inlet vanes aren’t a typical way to control screw compressors, and while hot gas bypass can reduce capacity, it’s generally less efficient and not the standard approach for displacement-based control. Using a variable-speed motor can also throttle capacity by changing speed, but that’s a different approach from the dedicated sliding-valve displacement control built into the screw design.

Capacity control in a helical-rotary screw compressor comes from changing how much gas the compressor moves in one revolution, its displacement. This is done with a sliding valve that moves axially inside the compressor to reduce the effective swept volume. As the valve shifts, part of the suction path is blocked or the active compression chamber is shortened, so less gas is drawn and compressed each cycle without altering the rotor speed. The result is lower flow at the same pressure ratio, giving smooth, efficient part-load operation.

Other methods are not the built-in mechanism for this type. Inlet vanes aren’t a typical way to control screw compressors, and while hot gas bypass can reduce capacity, it’s generally less efficient and not the standard approach for displacement-based control. Using a variable-speed motor can also throttle capacity by changing speed, but that’s a different approach from the dedicated sliding-valve displacement control built into the screw design.