By Mark Butterfield
The history of burners dates back to the early shipping days, when fuel oil first started replacing coal as the ships’ primary fuel source.
Since then, burner designs and construction have come a long way, but the principles behind their operation has remained the same.
All makes and types of burners have two things in common
They need to atomise the fuel They need to mix the fuel with the air needed for combustion
Why do we want to use a burner?
To convert water into steam.
To heat thermal oil
For direct firing in cooking and oxidising
To provide heat to dry products
Why is it necessary to atomise fuel?
Even though fuel oil is classified as a flammable liquid, most fuels will not burn easily in a liquid state. If you were to drop a lit match in a container of fuel oil, it would PROBABLY go out almost immediately (don’t try this!). In order for fuel oil to burn, it must first be transformed from a liquid to a vaporised state ~ atomised. Atomisation increases the exposure of the fuel to the oxygen in the air; this promotes combustion.
A nozzle rated at 0.60 US gallons per hour can generate as many as 50 million droplets of oil in an hour.
EFFECTS OF BAD ATOMISING
If atomisation is incomplete, the droplet sizes are too large for complete combustion. The larger droplets will escape the flame only partially burnt. This can usually be seen as “fire flies” when looking at the flame. This will not only result in a poor flame, but also soot deposits being formed inside the combustion chamber. In addition the combustion plant’s efficiency will reduce causing excessive fuel usage for the required energy output.
There are generally four types of burners, each of which atomise fuel in different ways.
Pressure Jet Air / Steam Atomised Rotary Cup Low Pressure Air Atomising
Pressure Jet Burner
The most common types of pressure jet burners in our industry are:
Riello Weishaupt Nu-Way FBR
How a pressure jet atomises fuel
To create the ‘fuel vapour’, the fuel is pressurised, and forced through the nozzles. This breaks the oil into mist-like droplets, that get mixed with the combustion air and ignited. On heavy fuel oil, the fuel atomising pressure might be as high as 3000kpa (30 bar), for light fuel oils as low as 600kpa (6 bar).
Higher Fuel Pressure Lowers the Average Droplet Size
Mean Drop Size (Micron)
20 7 7.5 8 8.5 9 9.5 10 10.5 Oil Pressure (kPa)
Selecting a Pressure Jet Burner
Single stage – single nozzle - on / off ~ no modulation, no turndown ratio. Multiple stage (either 2 or 3 nozzles), the burner switches between the stages, increasing / decreasing the fuel throughput as required by the plant load. Spillback type nozzle – one nozzle with a variable throughput.
PRESSURE JET ADVANTAGES:
Good atomisation Relatively low initial investment – low Capex Package type: “plug and play” Nozzles and spares are relatively inexpensive Maintenance is easily performed Reliable
PRESSURE JET DISADVANTAGES:
Poor turndown ratio A small drop in atomising pressure or increase in fuel viscosity will increase the droplet sizes, causing a reduction of the combustion performance Limited range of fuels can be used Generally not suitable for very large appliances
AIR / STEAM-ATOMISED BURNER
HOW A STEAM-ATOMISED BURNER ATOMISES FUEL
The fuel is supplied to the nozzle at a lower pressure than that of the conventional pressure jet burners (less than 600kPa ~ 6bar). Steam or compressed air is also discharged into the nozzle. These mix uniformly in the nozzle. As this mixture leaves the nozzle, the expansion of the ‘gases’ produces a spray of finely atomised oil.
ADVANTAGES OF STEAM ATOMISING
Steam atomising is tolerant to quality changes in the fuel; can successfully burn wide range of fuels These burners have better turndown...