(2) Boiler combustion cycle

Boiler systems use gaseous fuel such as propane and town gas as well as liquid fuel, including kerosene and fuel oil A.
All of these are flammable at a relatively low ignition temperature and considered as hazardous substances. Therefore, adopting a safe combustion method is the top priority for these fuels.
This is referred to as the combustion cycle of boilers, which must be operated under the applicable laws and regulations.
We will now take a look at the combustion system of a boiler.
[Fig.1] shows the flow sheet of the water-supply and combustion system of a boiler.

As you can see in the figure, the boiler combustion is controlled by the pressure switch (PS).
When the steam pressure falls below the preset pressure level, the pressure switch turns on and activates the burner. As the fuel starts burning, the steam pressure increases.
When the steam pressure reaches the preset level, the pressure switch turns off and fuel burning stops.
At this time, the burner operates in the time cycle shown in [Fig.2].
As soon as the pressure switch turns on, the air blower starts rotating and feeding secondary air.
After the pre-purge period, the spark period starts. In this cycle, the fuel valve opens and the pump sprays the fuel into the burner. Then, a spark occurs and ignites the fuel.
Once it has been verified that the furnace lights up inside, the spark stops.
When the pressure switch has detected the fire extinguishing pressure, the hydraulic valve closes to extinguish fire. Even after the fire goes out, the air blower continues to feed air during the post-purge period.

The purge period provided prior to and after the combustion cycle ensures the safe operation by removing (purging) unburned gas that may still be inside the furnace.
Thus, frequently repeating the combustion cycling increases the purge ratio and heat loss.