In a three-element feedwater regulating system, which component serves as the analog input?

In a three-element feedwater regulating system, the component that serves as the analog input is the drum level. This is because the primary function of the three-element control system is to ensure that the water level in the boiler's drum is maintained at an optimal point.

The drum level is measured using a level sensor, which continuously provides real-time data to the control system, allowing for precise adjustments in the feedwater flow rate. By maintaining the appropriate drum level, the system can prevent issues like low-water conditions, which can lead to overheating and potential damage to the boiler. This control approach allows for a safe and efficient operation of the steam generation process.

The other components mentioned, while critical in a boiler system, do not serve as the primary analog input in this specific context. For instance, feedwater flow rate is an output that is adjusted based on the input from the drum level and other control elements. Combustion air flow and steam pressure are also important parameters but are not the primary inputs for regulating feedwater in this system.