Auxiliary equipment of the boiler plant. Boiler installations and auxiliary equipment. Gas equipment for boiler rooms
A modern boiler plant is a complex technical structure and consists of a boiler and auxiliary boiler equipment located in the boiler room or outside it and designed to produce steam with the required parameters or for heating hot water, or both at the same time.
The composition of the boiler includes: a furnace, a water economizer, an air heater, brickwork and a frame with stairs and platforms, as well as fittings and a headset.
To accessories for heating boiler include: draft and power devices, water treatment equipment, fuel supply, as well as instrumentation and automation systems.
The technological process of obtaining steam in a heating boiler is carried out in the following sequence. Fuel in the boiler with the help of burners is introduced into the boiler furnace, where it burns. The air necessary for the combustion of fuel is supplied to the furnace by a blower fan or sucked through the grate - with natural draft.
To improve the process of fuel combustion in the heating boiler and increase the efficiency of the boiler, the air can be preheated by flue gases in the air heater before being fed into the furnace.
The flue gases in the heating boiler, having given up part of their heat to the radiation heating surfaces located in the combustion chamber, enter the convective heating surface, cool down and are removed by a smoke exhauster through chimney in atmosphere.
Raw tap water of the heating boiler passes through cationite filters, softens and then enters the deaerator, where corrosive gases (02 and CO2) are removed from it and flows into the deaerated water tank. Feed water is taken from the tank by a feed pump and fed to the steam boiler.
After passing through the heating surfaces, the water heats up, evaporates and collects in the upper drum. From the boiler, steam is directed to the general boiler steam collector and then supplied to consumers.
According to the purpose, boiler plants are divided into heating, production and heating and energy.
Boiler - heat balance
When fuel is burned in a boiler, not all of the heat released in the furnace is usefully used to heat water or produce steam. Part of the heat is lost with gases leaving the boiler, with chemical and mechanical underburning, etc. The main task in the operation of the boiler is to reduce these losses to a minimum.
The heat balance of the boiler is the equality of the heat introduced into the boiler and the heat used, which is the sum of the useful heat used to generate steam (hot water) and the heat losses that occur during the operation of the boiler plant. The heat balance is compiled for 1 kg of solid (liquid) fuel or 1 m3 of gaseous fuel.
The simplified heat balance of the boiler is written as an equation;
when burned solid fuel, kJ/kgt
Qph = Q1 + Q2 +Q3 +Q4 +Q5 +Q6,
when burning liquid and gaseous fuels, kJ/kg(m3)t
Qph = Q1 + Q2 +Q3 +Q4 +Q5
If both parts of the equations are divided by Qph and multiplied by 100, then we get the balance equations expressed as a percentage:
100 = d1 + d2 + d3 + d4 + d5 + d6,
100 = d1 + d2 + d3 + d4
In the formulas Q1 ;q1 the used heat is useful.
Heat loss:
Q1; d2 - with outgoing flue gases;
Q2; d3 - from chemical incompleteness of combustion;
Q3; d4 - from mechanical incompleteness of combustion;
Q4; d5 - through the outer fences of the brickwork in environment:
Q5; d6 - with the physical heat of the slag.
Coefficient useful action- useful heat used in the boiler:
L \u003d d1 \u003d 100 - d2 - d3 - d4 - d5 - d6;
L \u003d d1 \u003d 100 - d2 - d3 - d4
The efficiency of the boiler depends on the amount of heat loss: the lower the loss, the higher the efficiency. The efficiency value can be in the range L = 0.93 - 0.7 (93-70%),. and the value of heat losses for low-power boilers is: q2 = 12-15%; d3 = 2-7%; d4 = 1-6%; d5 = 0.4-3.5%; d6 = 0.5-1.5%.
THE MAIN EQUIPMENT OF HEAT
POWER PLANTS
Chapter 7
BOILER PLANTS OF THERMAL POWER PLANTS
The boiler plant consists of a boiler and auxiliary equipment. Devices designed to produce steam or hot water of increased pressure due to the heat released during the combustion of fuel, or heat supplied from extraneous sources (usually with hot gases), are called boiler units. They are subdivided respectively into steam boilers and hot water boilers. Boiler units that use (i.e., utilize) the heat of exhaust gases from furnaces or other main and by-products of various technological processes are called waste heat boilers.
The composition of the boiler includes: a furnace, a superheater, an economizer, an air heater, a frame, a lining, thermal insulation, upholstery.
Auxiliary equipment includes: draft blowers, heating surface cleaning devices, fuel preparation and fuel supply equipment, slag and ash removal equipment, ash collecting and other gas cleaning devices, gas and air pipelines, water, steam and fuel pipelines, fittings, headset, automation, instruments and control devices and protection, water treatment equipment and chimney.
Valves include control and shut-off devices, safety and water test valves, pressure gauges, water-indicating devices.
The headset includes manholes, peepers, hatches, gates, dampers.
The building in which the boilers are located is called boiler room.
A complex of devices, including a boiler unit and auxiliary equipment, is called a boiler plant. Depending on the type of fuel burned and other conditions, some of the specified items of auxiliary equipment may not be available.
Boiler plants supplying steam to the turbines of thermal power plants are called power plants. In some cases, special industrial and heating boiler plants are created to supply industrial consumers with steam and heat buildings.
Natural and artificial fuels (coal, liquid and gaseous products of petrochemical processing, natural and blast-furnace gases, etc.), exhaust gases are used as heat sources for boiler plants. industrial ovens and other devices.
