For capacity control, a centrifugal compressor uses what component?

In a centrifugal compressor, the use of guide vanes is essential for capacity control. Guide vanes are strategically placed within the compressor to direct the flow of air or gas and to adjust the angle at which it enters the impeller. This adjustment allows for modulation of the compressor's performance by changing the flow characteristics, which can effectively control the capacity delivered by the compressor.

When the guide vanes are adjusted, they can either increase or decrease the airflow through the compressor depending on the system demand. This capacity control is particularly important in systems where variable loads are present, allowing the compressor to operate more efficiently by matching its output to the required load.

In contrast, the other components listed do not serve the same direct function in controlling the capacity of a centrifugal compressor. A variable frequency drive primarily adjusts the speed of the motor driving the compressor, impacting overall performance but not specifically focusing on airflow direction. An accumulator is used for storing and managing pressure in piping systems, and an expansion valve typically regulates the flow of refrigerants in cooling systems rather than controlling compressor capacity directly. Hence, guide vanes are the correct choice for capacity control in a centrifugal compressor.