Optimum combustion ratio control systems must be “closed-loop” and have precise components. Two types of process control occur during boiler control: closed-loop and open-loop. Closed-loop control systems measure the process variable that is to be controlled (oxygen). Open-loop control systems respond to demands in a process variable that is not directly affected by the process change.
For example, if a boiler needs to produce a given steam pressure, the combustion control system, responding to a pressure signal, causes gas to be burned at a higher rate. While this produces more heat to satisfy the demand, it does not measure or control the ratios of fuel and air required to produce the heat. As long as steam pressure is maintained, the process demand is satisfied. The process is closed-loop with respect to steam pressure but open-loop with respect to controlling fuel-air ratios for efficient combustion. Nearly all boilers have open-loop combustion control systems.