The technological scheme of the boiler plant with a drum steam boiler operating on pulverized coal is shown in fig. 7.1. Fuel from the coal storage after crushing is fed by a conveyor to the fuel bunker 3, from which it is sent to the pulverizing system with a coal-pulverizing mill 1 . Pulverized fuel with a special fan 2 is transported through pipes in the air flow to the burners 3 of the furnace of the boiler 5 located in the boiler room 10. Secondary air is also supplied to the burners by a blower fan. 15 (usually through an air heater 17 boiler). Water to feed the boiler is supplied to its drum 7 by a feed pump 16 feed water tank 11, having a deaeration device. Before water is supplied to the drum, it is heated in a water economizer. 9 boiler. Evaporation of water occurs in the pipe system 6. Dry saturated steam from the drum enters the superheater 8 , then sent to the consumer.
Rice. 7.1. Technological scheme of the boiler plant:
1 - coal mill; 2 - mill fan; 3 - fuel bunker; 7 - burner; 5 - contour of the furnace and gas ducts of the boiler unit; 6 - pipe system - furnace screens; 7 - drum; 8 - superheater; 9 - water jonomizer; 10 - contour of the boiler house building (boiler room); 11 - water storage tank with deaeration device; 12 - chimney; 13 - pump; 14- ash collecting device; 15- fan; 16- nutrient cicoc; 17 - air heater; 18 - pump for pumping ash and slag pulp; / - water path; b- superheated steam; in- fuel path; G - the path of air movement; d - path of combustion products; e - path of ash and slag
The fuel-air mixture supplied by the burners to the combustion chamber (furnace) of the steam boiler burns out, forming a high-temperature (1500 ° C) torch that radiates heat to the pipes 6, located on inner surface furnace walls. These are evaporative heating surfaces called screens. Having given part of the heat to the screens, flue gases with a temperature of about 1000 ° C pass through upper part the rear screen, the pipes of which are located here at large intervals (this part is called the festoon), and wash the superheater. Then the combustion products move through the water economizer, air heater and leave the boiler with a temperature slightly higher than 100 °C. The gases leaving the boiler are cleaned of ash in the ash collector 14 and smoke exhauster 13 released into the atmosphere through a chimney 12. Caught from flue gases pulverized ash and slag that has fallen into the lower part of the furnace are removed, as a rule, in a stream of water through channels, and then the resulting pulp is pumped out by special bager pumps 18 and removed through pipelines.
The drum boiler unit consists of a combustion chamber and; gas ducts; drum; heating surfaces under pressure of the working medium (water, steam-water mixture, steam); air heater; connecting pipelines and air ducts. The pressurized heating surfaces include the water economizer, the evaporative elements, formed mainly by the firebox screens and festoon, and the superheater. All heating surfaces of the boiler, including the air heater, are usually tubular. Only a few powerful steam boilers have air heaters of a different design. The evaporating surfaces are connected to the drum and together with the downcomers connecting the drum to the bottom collectors of the screens form a circulation circuit. In the drum, steam and water are separated, in addition, a large supply of water in it increases the reliability of the boiler.
The lower trapezoidal part of the furnace of the boiler unit (see Fig. 7.1) is called a cold funnel - it cools the partially sintered ash residue falling out of the torch, which falls into a special receiving device in the form of slag. Oil-fired boilers do not have a cold funnel. The gas duct, in which the water economizer and the air heater are located, is called convective (convective shaft), in which heat is transferred to water and air mainly by convection. The heating surfaces built into this gas flue and called tail ones allow reducing the temperature of combustion products from 500...700 °C after the superheater to almost 100 °C, i.e. more fully use the heat of the burned fuel.
The entire piping system and the boiler drum are supported by a framework consisting of columns and cross beams. The furnace and gas ducts are protected from external heat losses by lining - a layer of refractory and insulating materials. FROM outer side boiler wall linings are gas-tight sheathed with steel sheet in order to prevent excess air from sucking into the furnace and knocking out dusty hot combustion products containing toxic components.
7.2. Purpose and classification of boiler units
The boiler unit is called energy device productivity D(t/h) to produce steam at a given pressure R(MPa) and temperature t(°C). Often this device is called a steam generator, because steam is generated in it, or simply steam boiler. If the end product is hot water of specified parameters (pressure and temperature) used in industrial technological processes and for heating industrial, public and residential buildings, then the device is called hot water boiler. Thus, all boilers can be divided into two main classes: steam and hot water.
According to the nature of the movement of water, steam-water mixture and steam, steam boilers are divided as follows:
drums with natural circulation(Fig. 7.2,a);
drum with repeated forced circulation(Fig. 7.2, b);
direct-flow (Fig. 7.2, in).
AT drum boilers with natural circulation(Fig. 7.3) due to the difference in densities of the steam-water mixture in the left pipes 2 and liquids in the right pipes 4 there will be a movement of the steam-water mixture in the left row - up, and the water in the right row - down. The pipes of the right row are called lowering, and the left - lifting (screen).
The ratio of the amount of water passing through the circuit to the steam capacity of the circuit D for the same period of time is called circulation ratio K c . For boilers with natural circulation K c ranges from 10 to 60.
Rice. 7.2. Steam generation schemes in steam boilers:
a- natural circulation; b- multiple forced circulation; in- once-through scheme; B - drum; ISP - evaporative surfaces; PE - superheater; EK - water economizer; PN - feed pump; TsN - circulation pump; NK - lower manifold; Q- heat supply; OP - downpipes; POD - lifting pipes; D p - steam consumption; D pv - feed water consumption
The difference in the weights of two columns of liquids (water in the downcomer and steam-water mixture in the riser pipes) creates a driving pressure D R, N / m 2, water circulation in the boiler pipes
where h- contour height, m; r in and r cm - density (volumetric mass) of water and steam-water mixture, kg / m 3.
