What type of automation control would work best if the chlorine demand varies but the flow is constant?

• A. residual control
• B. combined flow and residual control
• C. start-stop control
• D. proportional pacing

Answer: (A)

Maintaining a steady chlorine residual through feedback control is the idea here. When the demand for chlorine varies but the flow is constant, the amount of chlorine that needs to be added changes to keep the downstream residual at the target level. A residual control system uses a chlorine sensor downstream to measure the actual residual and automatically adjusts the dosing rate to hold that setpoint. If the demand increases and the residual tends to drop, the controller raises the dose; if demand drops and residual rises, it lowers the dose. This continuous adjustment ensures a consistent disinfecting concentration despite changes in demand.

Combining flow with residual control isn’t needed when flow is constant, since flow doesn’t vary. Start-stop control would cause on/off dosing rather than a smooth, constant residual, leading to instability. Proportional pacing is a form of residual-focused control, but the essential approach you’re looking for is maintaining a set residual via a feedback-controlled dosing system.