The main purpose of a boiler burner is to break the oil into fine droplets i.e. to atomize the fuel in the correct manner so that efficient combustion can be achieved. In marine boilers 3 types of burners are used,
>Pressure Jet Burner
> Air or Steam Blast Atomiser
> Rotary Cup Burner
Pressure Jet Burner
This is the simplest and most common design of burner. Atomisation of the fuel is achieved by forcing the fuel under pressure through an orifice at the end of the burner, the pressure energy in the fuel is converted to velocity. Spin is given to the fuel prior to the orifice imparting centrifugal force on the spray of fuel causing it to atomise.
The fuel pressure required for a pressure jet burner may vary from 7 bar to 15 bar depending upon the design of burner and load of the boiler. The maximum required viscosity at the burner inlet is normally 15 cst and both diesel and heavy oil can be used as a fuel.
The disadvantage of this burner is its low 'Turn-Down' ratio (in the region of 3.5). The advantage is that it does not require any assistance other than supplying the fuel at the correct pressure
Air or Steam Blast Atomiser
This type of burner is similar in working as a pressure jet, with an addition of high pressure steam supply arrangement incorporated in it. The oil is thus sprayed in the path of this high pressure air or steam which helps the fuel for atomization. Normally air is used during the initial starting of the burner and then steam takes over the operation.
The steam side has a tangential nozzle which provides rotatory motion to the fuel to ensure efficient combustion.
Rotary Cup Burner
As the name suggests, this burner comprises a burner nozzle which is covered by a rapidly rotating cone. The fuel oil is carried on to a nozzle which is centrally located within the rotating cone. As the fuel oil moves along the cup due to absence of centripetal force, the oil film becomes thinner in its course as the circumference of the cup increases. Ultimately, the fuel is discharged from the tip of the rotating cone in the form of fine atomized spray.
The spinning cup offers the following advantages;
>Wider turn down ratio with lower excess air
> Low O2 levels
> No requirement for atomising air or steam
> Low fuel pressure requirements to an extent that gravity flow is sufficient
> Stable flames achievable with very low fuel flows although maximum flow limited by size of cup. This, allied to being limited to side firing making the design more suitable for smaller installations.
Pilot or Ignition Burner:
In some marine boilers with the main burner firing in heavy fuel oil, it is very difficult to initially start the boiler with the main burner. For start up of the boiler, a separate pilot or ignition burner is provided which uses diesel oil as a fuel. This enables the pilot burner to ignite even at the coldest condition the ship faces at sea.
The Pilot burner has a separate diesel oil piping and pump. The heat source is provided by two electrodes igniting the pilot flame, which are fitted to a high voltage ignition transformer. The pilot burner is allotted with a limited period of ignition during which it acts as a source of heat for the main burner and once the time is over, the pilot flame goes off.