In boilers with forced circulation, the movement of water and steam-water mixture (see Fig. 7.2, b) is enforced with the help of circulation pump TsN, the driving pressure of which is designed to overcome the resistance of the entire system.
Rice. 7.3. Natural circulation of water in the boiler:
1 - lower manifold; 2 - left pipe; 3 - boiler drum; 4 - right trumpet
In once-through boilers (see Fig. 7.2, in) there is no circulation circuit, there is no multiple circulation of water, there is no drum, water is pumped by the feed pump PN through the economizer EK, the evaporating surfaces of the ISP and the steam exchanger PE connected in series. It should be noted that once-through boilers use water of a higher quality, all water entering the evaporation path is completely converted into steam at the exit from it, i.e. in this case, the circulation ratio K c = 1.
The steam boiler unit (steam generator) is characterized by steam capacity (t/h or kg/s), pressure (MPa or kPa), temperature of produced steam and feed water temperature. These parameters are listed in Table. 7.1.
Table 7.1. Summary table of boiler units manufactured by the domestic industry, indicating the scope
| Pressure, MPa(at) | Boiler steam output, t/h | Steam temperature, °C | Feed water temperature, °C | Application area |
| 0,88 (9) | 0,2; 0,4; 0,7; 1,0 | Saturated | Satisfaction of technological and heating needs of small industrial enterprises | |
| 1,37 (14) | 2,5 | Saturated | Satisfaction of technological and heating needs of larger industrial enterprises | |
| 4; 6,5; 10; 15; 20 | Saturated or superheated, 250 | Quarterly heating boiler houses | ||
| 2,35 (24) | 4; 6,5; 10; 15; 20 | Saturated or superheated, 370 and 425 | Meeting the technological needs of some industrial enterprises | |
| 3,92 (40) | 6,5; 10; 15; 20; 25; 35; 50; 75 | Supply of steam to turbines with a capacity of 0.75 to 12.0 MW at small power plants | ||
| 9,80 (100) | 60; 90; 120; 160; 220 | Supply of steam to turbines from 12 to 50 MW in power plants | ||
| 13,70 (140) | 160; 210; 320; 420; 480 | Supply of steam to turbines with a capacity of 50 to 200 MW at large power plants | ||
| 320; 500; 640 | ||||
| 25,00 (255) | 950; 1600; 2500 | 570/570 (with secondary superheat) | Steam supply for 300, 500 and 800 MW turbines at the largest power plants |
According to steam capacity, boilers of low steam capacity (up to 25 t/h), medium steam capacity (from 35 to 220 t/h) and high steam capacity (from 220 t/h or more) are distinguished.
According to the pressure of the produced steam, boilers are distinguished: low pressure(up to 1.37 MPa), medium pressure (2.35 and 3.92 MPa), high pressure (9.81 and 13.7 MPa) and supercritical pressure (25.1 MPa). The boundary separating low-pressure boilers from medium-pressure boilers is conditional.
Boiler units produce either saturated steam or steam superheated to different temperatures, the value of which depends on its pressure. Currently, in high-pressure boilers, the steam temperature does not exceed 570 °C. The feed water temperature, depending on the steam pressure in the boiler, ranges from 50 to 260 °C.
Hot water boilers are characterized by their heat output (kW or MW, in the MKGSS system - Gcal / h), temperature and pressure of heated water, as well as by the type of metal from which the boiler is made.
7.3. The main types of boiler units
Power boiler units. Boiler units with a steam capacity of 50 to 220 t/h at a pressure of 3.92 ... 13.7 MPa are made only in the form of drum units operating with natural water circulation. Units with a steam capacity of 250 to 640 t/h at a pressure of 13.7 MPa are made both in the form of drum and direct-flow, and boiler units with a steam capacity of 950 t/h or more at a pressure of 25 MPa - only in the form of a direct-flow, since at supercritical pressure natural circulation cannot be carried out.
A typical boiler unit with a steam capacity of 50 ... 220 t / h for a steam pressure of 3.97 ... 13.7 MPa at an overheating temperature of 440 ... 570 ° C (Fig. 7.4) is characterized by the layout of its elements in the form of the letter P, in resulting in two flue gas passes. The first move is a shielded furnace, which determined the name of the type of boiler unit. The screening of the furnace is so significant that all the heat required to convert the water entering the boiler drum into steam is transferred to the screen surfaces in it. Coming out of the combustion chamber 2, flue gases enter a short horizontal connecting gas duct, where the superheater is located 4, separated from the combustion chamber only by a small festoon 3. After that, the flue gases are sent to the second - descending gas duct, in which water economizers 5 and air heaters are located in a cut. 6. Burners 1 can be both swirling, located on the front wall or on the side walls opposite, and angular (as shown in Fig. 7.4). With a U-shaped layout of the boiler unit operating with natural water circulation (Fig. 7.5), the drum 4 the boiler is usually placed relatively high above the firebox; steam separation in these boilers is usually carried out in remote devices - cyclones 5.
