The primary way of achieving capacity control of a centrifugal compressor is:

• A. Varying Condenser Water Temperature
• B. Modulating Suction Pressure
• C. Changing Refrigerant Type Mid-operation
• D. Adjustment of the Impeller Inlet Pre-rotation Vanes

Answer: (D)

Controlling how much gas enters the compressor is the key to adjusting its capacity. The most direct and effective way to do this on a centrifugal compressor is to set the impeller inlet pre-rotation vanes, also called inlet guide vanes. By changing the angle of these vanes, you pre-swirl and regulate the flow that actually enters the impeller. When the vanes are opened more, more gas gets into the impeller, increasing mass flow and capacity. When the vanes are tightened, less gas enters, reducing capacity. This method provides fast, smooth control over the compressor’s output without altering speeds or the overall pressure ratio.

Other options don’t control the compressor flow as directly. Varying condenser water temperature mainly changes condenser cooling load and system pressures rather than the compressor’s air or gas flow. Modulating suction pressure affects the system conditions but isn’t the direct, primary means of throttling the compressor’s capacity. Changing the refrigerant type mid-operation isn’t practical. So adjusting the inlet pre-rotation vanes is the best way to achieve capacity control.