What type of control system is best when both flow rate and chlorine demand vary?

The most suitable control system when both flow rate and chlorine demand are variable is compound PID loop control. This control system is designed to manage complex processes that require continuous adjustments based on varying inputs.

In this context, a compound PID loop control employs proportional, integral, and derivative parameters to regulate the desired output effectively, such as chlorine dosing in water treatment. By continuously measuring the system's response and adjusting accordingly, it can maintain the optimal levels of chlorine even as conditions change. This is particularly essential in scenarios where both the flow rate of water and the demand for chlorine can fluctuate significantly, allowing for precise control that adjusts to maintain water quality.

In contrast, other control methods may not provide the necessary responsiveness or adaptability. Open-loop control lacks feedback mechanisms, making it ineffective in dynamic environments. Simple on-off control may result in oscillations or inefficient dosing, as it would only provide a binary response without fine-tuning. Feedback control, while providing some adjustments based on output, may not be as sophisticated as compound PID control in handling multiple variables simultaneously. Therefore, compound PID loop control stands out as the best choice for dealing with variable flow rates and chlorine demands efficiently.