Rice. 7.4. Boiler unit with a steam capacity of 220 t/h, a steam pressure of 9.8 MPa and a superheated steam temperature of 540 °C:
1 - burners; 2 - combustion chamber; 3 - festoon; 4 - superheater; 5 - water economizers; 6 - air heaters
When burning anthracite, a semi-open, fully shielded furnace is used. 2 with opposite burners 1 on the front and back walls and a hearth designed for liquid slag removal. Studded screens insulated with refractory mass are placed on the walls of the combustion chamber, and open screens are placed on the walls of the cooling chamber. Often used combined steam superheater 3, consisting of a ceiling radiation part, semi-radiation screens and a convective part. In the descending part of the unit, in a cut, i.e., alternating, a water economizer is placed 6 second stage (in the direction of the water) and a tubular air heater 7 of the second stage (in the direction of the air), followed by a water economizer 8 w air heater 9 first step.
Rice. 7.5. Boiler unit with a steam capacity of 420 t/h, a steam pressure of 13.7 MPa and a superheated steam temperature of 570 °C:
1 - burners; 2 - shielded furnace; 3 ~- superheaters; 4 - drum;
5 - cyclone; 6, 8 - economizers; 7, 9 - air heaters
Boiler units with a steam capacity of 950, 1600 and 2500 t/h for a steam pressure of 25 MPa are designed to operate in a unit with turbines with a capacity of 300, 500 and 800 MW. The layout of the boiler units of the named steam capacity is U-shaped with an air heater placed outside the main part of the unit. Steam superheating double. Its pressure after the primary superheater is 25 MPa, the temperature is 565 °C, after the secondary - 4 MPa and 570 °C, respectively.
All convective heating surfaces are made in the form of packages of horizontal coils. Outside diameter pipes of heating surfaces is 32 mm.
Steam boilers for industrial boiler houses. Industrial boiler houses supplying industrial enterprises with low-pressure steam (up to 1.4 MPa) are equipped with steam boilers manufactured by the domestic industry with a capacity of up to 50 t / h. Boilers are produced for burning solid, liquid and gaseous fuels.
At a number of industrial enterprises, when technologically necessary, medium-pressure boilers are used. The single-drum vertical water-tube boiler BK-35 (Fig. 7.6) with a capacity of 35 t / h at an overpressure in the drum of 4.3 MPa (steam pressure at the outlet of the superheater is 3.8 MPa) and a superheat temperature of 440 ° C consists of two vertical gas ducts - lifting and lower, connected in the upper part by a small horizontal flue. This arrangement of the boiler is called U-shaped.
The boiler has a highly developed screen surface and a relatively small convective beam. Screen pipes 60 x 3 mm are made of steel grade 20. The pipes of the rear screen in the upper part are parted, forming a scallop. The lower ends of screen pipes are expanded in collectors, and the upper ends are expanded into a drum.
The main type of low-capacity steam boilers, widely used in various industries, transport, utilities and agriculture(steam is used for technological and heating and ventilation needs), as well as at low-capacity power plants, are DKVR vertical water-tube boilers. The main characteristics of the DKVR boilers are given in Table. 7.2.
Hot water boilers. It was previously mentioned that at CHPPs with a large heat load, instead of peak network water heaters, hot water boilers high power for centralized heat supply of large industrial enterprises, cities and individual areas.
Rice. 7.6. Steam single-drum boiler BK-35 with oil-gas furnace:
1 - oil-gas burner; 2 - side screen; 3 - front screen; 4 - gas supply; 5 - air duct; 6 - drop pipes; 7 - frame; 8 - cyclone; 9 - boiler drum; 10 - water supply; 11 - superheater collector; 12 - steam outlet; 13 - surface steam cooler; 14 - superheater; 15 - serpentine economizer; 16 - flue gas outlet; 17 - tubular air heater; 18 - back screen; 19 - combustion chamber
Table 7.2. The main characteristics of boilers DKVR, production
Uralkotlomash (liquid and gaseous fuel)
| brand | Steam capacity, t/h | Steam pressure, MPa | Temperature, °С | Efficiency, % (gas/fuel oil) | Dimensions, mm | Weight, kg | ||
| Length | Width | Height | ||||||
| DKVR-2.5-13 | 2,5 | 1,3 | 90,0/883 | |||||
| DKVR-4-13 | 4,0 | 1,3 | 90,0/888 | |||||
| DKVR-6; 5~13 | 6,5 | 1,3 | 91,0/895 | |||||
| DKVR-10-13 | 10,0 | 1,3 | 91,0/895 | |||||
| DKVR-10-13 | 10,0 | 1,3 | 90,0/880 | |||||
| DKVR-Yu-23 | 10,0 | 2,3 | 91,0/890 | |||||
| DKVR-10-23 | 10,0 | 2,3 | 90,0/890 | |||||
| DKVR-10-39 | 10,0 | 3,9 | 89,0 | |||||
| DKVR-10-39 | 10,0 | 3,9 | 89,0 | |||||
| DKVR-20-13 | 20,0 | 1,3 | 92,0/900 | 43 700 | ||||
| DKVR-20-13 | 20,0 | 1,3 | 91,0/890 | |||||
| DKVR-20-23 | 20,0 | 2,3 | 91,0/890 | 44 4001 |
Hot water boilers are designed to produce hot water of specified parameters, mainly for heating. They work in a straight line with constant expense water. The final heating temperature is determined by the conditions for maintaining a stable temperature in living and working premises heated by heating devices, through which the water heated in the boiler circulates. Therefore, with a constant surface heating appliances the temperature of the water supplied to them is increased with a decrease in the ambient temperature. Usually, the water of the heating network in boilers is heated from 70 ... 104 to 150 ... 170 ° C. Recently, there has been a tendency to increase the temperature of water heating up to 180 ... 200 °C.
