Vertical shell boilers mainly include vertical horizontal water tube boilers, vertical multi-horizontal water tube boilers, vertical straight water tube boilers, vertical elbow water tube boilers and vertical fire tube boilers. At present, the latter three are widely used. Due to the low thermal efficiency of vertical boiler, it is difficult to solve the problem of mechanized combustion, and the furnace water cooling degree is large, which is not suitable for burning inferior coal. At present, the output is gradually decreasing, which is limited to a small number of applications with low voltage and small capacity, lax environmental protection control and abnormal power supply.
2. Horizontal shell boiler
Horizontal shell boiler is one of the largest industrial boilers. At present, it has developed from the original maximum output of 4t/h (a small amount of 6t/h) to the production of 40t/h shell boiler.
1. Horizontal internal combustion shell boiler
Horizontal internal combustion shell boiler is small in height and volume, and suitable for assembly. When using micro-positive pressure combustion, the sealing problem is easy to solve, and the furnace shape is favorable for gas, so it is mostly used in oil (gas) boilers and less used in coal-fired boilers.
2. Horizontal external combustion shell boiler
This is the most widely used and common industrial boiler in China. According to the current industrial boiler model compilation method, the application code is WW, but at present, the domestic boiler industry is represented by the water tube boiler form code DZ.
The main difference between horizontal external combustion water-fire tube boiler and horizontal internal combustion water-fire tube boiler is that the horizontal external combustion water-fire tube boiler removes the combustion device from the boiler shell, increases the grate area and furnace volume, and adds water-cooled wall tubes on both sides of the boiler shell to form combustion chambers, which creates good conditions for coal burning, so the fuel adaptability is wide and the thermal efficiency is high.
Three. water-tube boiler
The water tube boiler is equipped with a water tube heating surface outside the drum. The high-temperature flue gas flows outside the tube to release heat, and the water absorbs heat inside the tube. Because the cross section inside the tube is smaller than that outside the tube, the velocity of steam and water is greatly increased, and the steam generated on the heating surface is washed away immediately, which improves the heat absorption rate of boiler water. Compared with shell boiler, water tube boiler has smaller drum diameter, higher working pressure, smaller boiler water capacity, lighter disaster, better boiler water circulation, higher evaporation efficiency, better adaptability to load change and higher thermal efficiency. Therefore, boilers with high pressure and evaporation are all water-tube boilers.
Common water tube boilers are horizontal straight water tubes with double drums, vertical water tube boilers with double drums and vertical water tube boilers with single drum.
Four. hot-water boiler
Hot water boiler refers to a kind of thermal equipment in which water does not change phase in the boiler body, that is, it does not produce steam. After the backwater is sent to the boiler, it absorbs the heat of flue gas through the heating surface and is input into the heating network before reaching the saturation temperature.
(A) the characteristics of hot water boilers
1. Boiler working pressure
The working pressure of hot water boiler depends on the flow resistance and constant pressure value of thermal system. The working pressure given on the nameplate of hot water boiler is only the allowable pressure indicating the boiler strength, but in actual operation, the boiler pressure is often lower than this value. Therefore, the safety margin of hot water boiler is relatively large.
2. The temperature difference between flue gas and boiler water is large, the scaling is small, the heat transfer effect is good, and the efficiency is high.
3. The effect of heating with hot water boiler is obvious. There is no steam loss of steam heating in hot water boiler heating, and the pollution loss is greatly reduced, and the leakage of system and trap is also greatly reduced, and the heat loss is also reduced. Therefore, hot water heating system can save about 20% fuel compared with steam heating system.
4. No vaporization is allowed in any part of the boiler, otherwise it will destroy the water circulation.
5. If the water is not deoxygenated, the problem of oxygen corrosion is prominent; The tail heating surface is easily corroded by low temperature acid.
6. Dissolved gas will be separated from boiler water during operation, and the problem of gas removal is considered structurally.
