The basic principle on which a rotary cup burner operates is the mechanical breaking of fuel into very fine particles. As the name suggests, this burner has a high-speed rotating cup operating at 5000 to 6000 rpm, depending on the burner manufacturer’s design, and oil is made to pass over it in the form of a thin film. As a result of the high-speed rotary motion, the fuel particles get thrown away from the center of the cup in particular geometry and as a result, break into very fine particles. These particles when atomizing in different directions form a conical shape which is ignited to burn the fuel.
In a rotary cup burner, there are separate ducts provided for primary and secondary air. The primary air is contributing mainly to the oil atomization process while secondary air provides sufficient air for complete combustion. The distribution of air quantity is usually 15-20% for primary air and 80 - 85% for secondary air.
The burner consists of an oil distribution cup mounted on a rotating shaft. The shaft is driven by an electric motor using a V-belt. The cup is placed at the tip of the wind box that also contains primary and secondary air registers. The primary air register is fed with air at high pressure (7-15 kPa). The secondary air register is located separately and feeds air at lower pressure (2.5 – 5 kPa). This is achieved by the geometry of the airflow path and vane-controlled dampers.
This burner requires pre-heating only to bring it into the easily flowable condition and is usually required to be heated up to 90 degC for furnace oil commonly found in India. Similarly, since the oil pressure does not take part in fuel atomization, 3.5 barg pressure is sufficient.
After entering the burner oil is firmly spread on the walls of the rotating cup through an oil distributor located inner center part of the cup. On its way through the cup, oil is formed into a thin sheet and when it leaves the rim of the cup, this sheet has a thickness of 100 microns. Swirling primary air hits the oil sheet at an axial velocity of about 100 m/s. The result is good atomization and secure flame stability. The rest of the combustion process is provided with secondary air.
The rotary cup is the most important part of this burner. The outer mantle is made out of hard chromium–plated low-carbon steel. Its center of gravity lies near the hub, minimizing the load on the whole structure. The hub is precision-manufactured in light alloy. The cup is driven by an electric motor through a V-belt. Oil is fed through a tube separate from both shaft and belt drive. The shaft is balanced and is free from vibrations.
Atomization of fuel by mechanical means using a high-speed rotating cup is found to be much more effective than that of pressure jet principle. The atomization process is further assisted by primary air distribution.
This makes the rotary cup burner more suitable to handle highly viscous heavy oils. This burner finds itself comfortably burn Furnace oil, LSHS, or heavy grade viscous oils found in European countries.
The most outstanding features of this technology are wide turndown range, savings in terms of electrical energy & fuel, and capability to handle difficult fuels. These features are elaborated further.
Effective Combustion, Reduced Emissions due to Rotary cup burner :
Micro Carbon Residue, Ramsbottom, and Conradson Carbon Residue are three different test Methods to check the same characteristic of a diesel fuel and a heavy fuel. This residue contains incompletely burned fuel particles and also the ash formed by the fuel upon combustion.
The Rotary cup burner when compared to a pressure atomized burner is more suited to give a reduction from the amounts of unburnt coming from the stack.
With a pressure atomized burner the oil droplet size range is larger than with the Rotary cup burner (50 - 300 microns for pressure jet as against 20 - 150 microns with a Rotary Cup). In addition to this, the pressure atomizing burner is dependant on oil pressure to keep within the droplet size range. As soon as the burner turns down the oil pressure falls off and this has a major impact on the atomizing quality and oil droplet size. This has an adverse effect on emissions and is a factor in the production of smutting, which is the common problem with pressure jet burners firing heavy fuel oils.
The lower the oil pressure from the pressure jet burner the worse the situation gets and at the low fire the atomizing quality is poor the burner is using more fuel than necessary and generally giving poor combustion figures.
Another factor to consider is the oil viscosity. Rotary Cup burners are very tolerant of viscosity changes with heavy fuel oils, which is quite common. Whereas in Pressure Jet Burners, any change in viscosity will severely affect the atomization quality. This again will contribute to poor quality of emissions if the viscosity varies. Because again the Rotary burner handles these viscosity changes better due to the mechanical means of atomization. One should also consider the fact that with heavy oil and pressure jet burners the atomizing temperature is a compromise as the actual atomizing temperature required on a pressure jet burner for heavy fuel to achieve the correct viscosity through the nozzle would be a lot higher than is used. This again results in poor combustion quality and higher incidents of smutting.
Because the Rotary cup burner is not dependant on oil pressure to atomize the oil and because the cup allows a very thin film of oil to be thrown into the path of the primary air, the atomizing quality stays the same as the burner turns down the modulation range. Together with the fact that the burner produces a much smaller range of droplet size and that the droplet size remains the same throughout the modulation range, this allows more fuel to be burnt. The result is a reduction of smutting, in most cases eliminating it altogether.
The combination of better quality atomizing, lower oil temperatures and lower oil pressures lower FD fan ratings all combine to allow the customer to enjoy better emissions and reduced running costs for the boiler plant.
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