In order to avoid condensation of water vapor from flue gases and the resulting external corrosion of heating surfaces, the water temperature at the inlet to the unit must be above the dew point for the combustion products. In this case, the temperature of the pipe walls at the point of water inlet will also not be lower than the dew point. Therefore, the inlet water temperature should not be lower than 60 °C when operating on natural gas, 70 °C when operating on low sulfur fuel oil and 110 °C when using high sulfur fuel oil. Since water can be cooled in the heating network to a temperature below 60 ° C, a certain amount of (direct) water already heated in the boiler is mixed with it before entering the unit.
Rice. 7.7. Gas-oil hot water boiler type PTVM-50-1
The gas-oil hot water boiler of the PTVM-50-1 type (Fig. 7.7) with a heat output of 50 Gcal / h has proven itself well in operation.
7.4. The main elements of the boiler unit
The main elements of the boiler are: evaporative heating surfaces (wall tubes and boiler bundle), superheater with steam superheat controller, water economizer, air heater and draft devices.
Evaporating surfaces of the boiler. Steam-generating (evaporative) heating surfaces differ from each other in boilers of various systems, but, as a rule, they are located mainly in the combustion chamber and perceive heat by radiation - radiation. These are screen pipes, as well as a convective (boiler) bundle installed at the outlet of the furnace of small boilers (Fig. 7.8, a).
Rice. 7.8. Evaporator layouts (a) and superheaters (b) surfaces of the drum boiler unit:
/ - the contour of the lining of the furnace; 2, 3, 4 - side screen panels; 5 - front screen; 6, 10, 12 - collectors of screens and convective beam; 7 - drum; 8 - festoon; 9 - boiler bundle; 11 - back screen; 13 - wall-mounted radiation superheater; 14 - screen semi-radiation superheater; 15 ~~ ceiling radiant superheater; 16 ~ overheating regulator; 17 - removal of superheated steam; 18 - convective superheater
The screens of boilers with natural circulation, operating under vacuum in the furnace, are made of smooth pipes (smooth-tube screens) with an inner diameter of 40 ... 60 mm. The screens are a series of vertical lifting pipes connected in parallel with each other by collectors (see Fig. 7.8, a). The gap between pipes is usually 4...6 mm. Some screen pipes are inserted directly into the drum and do not have upper headers. Each panel of screens, together with the downcomers placed outside the lining of the furnace, forms an independent circulation circuit.
The pipes of the rear screen at the exit point of the combustion products from the furnace are bred in 2-3 rows. This discharge of pipes is called festooning. It allows you to increase the cross section for the passage of gases, reduce their speed and prevents clogging of the gaps between the pipes, hardened during cooling by molten ash particles carried out by gases from the furnace.
In high-power steam generators, in addition to wall-mounted ones, additional screens are installed that divide the furnace into separate compartments. These screens are illuminated by torches from two sides and are called double-light. They perceive twice as much warmth as wall-mounted ones. Two-light screens, increasing the overall heat absorption in the furnace, allow reducing its size.
Superheaters. The superheater is designed to increase the temperature of the steam coming from the evaporative system of the boiler. It is one of the most critical elements of the boiler unit. With an increase in steam parameters, the heat absorption of superheaters increases to 60% of the total heat absorption of the boiler unit. The desire to obtain a high superheat of the steam makes it necessary to place a part of the superheater in the zone of high temperatures of the combustion products, which naturally reduces the strength of the pipe metal. Depending on the determining method of transferring heat from gases, superheaters or their individual stages (Fig. 7.8, b) are divided into convective, radiative and semi-radiative.
Radiation superheaters are usually made of pipes with a diameter of 22 ... 54 mm. At high steam parameters, they are placed in the combustion chamber, and they receive most of the heat by radiation from the torch.
Convective superheaters are located in a horizontal flue or at the beginning of a convective shaft in the form of dense packages formed by coils with a step along the width of the flue equal to 2.5...3 pipe diameters.
Convective superheaters, depending on the direction of steam movement in the coils and the flow of flue gases, can be counter-current, direct-flow and with a mixed flow direction.
The temperature of the superheated steam must always be kept constant, regardless of the operating mode and load of the boiler unit, since when it decreases, the steam humidity in the last stages of the turbine increases, and when the temperature rises above the calculated one, there is a risk of excessive thermal deformations and a decrease in the strength of individual elements of the turbine. Steam temperature is maintained at a constant level with the help of control devices - desuperheaters. The most widely used desuperheaters are injection type, in which the regulation is carried out by injecting demineralized water (condensate) into the steam stream. During evaporation, water takes away part of the heat from the steam and reduces its temperature (Fig. 7.9, a).
Typically, the injection desuperheater is installed between separate parts superheater. Water is injected through a series of holes around the circumference of the nozzle and sprayed inside a jacket, consisting of a diffuser and a cylindrical part that protects the body, which has a higher temperature, from splashing water from it in order to avoid cracking in the metal of the body due to a sharp change in temperature.
Rice. 7.9. Desuperheaters: a - injecting; b - surface with steam cooling by feed water; 1 – hatch for measuring instruments; 2 – cylindrical part of the shirt; 3 - desuperheater body; 4 - diffuser; 5 - holes for spraying water in steam; 6 - desuperheater head; 7- tube board; 8 - collector; 9 - a shirt that prevents steam from washing the tube plate; 10, 14 - pipes supplying and discharging steam from the desuperheater; 11 - remote partitions; 12 - water coil; 13 - a longitudinal partition that improves the steam washing of the coils; 15, 16 - pipes supplying and discharging feed water
In boilers of medium steam output, surface desuperheaters are used (Fig. 7.9, b), which are usually placed at the entrance of steam to the superheater or between its individual parts.