Structure form of hot water boiler
1. Tubular hot water boiler
There are two kinds of this kind of boiler: pipe rack and snake tube, the former is more common. The tubular hot water boiler forces the boiler water to flow through the pressure head of the circulating pump and directly heats the boiler water. This kind of boiler is mostly composed of small diameter cylinders (headers) and pipes, with compact structure, small volume, steel saving, simple processing and low cost. However, the water capacity of this kind of boiler is small, and in case of sudden power failure during operation, the boiler water is easy to vaporize, which may cause water hammer.
2. Drum type hot water boiler
This kind of hot water boiler was transformed from a steam boiler in the early days, and its boiler water naturally circulates in the boiler. In order to ensure the safety and reliability of boiler water circulation, it is required that the boiler should have a certain height, so this kind of boiler has a large volume, and the steel consumption and cost are relatively increased. However, due to the large outlet capacity of this boiler, it can maintain natural circulation, and when the circulating pump of the system suddenly stops running, it can effectively prevent the boiler water from vaporizing. It is for this reason that natural circulation hot water boilers have developed rapidly in China in recent years.
Section 2 Basic Structure and Structural Features
According to the given parameters such as evaporation or thermal power, working pressure, steam temperature or rated inlet and outlet water temperature, fuel characteristics and combustion mode, and in accordance with the relevant provisions of the Safety Technical Supervision Regulations for Steam Boilers, the Safety Technical Supervision Regulations for Hot Water Boilers and the strength calculation standards for boiler pressure components, the boiler structure is determined. A qualified boiler, no matter what form it belongs to, should meet the basic requirements of "safe operation, high efficiency and low consumption, smoke and dust elimination, production and heat preservation".
First, the basic requirements of laws and regulations for boilers
(1) Each pressure element shall be able to expand freely in the design direction during operation;
(2) Ensure the normal water circulation of each circulation loop, and the heating surfaces should be reliably cooled;
(3) Pressure components should have sufficient strength;
(4) The structural forms, openings and welds of compression elements and members should be arranged to avoid reducing composite stress and stress concentration;
(5) The structure of water-cooled fireplace wall should have enough bearing capacity;
(6) The furnace wall should be well sealed;
(7) Necessary manhole, hand hole, inspection hole, fire inspection door, ash removal door, etc. It should be convenient for installation, operation, maintenance and internal and external cleaning;
(8) There should be safety accessories and display instruments that meet the requirements to ensure the normal operation of the equipment;
(9) Boiler blowdown structure should be compatible with blowdown;
(10) Butt joints shall be adopted at the joints between the tank and tempering chamber (wet back type), the tank and rear tube plate (dry back type) and the tank and front tube plate (tempering type) of horizontal internal combustion boiler.
Second, the structural characteristics of oil (gas) boiler:
Compared with coal-fired boilers, oil-fired (gas-fired) boilers have the following structural characteristics due to the different fuels used:
(1) Fuel is injected into the boiler furnace through the burner, and the combustion is carried out in the fire room without grate facilities;
(2) The oil (gas) boiler has no slag outlet and slag discharge equipment because no ash is produced after oil and gas combustion;
(3) If the materialized oil and gas or the gas injected into the furnace is turned off or mixed with air in a certain range, it is easy to form explosive gas, so the oil (gas) boiler needs to adopt an automatic combustion system, including flame monitoring, flameout protection, explosion prevention and other safety facilities;
(4) Because the calorific value of oil and gas is much higher than that of coal, and its furnace thermal intensity is much higher than that of coal-fired boiler, compared with coal-fired boiler with the same capacity, the boiler is small in volume, compact in structure and small in occupied area;
(5) The combustion process of the oil-fired (gas-fired) boiler is suspended in the hearth, so the front and rear arches are arranged in the hearth, and the hearth structure is very simple.
3. The difference between oil-fired boiler and gas-fired boiler
(1) There is not much difference in furnace body structure between oil-fired boiler and gas-fired boiler, only because of the different calorific value of fuel, the heating surface is adjusted accordingly. That is, the radiation heating area of oil-fired boiler is larger, while the convection heating surface of gas-fired boiler is designed to be larger.
(2) The burner of oil-fired boiler must have a fuel materializer, while the burner of gas-fired boiler does not need a materializer.
