What is the primary method used to control capacity in a centrifugal compressor?

• A. Adjustment of the Impeller Inlet Pre-rotation Vanes
• B. Varying Refrigerant Charge
• C. Changing Condenser Water Temperature
• D. Modulating Suction Pressure with Throttling Valve

Answer: (A)

Capacity in a centrifugal compressor is controlled by how much flow enters the impeller. This is achieved with the inlet guide vanes (impeller inlet pre-rotation vanes). By adjusting these vanes, you change the entering angle and pre-rotation of the refrigerant, which directly sets the mass flow through the compressor at a given speed. Opening the vanes lets more refrigerant in and increases capacity; closing them reduces flow and capacity. This method keeps the compressor operating in a stable, efficient region and avoids the energy losses or instability that can come from throttling suction or altering system charges. Varying refrigerant charge, changing condenser water temperature, or throttling suction do affect system conditions but do not directly and efficiently control the compressor’s capacity at a fixed speed.