Basic Introduction to Burners

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This post was last edited by lyyifeng on 2016-3-30 at 11:49. As a highly automated mechatronic device, a burner can be divided into five major systems based on the functions it performs: the air supply system, the ignition system, the monitoring system, the fuel system, and the electronic control system. 1. Air supply system: The function of the air supply system is to deliver air into the combustion chamber at a certain wind speed and volume. Its main components include: housing, fan motor, fan impeller, air gun nozzle, damper controller, damper plates, and diffuser disk. Shell: It serves as the mounting bracket for the various components of the burner and is a key part of the fresh air intake passage. In terms of appearance, they can be divided into box-type and gun-type. Box-type burners usually have an outer cover made of injection-molded material, and their power is generally low; high-power burners mostly feature a split-shell design and are typically of the gun-type. The material used to construct the housing is generally a high-strength, lightweight alloy casting. Fan motor: It primarily provides power for the operation of the fan impeller and the high-pressure oil pump; some burners use a separate motor to drive the oil pump. Some low-power burners use single-phase motors, with relatively low power, while most burners employ three-phase motors; the motor must rotate in a specific direction for the burner to function properly. Fan impeller: It rotates at high speed to generate sufficient wind pressure to overcome the resistance in the furnace and chimney, and to blow enough air into the combustion chamber to meet the requirements of combustion. It consists of cylindrical wheels with blades set at a certain inclination angle; the material used for these wheels is usually high-strength, lightweight alloy steel, though injection-molded versions are also available. All qualified fan impellers possess good dynamic balance properties. Air gun barrel: It serves to guide the airflow and stabilize the air pressure; it is also part of the air intake channel, and typically features an external flange for connecting to the furnace opening. Its constituent materials are generally high-strength and high-temperature resistant alloy steels. Damper controller: It is a driving device that controls the rotation of the damper flap through mechanical links. There are generally two types: hydraulic drive controllers and servo motor drive controllers. The former operates stably and is less prone to failures, while the latter provides precise control with smooth variations in air volume. Vent damper: Its main function is to adjust the size of the air intake passage in order to control the amount of air that enters. Its constituent materials include injection-molded and alloy types; injection-molded partitions are generally in single-piece form, while alloy partitions come in various configurations such as single-piece, double-piece, and triple-piece. Diffuser disk: Its special structure generates rotating air currents, which helps to ensure thorough mixing of air and fuel, and it also serves to regulate the amount of secondary air. 2. Ignition system: The function of the ignition system is to ignite the mixture of air and fuel. Its main components include the ignition transformer, ignition electrode, and high-voltage ignition cable. Ignition transformer: It is a conversion element that generates high-voltage output, with output voltages typically ranging from 2×5KV to 2×7KV, and the output current is generally between 15 and 30mA. Ignition electrode: Converts high-voltage electrical energy into light and heat energy through arc discharge in order to ignite the fuel. There are generally two types: unitary and split-type. High-voltage power cable: Its function is to transmit electrical energy. 3. Monitoring System: The function of the monitoring system is to ensure the safe operation of the burner; its main components include flame detectors, pressure monitors, and temperature sensors. Flame monitor: Its main function is to monitor the formation of the flame and generate signals to report to the programmer. There are mainly three types of flame detectors: photoresistors, ultraviolet UV sensors, and ionization electrodes. A. Phototransistor: It is commonly used in light oil and heavy oil burners. Its function and working principle are as follows: The phototransistor is connected to a flame relay with three contacts. The resistance value of the phototransistor changes depending on the amount of light it receives; the more light it receives, the lower its resistance value becomes. When the voltage applied across the phototransistor remains constant, the current in the circuit increases. Once the current reaches a certain level, the flame relay is activated, allowing the burner to continue operating. When the photosensitive