(3) Oil-fired boilers must be equipped with complex oil supply systems (especially when burning heavy oil and residual oil), such as oil tanks, oil pumps and filtering heating pipes, which must occupy a certain space, while gas-fired boilers do not need to be equipped with gas storage devices. Just connect the gas pipeline to the gas supply network. Of course, the pipeline also needs to be equipped with pressure regulating devices, solenoid valves, buffer valves and other accessories to ensure the safe operation of the boiler.
In the third quarter, coal-fired boilers were changed to oil (gas).
Basic principle of boiler
1. Basic principles of changing coal-fired boilers into oil-fired (gas) boilers
(1) The reformed coal-fired boiler must meet the following conditions:
(1) The pressure components of the original boiler must be basically intact and have the value of continuous use;
(2) The original boiler water gas system, air supply system and induced air system must be basically intact.
(2) The reformed boiler shall achieve the following purposes:
(1) Keep the rated parameters of the original boiler (such as steam pressure, steam temperature, feed water and return water temperature, etc.). ) unchanged;
② Maintain or improve the output and efficiency of the original boiler.
(3) After transformation, smoke and dust can be eliminated, which meets the requirements of environmental protection.
(4) The boiler transformation scheme must be simple, feasible, with less investment, quick effect and short construction period. Therefore, the smaller the scope of boiler transformation, the better. Only the furnace and combustion device can be changed, and the transformation part does not exceed the basic structure of the boiler body.
2. Problems that should be paid attention to when changing coal-fired boilers into oil-fired (gas) boilers.
(1) Mechanized layered coal-fired boilers should be changed to oil-fired (gas-fired) boilers. First of all, the front and rear arches should be removed, at the same time, the bottom heating surface should be increased and the grate should be replaced to prevent the grate from overheating and burning out.
(2) For small boilers, it is necessary to pay attention to the structure and sealing of the furnace wall when changing from coal combustion to oil (gas) combustion, that is, from the original negative pressure combustion to the present micro-positive pressure combustion.
(3) The selection and arrangement of burners are closely related to the furnace type, so that the flame plumpness in the furnace is better and no dead angle of airflow is formed; Avoid the mutual interference of the flames of adjacent burners; Keep the flame in the center of the furnace at low load to avoid the flame center deviating from the symmetrical center of the furnace; Unburned gas-air mixture should not contact the heating surface to avoid incomplete combustion of gas; The high-temperature flame should avoid high-speed scouring of the heating surface, so as to avoid overheating of the pipe wall caused by the high thermal intensity of the heating surface. The burner layout should also consider the reasonable layout of gas pipelines and air ducts, as well as the convenience of operation, inspection and maintenance.
(4) The flue gas velocity of convection heating surface of oil-fired and gas-fired boiler will not be limited by fly ash abrasion, so the heat transfer coefficient of convection heating surface can be increased by increasing the flue gas velocity appropriately, and the boiler pressure can be increased without increasing the boiler heating surface. At this time, attention should be paid to the capacity of the steam-water separation device in the boiler to ensure the steam quality, which is especially important for boilers with hot air.
(5) In order to prevent high temperature corrosion, the fuel is changed from coal to fuel. Because the fuel oil contains organic metal elements such as sodium and vanadium, the melting point of oxide molten crystals generated after combustion is very low, generally around 600℃, or even lower. After sublimation at high temperature in the furnace, these oxides condense on the heating surface with relatively low temperature, forming corrosive high-temperature ash deposit. The higher the temperature, the faster the corrosion. Therefore, special coatings such as ceramics and silicon carbide should be coated on the surface of the heating surface which is susceptible to high temperature corrosion, and materials with good high temperature corrosion resistance can also be selected to improve its high temperature corrosion resistance.
(6) Prevent furnace explosion. When the coal-fired furnace is changed into an oil-fired (gas) furnace, oil droplets (gas) with poor fuel atomization or incomplete combustion will gather in the furnace or the heating surface at the tail, which will lead to fire or explosion. Therefore, explosion-proof doors should be installed in appropriate parts of the boiler, and ignition program control and flameout protection devices should be added to the automatic control to ensure the safe operation of the boiler.