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1. Combustion control system correct the proportion the quantities of air and fuel being burnt. 
2. Ensure complete combustion
3. The system measure the flow rates of fuel oil and air in order to correctly regulate their proportions.
4. Two control elements are used, ‘steam flow’ and ‘steam pressure’. 
5. The steam pressure signal is fed to a controller and is compared with the desired value.
6.  Any deviation results in a signal.
7. A high or low signal selector is present to ensure that when a load change occurs the combustion air flow is always in excess of the fuel requirements. 
8. This prevents poor combustion and black smoke
9. If the master signal is for an increase in steam flow, then when it is fed to the low signal selector it is blocked since it is the higher input value. 
10. When the master signal is input to the high signal selector it passes through as the higher input.
11.  This master signal now acts as a variable desired value for the combustion air and brings about an increased air flow. 
12. When the increased air flow is established, its measured value is now the higher input to the low signal selector. 
13. The master signal will now pass through to bring about the increased fuel supply to the boiler via the fuel supply sub-loop. 
14. The air supply for an increase in load is therefore established before the increase in fuel supply occurs. 

Drawing: Boiler ACC

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