Why are the inlet vanes of a centrifugal refrigeration compressor closed during startup?

During the startup of a centrifugal refrigeration compressor, closing the inlet vanes is primarily done to reduce the starting torque required by the compressor. Centrifugal compressors operate by converting kinetic energy from the impeller into pressure energy. When the compressor is started with the vanes fully open, it has to handle a full flow of refrigerant, which can create significant resistance, leading to high starting torque.

By closing the inlet vanes, the flow of refrigerant is restricted, resulting in less load and reduced torque requirements on the motor during the initial startup phase. This allows the compressor system to start more smoothly and helps prevent potential issues related to excessive motor load that could cause wear or failure.

While other responses might relate to different operational concerns, the key reason for this practice is the specific need to minimize the torque on startup, ensuring a more manageable and safer commissioning process for the compressor.