Steam is supplied to the collector and discharged through coils. Inside the collector are coils through which feed water flows. The steam temperature is controlled by the amount of water entering the desuperheater.
Water economizers. These devices are designed to heat the feed water before it enters the evaporative part of the boiler by using the heat of the exhaust gases. They are located in a convective flue and operate at relatively low temperatures of combustion products (flue gases).
Rice. 7.10. Steel coil economizer:
1 - lower manifold; 2 - upper collector; 3 - support stand; 4 - coils; 5 -- support beams (cooled); 6 - descent of water
Most often, economizers (Fig. 7.10) are made from steel pipes with a diameter of 28 ... 38 mm, bent into horizontal coils and arranged in packages. Pipes in packages are staggered quite tightly: the distance between the axes of adjacent pipes across the flue gas flow is 2.0 ... 2.5 pipe diameters, along the flow - 1.0 ... 1.5. The fastening of the pipes of the coils and their spacing are carried out by support posts, fixed in most cases on hollow (for air cooling), insulated from the side of hot gases frame beams.
Depending on the degree of water heating, economizers are divided into non-boiling and boiling. In a boiling economizer, up to 20% of the water can be converted into steam.
The total number of pipes operating in parallel is selected based on a water velocity of at least 0.5 m/s for non-boiling and 1 m/s for boiling economizers. These speeds are due to the need to flush air bubbles from the pipe walls, which contribute to corrosion and prevent the separation of the steam-water mixture, which can lead to overheating of the upper wall of the pipe, which is poorly cooled by steam, and its rupture. The movement of water in the economizer is necessarily upward. The number of pipes in the package in the horizontal plane is chosen based on the velocity of the combustion products 6 ... 9 m / s. This speed is determined by the desire, on the one hand, to protect the coils from drifting with ash, and on the other hand, to prevent excessive ash wear. The heat transfer coefficients under these conditions are usually 50 ... 80 W / (m 2 - K). For the convenience of repairing and cleaning pipes from external contaminants, the economizer is divided into packages 1.0 ... 1.5 m high with gaps between them up to 800 mm.
External contaminants are removed from the surface of the coils by periodically switching on the shot cleaning system, when the metal shot is passed (falls) from top to bottom through the convective heating surfaces, knocking down deposits adhering to the pipes. Ash sticking can be the result of dew from the flue gases on the relatively cold surface of the pipes. This is one of the reasons for the preheating of feed water supplied to the economizer to a temperature above the dew point of water vapor or sulfuric acid vapor in flue gases.
The upper rows of economizer pipes during solid fuel boiler operation, even at relatively low gas velocities, are subject to noticeable ash wear. To prevent ash wear, these pipes are attached various kinds protective pads.
Air heaters. They are installed to preheat the air sent to the furnace in order to increase the efficiency of fuel combustion, as well as to the coal-grinding devices.
The optimal amount of air heating in the air heater depends on the floor of the fuel being burned, its humidity, type of combustion device and is 200 °C for hard coal, burned on a chain grate (to avoid overheating of the grate), 250 ° C for peat burned on the same grates, 350 ... 450 ° C for liquid or pulverized fuel burned in chamber furnaces.
For getting high temperature air heating, two-stage heating is used. To do this, the air heater is divided into two parts, between which (“in a cut”) a part of the water economizer is installed.
The temperature of the air entering the air heater must be 10 ... 15 °C above the dew point of the flue gases to avoid corrosion of the cold end of the air heater as a result of condensation of water vapor contained in the flue gases (when they come into contact with the relatively cold walls of the air heater), and also clogging the passage channels for gases with ash adhering to wet walls. These conditions can be met in two ways: either by increasing the temperature of the exhaust gases and losing heat, which is economically unprofitable, or by installing special devices for heating the air before it enters the air heater. For this, special heaters are used, in which the air is heated by selective steam from turbines. In some cases, air heating is carried out by recirculation, i.e. part of the air heated in the air heater returns through the suction pipe to the blower fan and mixes with cold air.
According to the principle of operation, air heaters are divided into recuperative and regenerative. In recuperative air heaters, heat from gases to air is transferred through a fixed metal pipe wall separating them. As a rule, these are steel tubular air heaters (Fig. 7.11) with a tube diameter of 25 ... 40 mm. The tubes in it are usually located vertically, combustion products move inside them; the air washes them with a transverse flow in several passages, organized by bypass air ducts (ducts) and intermediate partitions.
The gas in the tubes moves at a speed of 8 ... 15 m / s, the air between the tubes is twice as slow. This makes it possible to have approximately equal heat transfer coefficients on both sides of the pipe wall.
Thermal expansion of the air heater is perceived by the lens compensator 6 (see Fig. 7.11), which is installed above the air heater. With the help of flanges, it is bolted from below to the air heater, and from above - to the transition frame of the previous flue of the boiler unit.