resistor does not detect sufficient light, the flame relay does not function, and the burner will stop working. Phototransistors are not suitable for gas burners because the flame is not bright enough when gas burns. B. Ionization electrode: commonly used in gas burners. The controller supplies 220V voltage to the ignition transformer; one of the two high-voltage output wires is grounded, while the other is connected to the ignition electrode. A discharge occurs between the electrode and the ground, generating an electric spark that ignites the mixture of gas and air. The controller also provides power to the ionization electrode; if no flame is present, the power supply to the electrode is stopped. If a flame is present, the gas is ionized by its own high temperature, and an ion current flows between the electrodes, the flame, and the burner head. This ion current is rectified into direct current and sent through the grounded burner housing to activate the flame relay, thereby ensuring the proper operation of the burner. If the ionization electrode becomes grounded, the current generated will be alternating rather than direct current; the flame relay will not function, and the programmer will lock up. Furthermore, the ionization current and the ignition current flow through the same grounding circuit. Since the ignition current is much stronger than the ionization current, if these two currents flow in opposite directions, the ionization current will be blocked by the ignition current, resulting in the burner becoming open-circuited after a flame is formed. This defect can be compensated for by reversing the connection of the ignition transformer; by reversing the wires, the direction of the alternating current in the ignition transformer changes by 180°, which in turn causes the direction of the ignition current to also change by 180°. As a result, the directions of both currents become the same, thereby overcoming the aforementioned defect. Additionally, flame instability in the ionization zone can also cause the burner to disconnect while the flame is still present. This may be due to an inappropriate air-to-gas ratio, which can be resolved by adjusting the amount of air or gas; it could also be caused by uneven distribution of air and gas at the burner tip, which can be fixed by adjusting the position of the burner tip. C. Ultraviolet UV sensor: Generally used in gas-oil dual-fuel burners. This sensor can only detect the ultraviolet rays present in the flame (with a spectral range of 190–270 nanometers). The UV tube does not respond to the light from the refractory materials inside the furnace, ordinary light, or any glowing substances within the furnace. Under an ambient temperature of no more than 50°C, the lifespan of the UV tube is approximately 10,000 hours; higher ambient temperatures have a significant impact on its lifespan. If it receives a sufficient amount of ultraviolet light, it can generate an electric current, which, after appropriate amplification, can cause a relay or flame relay to close. If the UV tube’s power is depleted, it will still indicate that ultraviolet light is present, even though none actually exists. To overcome this issue, the controller applies an appropriate voltage across its terminals before each activation; in this way, even if the power is exhausted, the signal will indicate the absence of a flame, causing the controller to stop working immediately. To test the effectiveness of the UV sensor, it is removed from its original position for at least one minute after ignition; once the UV sensor is removed, the ultraviolet rays emitted by the flame can no longer be detected, the corresponding relay is disconnected, and the burner stops working. Even a small amount of oil contamination can block the path for ultraviolet light to reach the phototube, preventing the internal sensing elements from receiving enough ultraviolet light and thus preventing them from functioning. Therefore, the phototube must be thoroughly cleaned. A UV tube cannot detect light from the sun or ordinary lighting; its sensitivity can be tested using a flame or the spark generated between the two electrodes of an ordinary ignition transformer. To ensure the proper operation of the burner, its current must be stable and cannot be lower than the current required by the controller. This current can be measured using a microammeter, and its value must not be lower than that of the pressure monitor. Pressure monitors are generally used in gas burners; they are primarily used for monitoring high and low gas pressures as well as air pressure. If the burner is used in a steam boiler, steam pressure monitoring is also necessary. Temperature monitors: They are primarily used for monitoring and controlling the temperature of flue gas, as well as the temperature of fuel (heavy oil). They also serve to monitor and control the temperature of system water and medium water. 