TYPES OF BURNERS

 

Classification on the basis of Construction

MONOBLOC BURNERS : 

                               In these burners, the oil pump and the blower impeller are driven by the same prime mover (electric motor). The impeller is mounted on the shaft of the motor and the pump is coupled to this shaft.

. This means that the oil pump and the FD fan are driven by a common electric motor. The arrangement is made in such a manner that these three parts are mounted on a single common shaft. All parts of the burner are housed in a single unit. 

DUALBLOC BURNERS:

In these types of burners, the impeller and the gear pump are driven by separate prime movers. Both the pump and impeller are mounted on different units and are connected using a duct or a skid.

Classification according to Modulation

   

ON/OFF:   

                      The lance assembly houses only one nozzle. As the name suggests the nozzle is either on (firing) or off. The construction and operation are as described earlier. The high-pressure cut-off at which the nozzle stops firing is defined by the user on the basis of steam loads. The pressure switch senses the pressure change and actuates the solenoid valve which turns the nozzle on or off.

HIGH/LOW (2-STAGE): 

                     This burner consists of two nozzle atomizers housed in the lance assembly which are backed by two solenoid valves. The valves are actuated by two pressure switches, which are set at different pressures as per the requirement. Consider a situation when the boiler is under low fire operation, as the load increases the pressure in the boiler decreases. As soon as the pressure drops below the setpoint, the second nozzle fires and the pressure starts increasing.

3-STAGE BURNER: 

                     This pressure jet burner has three nozzles backed by three solenoid valves. There are three stages of burner firing- low, medium, and high. These are actuated by three different pressure switches set as per the customer’s requirements.  

 When the steam demand increases and excess steam is drawn from the boiler, the pressure drops. When the drop crosses the first set point the pressure switch actuates the mid-fire nozzle. If the demand further increases and the pressure drops down the next set-point the third nozzle starts firing. This is a high fire position. The air damper is also set accordingly at all the 3 positions with respect to the firing conditions.

STEPLESS MODULATION:

                       In this burner, there is a single nozzle atomizer and control valve mounted behind it. The rest of the construction is similar except for the modulating mechanism. This mechanism simultaneously controls the flow of the fuel and the air.

            The driving unit for the air damper is a reversing modulating motor regulated by a proportional controller.  The pressure of the steam in the boiler is sensed by a Pressuretrol. This Pressuretrol transmits the pressure changes to the control valve behind the nozzle and the motor which controls the air damper.  Thus in line with the pressure changes, the amount of air and fuel vary till we get the correct air-fuel ratio.

Classification on the basis of the principle of atomization

ROTARY CUP BURNERS: 

                           In these burners, the fuel is atomized mechanically with the help of centrifugal force. The fuel is broken into tiny particles by the air flowing over the oil film formed in the high-speed rotary cup.

         The rotary cup is an oil distribution cup mounted on a rotating shaft. This shaft is driven by an electric motor using a V-belt. The cup is placed at the tip of the wind box along with primary and secondary air registers. The fuel is preheated just to make it less viscous (90-Degree Celsius). As the fuel pressure doesn’t take part in the atomization, so 3.5g pressure is sufficient.

          A particularly fine and homogeneous oil spray is achieved through rotary cup and air atomization, which means this burner provides optimum combustion with low excess air. Liquid fuel at low pressure is introduced at the rear of the rotating atomizing cup. The combination of the shape of the cup and the centrifugal force generates a thin and homogeneous film of fuel. This film spreads over the interior surface of the cup towards the rim. Oil spins off the cup and disperses into an extremely fine mist. Finally, the primary air discharged concentrically around the cup and controlled by swirl vanes complete the distribution of fuel droplets which is a pre-requisite for excellent combustion. It also provides a wide turndown and control range. The thinness of the oil film and the quality of atomization ensure that the burner needs a minimum of low-pressure primary air. Combustion takes place a short distance from the cup rim and is stabilized by a flame baffle.

STEAM ATOMISED BURNERS:

In these burners, the fuel is atomized using a high-speed jet of steam.

PRESSURE JET BURNERS:

                           The fuel is pressurized using a fuel pump and this pressurized fuel is broken into tiny droplets using a nozzle (atomizer). Air is blown in to get a proper air-fuel ratio. The spark is generated between the ignition electrodes and combustion takes place.

      The fuel is pressurized up to a pressure of 21 - 25 kg/sq.cm using a gear pump and is atomized into tiny droplets using an atomizer nozzle. The oil is supplied to the gear pump from the day oil tank. The electric heating assembly connected to the gear pump consists of a heater element, which heats the oil to temperatures of about 120 degrees Celsius (In the case of Furnace oil). The fuel is heated to make the fuel less viscous (thinner). Heating is not required in case LDO or HSD is used, as they are comparatively less viscous than FO. The gear pump circulates the oil through the oil circuit. Oil circulation continues till the desired temperature is attained. 

              There are pressure and temperature gauges mounted on the fuel circuit which continuously monitor the fuel temperature and pressure. Along with this FD, the fan starts and draws air through an acoustically lined air duct and delivers to the diffuser/punker plate where atomized fuel and air mix. The primary and secondary airs suffice flame establishment and flame stability respectively. A damper motor depending on the firing condition governs the air quantity. 

             Once the temperature is attained the flame is established with the help of a spark produced across the two ignition electrodes. The high is generated using a step-up ignition transformer. There is a pressure switch that senses the pressure in the boiler and actuates the solenoid valves behind each of the nozzles. The number of nozzles depends on whether the burner is ON/OFF, HIGH LOW (2 Stage), or MODULATING (3 Stage).  Once the flame is established it is registered by a photocell (flame detector) in the lance assembly.