Rice. 7.11. Tubular air heater:
1 - Column; 2 - support frame; 3, 7 - air ducts; 4 – steel
pipes 40´1.5 mm; 5, 9 – upper and lower tube plates 20...25 mm thick;
6 - thermal expansion compensator; 8 – intermediate tube plate
In a regenerative air heater, heat is transferred by a metal nozzle, which is periodically heated by combustion gases, after which it is transferred to the air flow and gives it the accumulated heat. The regenerative air heater of the boiler is a slowly rotating (3 ... 5 rpm) drum (rotor) with a packing (nozzle) made of corrugated thin steel sheets, enclosed in a fixed housing. The body is divided by sector plates into two parts - air and gas. When the rotor rotates, the packing alternately crosses either the gas or the air flow. Despite the fact that the packing works in a non-stationary mode, the heating of the continuous air flow is carried out continuously without temperature fluctuations. The movement of gases and air is countercurrent.
The regenerative air heater is compact (up to 250 m2 of surface per 1 m3 of packing). It is widely used in powerful power boilers. Its disadvantage is large (up to 10%) air flows into the gas path, which leads to overloads of blowers and smoke exhausters and an increase in losses with exhaust gases.
Draft-blowing devices of the boiler unit. In order for fuel to burn in the furnace of the boiler unit, air must be supplied to it. To remove gaseous products of combustion from the furnace and ensure their passage through the entire system of heating surfaces of the boiler unit, draft must be created.
Currently, there are four schemes for supplying air and removing combustion products in boiler plants:
with natural draft created by the chimney, and natural suction of air into the furnace as a result of rarefaction in it, created by the draft of the pipe;
·artificial draft created by the exhauster, and suction of air into the furnace, as a result of the rarefaction created by the exhauster;
·artificial draft created by a smoke exhauster and forced air supply to the furnace by a blower fan;
supercharging, in which the entire boiler plant is sealed and placed under some excess pressure created by the blower fan, which is enough to overcome all the resistances of the air and gas paths, which eliminates the need to install a smoke exhauster.
The chimney is preserved in all cases of artificial draft or pressurized operation, but the main purpose of the chimney is the removal of flue gases into higher layers of the atmosphere in order to improve the conditions for their dispersion in space.
In boiler plants with high steam capacity, artificial draft with artificial blast is widely used.
Chimneys are brick, reinforced concrete and iron. Pipes up to 80 m high are usually constructed from bricks. Higher pipes are made of reinforced concrete. Iron pipes are installed only on vertically cylindrical boilers, as well as on powerful steel tower-type hot water boilers. To reduce costs, one common chimney is usually built for the entire boiler house or for a group of boiler plants.
The principle of operation of the chimney remains the same in installations operating with natural and artificial draft, with the peculiarity that with natural draft the chimney must overcome the resistance of the entire boiler installation, and with artificial it creates additional draft to the main one created by the smoke exhauster.
On fig. 7.12 shows a diagram of a boiler with natural draft created by a chimney 2 . It is filled with flue gases (combustion products) with a density of r g, kg / m 3, and is communicated through the boiler flues 1 With atmospheric air, the density of which is r in, kg / m 3. It is obvious that r in > r r.
With chimney height H air column pressure difference gH r in and gases gH r g at the level of the base of the pipe, i.e. the value of the thrust D S, N/m 2 has the form
where p and Rg are the densities of air and gas under normal conditions, kg/m; AT- barometric pressure, mm Hg Art. Substituting the values of r into 0 and r g 0 , we get
From equation (7.2) it follows that natural draft is the greater the greater the height of the pipe and the flue gas temperature and the lower the ambient air temperature.
The minimum allowable pipe height is regulated for sanitary reasons. The diameter of the pipe is determined by the rate of flue gases flowing out of it at the maximum steam output of all boiler units connected to the pipe. With natural draft, this speed should be within 6 ... 10 m / s, not becoming less than 4 m / s in order to avoid disturbing the draft by the wind (pipe blowing). With artificial draft, the speed of flue gases outflow from the pipe is usually assumed to be 20 ... 25 m / s.
Rice. 7.12. Scheme of a boiler with natural draft created by a chimney:
1 - boiler; 2 - chimney
Centrifugal smoke exhausters and draft fans are installed for boiler units, and for steam generators with a capacity of 950 t / h and more - axial multi-stage smoke exhausters.
Smoke exhausters are placed behind the boiler unit, and in boiler plants intended for burning solid fuels, smoke exhausters are installed after ash removal in order to reduce the amount of fly ash passing through the exhaust fan, and thereby reduce ash abrasion of the exhaust fan impeller. n
The vacuum that must be created by the smoke exhauster is determined by the total aerodynamic resistance of the gas path of the boiler plant, which must be overcome provided that the flue gas rarefaction at the top of the furnace is 20 ... 30 Pa and the necessary velocity pressure is created at the flue gas outlet from the flue pipes. In small boiler installations, the vacuum created by the smoke exhauster is usually 1000 ... 2000 Pa, and in large installations 2500 ... 3000 Pa.
Blow fans installed in front of the air heater are designed to supply unheated air into it. The pressure created by the fan is determined by the aerodynamic resistance of the air path, which must be overcome. Usually it consists of the resistances of the suction duct, the air heater, the air ducts between the air heater and the furnace, as well as the resistance of the grate and the layer of fuel or burners. In sum, these resistances are 1000 ... 1500 Pa for low-capacity boiler plants and increase to 2000 ... 2500 Pa for large boiler plants.
7.5. Thermal balance of the boiler unit
Thermal balance of the steam boiler. This balance consists in establishing equality between the amount of heat supplied to the unit during fuel combustion, called available heat Q p p , and the amount of heat used Q 1 and heat losses. Based on the heat balance, efficiency and fuel consumption are found.