4. Fuel system: The function of the fuel system is to supply the fuel required for combustion in the burner. The fuel system of a fuel burner mainly includes: fuel pipes and connectors, fuel pumps, solenoid valves, nozzles, and heavy oil preheaters. A gas burner mainly consists of a filter, a pressure regulator, a solenoid valve assembly, and an ignition solenoid valve assembly. Fuel pipes and fittings: used for transporting fuel. Oil pump: A mechanism that generates pressurized oil, with an output pressure of generally over 10 bar to meet the requirements for atomization and oil injection volume. It comes in two types: single-tube output and double-tube output. Some burner oil pumps are connected coaxially with the fan motor, while others are driven by a separate oil pump motor. Common oil pumps include the J type, E type, and TA type, which are suitable for single-tube and double-tube oil systems. The oil pumps are equipped with filters, pressure control valves, and stop valves. The filter serves primarily to protect the transmission mechanism. The filter of the E-type pump has larger mesh openings; when the filter becomes clogged, it can lead to excessive vacuum. The filter needs to be cleaned regularly, and after cleaning or replacing it, it is essential to ensure that the pump cover is tightly sealed. Before operating the oil pump, oil must be poured into the pump’s overflow port through the suction side pipe; otherwise, the pump will be damaged due to dry operation. The suction resistance at the oil pump’s inlet should not exceed 0.4 bar, while the pressure at the outlet is generally between 10 and 24 bar. The maximum oil supply pressure for J-type pumps is 20 bar, while that for E-type and TA-type pumps is 40 bar; the maximum oil supply temperature is 90°C. Solenoid valve: Used to control the opening and closing of oil circuits, usually two-way or three-way valves. Nozzle: Its main function is to atomize oil droplets. The main parameters of the fuel injector include the injection angle (30°, 45°, 60°, 80°), injection mode (solid, hollow, semi-hollow), and fuel injection volume. Under the same pressure, nozzles with a smaller fuel injection volume achieve a better atomization effect. Commonly used nozzles include simple mechanical atomization nozzles and return-oil type mechanical atomization nozzles. The former feature a simple structure, a straightforward system, and are relatively reliable; they are generally used in burners with lower loads. The latter have a more complex structure and system, but they offer better adjustment capabilities and are suitable for use in boilers where the load needs to be adjusted over a wide range. Simple mechanical atomization nozzles come in tangential-groove and tangential-hole types; the former has a larger atomization angle and produces smaller atomized particles. Heavy oil preheater: A device specific to heavy oil burners, used to heat the heavy oil to a certain temperature in order to reduce its viscosity and thereby improve the atomization effect of the oil. Its temperature control mechanism is interlocked with the burner’s control circuit. Filter: Its function is to prevent impurities from entering the solenoid valve assembly and the burner. Voltage regulator: Its main function is to reduce and stabilize voltage; it is generally used in high-pressure gas supply systems, where the inlet pressure must be no lower than 100 mbar. Solenoid valve assembly: It generally consists of a safety solenoid valve and a main solenoid valve, and is available in split and integrated types. Integrated solenoid valve assemblies usually also include a pressure stabilizing valve and a filter screen. Safety solenoid valves are generally of the quick-open and quick-close type. The main solenoid valve is generally of two-stage type, and can be divided into types with fast opening and closing as well as types with slow opening and closing. Solenoid valve leakage detector: Its function is to detect whether the solenoid valve assembly is sealed properly. It is generally used in burners with a power greater than 1400 kW. Ignition solenoid valve assembly: It generally consists of a manual ball valve, a pressure regulator, and solenoid valves. It is mainly used in burners with high power. Among them: 1–Hand valve; 2–Filter; 3–Pressure regulator; 4–Pressure switch; 5–Solenoid valve; 6–Leak detection; 7–Air-gas pressure balance regulator; 8–Nozzle; 9–Dampener regulator; 10–Blower; 11–Ignition gas valve. Fuel supply system: Gas supply system: 5. Electronic control system: The electronic control system serves as the command and coordination center for all the aforementioned systems. Its main control element is the programmable controller, with different programmable controllers being used for various burners. Commonly used programmable controller series include LFL, LAL, LOA, and LGB; the main difference between these series lies in the timing of various program steps.