Industrial furnace is a thermal equipment that uses the heat of fuel combustion or electric energy conversion to heat materials or workpieces in industrial production. The main components of industrial furnaces are: industrial furnace masonry, industrial furnace exhaust system, industrial furnace preheater and industrial furnace combustion device.
Industrial furnace masonry
The function of the masonry is to make the industrial furnace bear high temperature load during the heating or smelting process, reduce heat loss, resist chemical corrosion and have a certain structural strength to ensure the heat exchange process in the furnace.
The masonry consists of a refractory layer and a thermal insulation layer. In order to ensure the strength and air tightness of the masonry, a steel structure (called a furnace frame) is used to fasten the masonry on the periphery of the masonry. The refractory layer directly bears high temperature load and mechanical impact, and at the same time bears the chemical erosion of furnace gas or molten liquid, and is mostly made of standard refractory bricks with specified dimensions.
The brick joints of the masonry are generally staggered with each other, leaving an appropriate size expansion joint within a certain distance. The chemical composition and thermal properties of refractory mud for bricklaying should be compatible with refractory bricks, and have suitable consistency and plasticity to meet construction requirements.
The outside of the refractory layer is an insulating layer, which is used to keep the refractory layer insulated to reduce the heat loss of the furnace wall and lower the outer surface temperature of the furnace wall. Standard insulation bricks with low density and low thermal conductivity or fiber materials such as cotton and felt are often used. composition.
Industrial furnace exhaust system
Industrial furnace smoke exhaust system is a system that uses chimneys or mechanical devices to exhaust the flue gas in the furnace chamber of industrial furnaces out of the furnace. Ensuring smooth flue exhaust is an important condition for the normal use of industrial furnaces. When the flue exhaust is not smooth, the furnace pressure will increase, and a large amount of flue gas will escape from the gaps around the furnace, which will increase the heat loss of the furnace and affect the uniform distribution of airflow in the furnace. Reduce furnace temperature uniformity and worsen the operating environment.
The smoke exhaust system is composed of a smoke exhaust device that generates suction and a flue that exhausts the flue gas. Commonly used smoke exhaust devices include chimneys, induced draft fans or jet pipes.
Chimney exhaust is based on the buoyancy generated by the density of hot smoke flowing into the chimney that is less than the density of the air outside the chimney to overcome the resistance of the flue. The flue gas can also be discharged by the induced draft fan, or a jet pipe is installed in a certain part of the smoke exhaust system to discharge the flue gas with the negative pressure generated by the high-speed jet gas. Chimney exhaust does not consume power, and the exhaust temperature is not limited. When the smoke exhaust resistance is very large and the industrial furnace runs intermittently, the induced draft fan or jet pipe can be used to exhaust smoke. The jet pipe is suitable for removing high-temperature flue gas; the induced draft fan is suitable for removing low-temperature flue gas.
Chimneys are divided into brick chimneys, concrete chimneys and steel plate chimneys. There are two types of flue: underground flue and overhead flue. The underground flue is mostly made of bricks, and the overhead flue should be made of steel plate lined with refractory materials.
In order to reduce the pollution of the flue gas to the environment, or to install a preheater in the flue for energy saving, it is necessary to increase the height of the chimney and increase the flue gas flow rate at the chimney exit to make it greater than the local maximum wind speed or at least not less than 3 meters per second to prevent harmful gases and smoke in the flue gas from spreading to the ground.
Industrial furnace preheater
A device that uses the waste heat of the flue gas discharged from an industrial furnace to heat the combustion air and gas fuel. After installing a preheater on the industrial furnace, due to the heat recovery, fuel can be saved and the furnace temperature can be easily increased to speed up the heating rate. Industrial furnace preheaters are divided into two types: heat exchange type and heat storage type.
1. Heat exchange preheater
Heat exchange preheaters are divided into two types: metal preheaters and ceramic preheaters. They all use the waste heat of the flue gas discharged from the furnace to heat the preheater wall through radiation heat exchange and convection heat exchange, and then heat the air or gas flowing through the other side of the wall in the same way, that is, preheat.
The wall of the metal preheater has a large thermal conductivity, the wall can be very thin, and the air tightness is good. It can preheat the air to about 600°C. It is a widely used preheater. The wall thermal conductivity of the ceramic preheater is small, but it can withstand higher flue gas temperature and can also preheat the air to about 600°C.
In the early 1920s, cast iron tubular or needle-shaped preheaters were mostly used in industrial furnaces. After the 1940s, tubular preheaters, cylindrical radiant preheaters, jet preheaters, and cast iron blocks made of steel were mostly used. There are block preheaters for steel pipes and so on.
The flow modes of flue gas and air in the preheater are divided into three types: forward flow, counter flow and cross flow. From the perspective of improving the heat transfer performance, it is better to adopt the counter-current method to obtain a higher preheating temperature; from the perspective of reducing the wall temperature and increasing the service life of the preheater, it is better to adopt the downstream method; Between downstream and upstream. The jet preheater has a unique flow mode. The preheated gas is jetted from the small holes densely arranged on the inner tube at high speed to flush the heat exchange surface of the outer tube and make the fluid boundary layer have turbulent properties, thereby generating strong heat exchange. .
