Boiler is a pressurized part containing water and steam, including drum, heating surface, header (also called header) and pipeline. In which water heating, evaporation and steam-water separation processes are carried out.
1. The function of steam drum is to collect, store, purify steam and supplement water supply. In water-tube boilers, there are usually 1~2 drums, the upper drum is called steam drum, and the lower drum is called water drum or mud drum. The upper drum is internally provided with a water distribution tank with uniform water supply, a steam-water separation device and a continuous blowdown device. There are assembled pipes and baffles in the drum of hot water boiler. Connecting pipes such as main steam valve, auxiliary steam valve, safety valve, air valve, water level gauge, sewage pipe and pressure gauge are installed outside the upper drum. Manhole door at one end or top of the upper drum.
One end of the lower drum is provided with a manhole, and the bottom is provided with a regular blowdown device. The upper and lower drums are connected by many ascending pipes and descending pipes (bundles), and the whole drum is elastic.
The drum of shell boiler not only plays the role of loading and unloading the drum of water-tube boiler, but also has smoke pipes arranged inside it, and some have heating surfaces such as combustion chamber hearth, and the drum is also a part of the heating surface.
The flat head of a drum with a flue pipe is called a tubesheet. The tube plate is flat, and the peripheral flange is welded with the cylinder. There are many pipe holes on the pipe plate, which are connected with the smoke pipe by expansion or welding. The part of the tubesheet without smoke pipe is the weak link in compression, which is usually strengthened by diagonal braces or circular diagonal braces. The bracket is welded between the cylindrical joint and the tubesheet.
At present, a new connection mode between tubesheet and convex tube group has been developed, which further reduces the thickness and water flow of tubesheet and improves the stress situation.
Second, the water wall heating surface
The water wall is a radiant heating surface arranged around the furnace. In industrial boilers, it is generally made of corroded φ5 1×3~φ63.5×3.5mm boiler steel pipe, which is the main heating surface of water-tube boilers. The radiant heating surface absorbs the radiant heat of the flame and shields the furnace wall, thus protecting the furnace wall; At the same time, because of absorbing more heat, the furnace temperature can be reduced and coking of burning coal seam can be prevented.
The upper part of the water wall is directly connected with the upper boiler or connected with the upper boiler drum through the upper header first. Boiler water from the upper drum is pumped into the lower drum and the lower header through the downcomer, absorbed by the water wall to form a steam-water mixture, and rises to the upper drum to form a closed natural circulation.
In the past, fins (also called fins) were welded on both sides of the water wall tubes of some fast-loading boilers. Its purpose is to receive more radiant heat from the furnace and increase the shielding area of the furnace wall. Because of its difficult processing and poor thermal conductivity, it is easy to crack and tear the water wall tube in serious cases, so it has been seldom used and gradually replaced by special-shaped steel tube.
Third, the header is the main element of the arrangement and connection of the collector tubes, which has the function of distributing water and leading out. According to its position, there are upper and lower headers or inlet and outlet headers.
The upper header is located at the upper part of the furnace tube, which collects the water vapor mixture of the ascending tube bundle and leads it to the steam drum through the conduit. Some upper headers are installed outside the furnace wall, and there are rows of hand holes in the position opposite to the furnace tube to clean the inside of the furnace tube.
The lower header is located at the lower part of the furnace tube and connected with the lower drum to supply water and distribute it to the ascending furnace tube. The lower header located on both sides of the grate can prevent the furnace walls on both sides of the grate from burning out or hanging coke, which is called anti-coke box. There is a drain pipe and a hand hole at the end of the lower header for checking and cleaning the inside of the header.
In addition to the header of the boiler body, there are also corresponding headers on the economizer, superheater and other components.
The header is generally welded by seamless steel pipe with large diameter and two end covers. In recent years, some manufacturers have spun the pipe end instead of welding the end cover, which makes the structure more reasonable.
Four. Convection tube bundle heating surface
Convection tube bundle, also known as convection tube bundle, is located between the upper and lower drums and generally consists of φ5 1×3mm boiler steel tubes, which are the main heating surfaces of water-tube boilers. In order to fully absorb heat, a trap wall is usually used in the middle of the convection tube bundle to form a back-and-forth or turning flue, increase the flue gas flow, and laterally scour the tube bundle at high smoke velocity. Usually, the tube bundle washed with flue gas first becomes an ascending tube because of the high flue gas temperature, more heat transfer and low density of steam-water mixture in the tube, while the flue gas temperature is low and less heat transfer becomes a descending tube.
Flue gas washing tube bundle generally adopts transverse washing. Because of transverse scouring, the thermal effect is better than that of longitudinal scouring. Staggered row (or staggered row) has better heat transfer effect than straight row, but with the improvement of heat transfer effect, the flow resistance of flue gas increases accordingly.
Verb (abbreviation of verb) heating surface of pyrotechnic tube Smoke tube is the main heating surface of smoke tube boiler and water-fire tube combined boiler. Its function is to continuously heat the boiler water outside the furnace when the high-temperature flue gas generated by furnace combustion flows through the tube, so that it gradually becomes steam (or hot water). The flue pipe is generally installed in the boiler shell and fixed on the tube plates at both ends by welding or expansion joints. Because its installation quantity is limited by the boiler shell, the pipeline is easily blocked by dust and scale, which is not suitable for larger boilers. But in recent years, large-scale water-fire tube boilers are also under study, and some progress has been made.
In recent years, a new type of flue pipe, namely threaded flue pipe, has been popularized and applied. It presses a threaded groove with a depth of about 2mm on the flue pipe, which improves the heat dissipation of flue gas to the pipe wall, and the effect is remarkable. Compared with the common flue pipe, the heat transfer effect of this threaded flue pipe can be improved by nearly 2 times.
Six, superheater economizer and air preheater. All belong to convection heating surface. Superheaters include convection superheaters and radiation superheaters, and the remaining economizer and preheater are equipped with boiler tail flue. Their main function is to further reduce the exhaust gas temperature and improve the thermal efficiency of the boiler.
The furnace is located in front of the boiler body, where the fuel is burned. It includes combustion equipment and a stove. Because of different fuels, the combustion modes are different, mainly including layer combustion furnace, chamber combustion furnace, cyclone furnace, boiling furnace and so on. Water wall pipes are installed around their stoves. In order to enhance fuel combustion, the layer furnace also has a front arch and a rear arch. Different furnaces have different structures, but they all have room for organizing fuel combustion and corresponding combustion equipment.
The boiler and the furnace are connected through the heat transfer process, and the heating surface is the interface between the boiler and the furnace. Whenever there is an exothermic medium (flame, smoke) on one side, there is a heated medium (water, steam, air) on the other side for heat transfer, and the heat exchange between them depends on the heated surface.
The boiler body consists of drum, heating surface, connecting pipe and flue, combustion equipment and slag tapping equipment, furnace wall and frame (including platform and escalator).
A complete set of equipment consisting of boiler body, water, steam, smoke, wind, fuel pipeline and its auxiliary equipment, measuring instruments and other boiler auxiliary machinery within the scope of boiler is called boiler unit.