How is capacity control typically achieved in a reciprocating compressor?

Capacity control in a reciprocating compressor is commonly achieved by manipulating the suction valves. This method allows the compressor to modulate its performance based on the demand for air or gas. When the suction valves are opened to a certain degree, the volume of the gas being drawn into the compressor increases, allowing it to handle more flow and thus increase its capacity. Conversely, if the demand is lower, the valves can be partially closed to decrease the intake, which in turn lowers the capacity of the system.

This control strategy is effective because it provides a responsive way to adjust the output without requiring significant changes to the fundamental operation of the compressor. This can help maintain efficiency and reduce wear on the compressor components, as it minimizes the need for rapid cycling or drastic changes in other operational parameters.

In contrast, the other options either do not provide an effective method for controlling compressor capacity or could lead to inefficiencies or operational issues. Decreasing the frequency of operation may reduce capacity but could also lead to instability in operation. Increasing discharge pressure may indicate a problem rather than control capacity and does not allow for tailored adjustments based on workload. Manually adjusting the compressor speed is generally less efficient and more cumbersome than using suction valve control.