Regenerative preheater
The regenerative preheater is the regenerative chamber, which is a checker brick body made of refractory bricks. In order to enable continuous preheating of air, a furnace needs to be equipped with two regenerators, which are respectively in the heat storage or preheating working state.
The heat transfer process is: the flue gas is introduced into the regenerator, part of the heat of the flue gas is absorbed by the checker bricks (heat storage), after 10-30 minutes, the flue gas is automatically cut off by the reversing device, and air is introduced instead. The heat storage of the brick body heats the air (preheating); also after 10 to 30 minutes, the air is cut off, and then the flue gas is introduced. This is a reversing cycle. The regenerator used in the heating furnace can preheat the air to 600-700°C and has a long service life.
Industrial furnace combustion device
A device used to realize the fuel combustion process in an industrial furnace that uses fuel as a heat source. According to the heating requirements of the flame furnace, various combustion devices should ensure:
① Ensure the complete combustion of fuel under the specified thermal load conditions;
②The combustion process is stable and can continuously supply heat to the furnace;
③The flame direction, shape, rigidity and spreadability meet the requirements of furnace type and heating process;
④Simple structure, easy to use and maintain.
The combustion process of various fuels is different, so the structure of the combustion device is also different. Combustion devices can be divided into several types of gas, liquid, and solid fuel.
1. Gas fuel combustion device
Usually called a burner, its main function is to send gas and air to the furnace for combustion (also burned inside the burner) according to a certain proportion and certain mixing conditions, and to meet the flame requirements of the furnace heating process. According to the mixing situation of gas and air in the burner, it is divided into flame and flameless burner.
The characteristic of the flame burner is that the gas and air are not mixed or only partially mixed in the burner, and then burned while being mixed after being sprayed into the furnace, so the flame is longer and has a clear outline. When using a flame burner, the main means of intensifying combustion and organizing flames is to change the mixing conditions of gas and air, such as dividing the gas and air into many small streams, making the gas flow and air flow intersect at a certain angle, or using a swirling device Promote air flow to accelerate mixing, etc. Figure 1 shows a gas burner with a single tube.
Single tube gas burner
The characteristic of the flameless burner is that the gas and air are evenly mixed inside the burner, and can be burned immediately after being sprayed out of the burner. The flame is very short and there is no obvious flame contour. The flameless burner commonly used in industrial furnaces is a jet burner, which draws in the required combustion air directly from the atmosphere by the jet effect of gas, mixes it evenly in the mixing tube, and then enters the combustion channel made of refractory materials. Complete the combustion reaction.
Beginning in the 1960s, in order to meet the needs of new heating processes, high-speed burners with gas outlet speeds of more than 100 m/s, flat flame burners with disc-shaped flames, burners and preheaters, and exhaust gas outlets appeared successively. The smoke device constitutes an integral self-preheating burner. In order to reduce the pollution of harmful gas NOX to the environment, various new types of combustion devices such as low nitrogen oxide burners have also been developed.
2. Liquid fuel combustion device
Usually called a grease nipple, or nozzle. Fuel oil needs to be atomized and then burned. Therefore, in addition to the basic performance of a general combustion device, the fuel nozzle should also have a good atomization ability to ensure the complete combustion of the fuel. According to the atomization method, the nozzles can be divided into low pressure nozzles, high pressure nozzles, mechanical nozzles and rotary cup nozzles. Among them, low pressure nozzles and high pressure nozzles are widely used.
The low-pressure nozzle uses all combustion-supporting air as the atomization medium, and atomizes the oil by the momentum of the airflow. The atomization particle size is 80-100 microns, the air pressure is generally 2940-7840 Pa, and the flame during combustion is generally 600-1400 mm.
The high-pressure nozzle uses steam or compressed air as the atomizing medium, and the pressure is generally as high as (3～12)×105 Pa. Because the pressure of the atomizing medium is high, the ejection speed can reach or exceed the speed of sound, so the atomization capacity of the high-pressure nozzle is lower than that of the low pressure. The oil nozzle is strong, and the atomized particle size can reach 20-30 microns, but it needs to add a channel for conveying combustion air and corresponding airflow guide facilities.
3. Solid fuel combustion device
For industrial furnaces that use solid fuels, the lump coal bed combustion method and the pulverized coal jet combustion method are commonly used. The combustion device using the lump coal layered combustion method is referred to as the combustion chamber, which is divided into an artificial coal combustion chamber and a mechanical coal combustion chamber. The lump coal is stacked on the grate by manual or mechanical devices, and the combustion-supporting air passes through the coal seam from the bottom of the grate from bottom to top to complete the combustion reaction. The mechanical coal combustion chamber of the reciprocating grate.