In the steady state operation of the unit, the heat balance for 1 kg or 1 m 3 of fuel burned is as follows:
where Q p p - available heat per 1 kg of solid or liquid fuel or 1 m 3 of gaseous fuel, kJ / kg or kJ / m 3; Q 1 - used heat; Q 2 - heat loss with gases leaving the unit; Q 3 - heat loss from chemical incompleteness of fuel combustion (underburning); Q 4 - heat loss from mechanical incompleteness of combustion; Q 5 - heat loss to the environment through the external enclosure of the boiler; Q 6 - heat loss with slag (Fig. 7.13).
Usually, the calculations use the heat balance equation, expressed as a percentage in relation to the available heat, taken as 100% ( Q p p = 100):
where q 1 = Q 1 × 100/Q p p; q2= Q 2 × 100/Q p p etc.
Available heat includes all types of heat introduced into the furnace together with fuel:
where Q nr – lower working calorific value of fuel combustion; Q ft is the physical heat of the fuel, including that obtained during drying and heating; Q v.vn - the heat of the air received by it when heated outside the boiler; Q f is the heat introduced into the furnace with atomizing nozzle steam.
The heat balance of the boiler unit is made relative to a certain temperature level or, in other words, relative to a certain starting temperature. If we take as this temperature the temperature of the air entering the boiler unit without heating outside the boiler, we do not take into account the heat of the steam blast in the nozzles and exclude the value Q ft, since it is negligible compared to the calorific value of the fuel, we can take
Expression (7.5) does not take into account the heat introduced into the furnace by the hot air of its own boiler. The fact is that the same amount of heat is given off by the products of combustion to the air in the air heater within the boiler unit, that is, a kind of recirculation (return) of heat is carried out.
Rice. 7.13. The main heat losses of the boiler unit
Heat used Q 1 is perceived by the heating surfaces in the combustion chamber of the boiler and its convective gas ducts, is transferred to the working fluid and is spent on heating water to the phase transition temperature, evaporation and overheating of steam. The amount of heat used per 1 kg or 1 m 3 of burned fuel,
where D 1 , D n, D pr, - respectively, the performance of the steam boiler (superheated steam consumption), saturated steam consumption, boiler water consumption for blowing, kg / s; AT- fuel consumption, kg / s or m 3 / s; i pp, i", i", i pv - respectively, the enthalpies of superheated steam, saturated steam, water on the saturation line, feed water, kJ / kg. With a purge rate 
and the absence of saturated steam flow, formula (7.6) takes the form
For boiler units that are used to produce hot water (hot water boilers),
where G c - hot water consumption, kg / s; i 1 and i 2 - respectively, the specific enthalpies of water entering the boiler and leaving it, kJ / kg.
Heat loss steam boiler. The efficiency of fuel use is determined mainly by the completeness of fuel combustion and the depth of cooling of combustion products in the steam boiler.
Heat loss with flue gases Q 2 are the largest and are determined by the formula
where I ux - enthalpy of flue gases at flue gas temperature q ux and excess air in flue gases α ux, kJ/kg or kJ/m 3 ; I hv - enthalpy of cold air at the temperature of cold air t xv and excess air α xv; (100- q 4) is the share of burned fuel.
For modern boilers, the value q 2 is within 5...8% of available heat, q 2 increases with an increase in q ux, α ux and the volume of exhaust gases. A decrease in q ux by about 14 ... 15 ° C leads to a decrease q 2 to 1%.
In modern power boiler units, q uh is 100 ... 120 °С, in industrial heating units - 140 ... 180 °С.
Heat loss from chemical incomplete combustion of fuel Q 3 is the heat that remained chemically bound in the products of incomplete combustion. It is determined by the formula
where CO, H 2 , CH 4 - volumetric content of products of incomplete combustion in relation to dry gases,%; the numbers before CO, H 2 , CH 4 - reduced by a factor of 100 the calorific value of 1 m 3 of the corresponding gas, kJ / m 3.
Heat losses from chemical incomplete combustion usually depend on the quality of mixture formation and local insufficient amounts of oxygen for complete combustion. Consequently, q 3 depends on α t. Smallest valuesα t , under which q 3 are practically absent, depending on the type of fuel and the organization of the combustion regime.
Chemical incompleteness of combustion is always accompanied by soot formation, which is unacceptable in the operation of the boiler.
Heat loss from mechanical incomplete combustion of fuel Q 4 - this is the heat of the fuel, which, during chamber combustion, is carried away together with the products of combustion (entrainment) into the gas ducts of the boiler or remains in the slag, and during layer combustion, in the products that fall through the grate (dip):
where a shl+pr, a un - respectively, the proportion of ash in the slag, dip and entrainment, is determined by weighing from the ash balance a sl+pr +a un = 1 in fractions of a unit; G shl+pr, G un - the content of combustibles, respectively, in the slag, dip and entrainment, is determined by weighing and afterburning in laboratory conditions samples of slag, dip, entrainment,%; 32.7 kJ/kg - calorific value of combustibles in slag, dip and entrainment, according to VTI data; A r - ash content of the working mass of fuel, %. Value q 4 depends on the combustion method and slag removal method, as well as the properties of the fuel. With a well-established process of burning solid fuel in chamber furnaces q 4 » 0.3 ... 0.6 for fuels with a high volatile content, for anthracite fines (ASh) q 4 > 2%. In stratified combustion for bituminous coals q 4 = 3.5 (of which 1% is due to losses with slag, and 2.5% - with entrainment), for brown - q 4 = 4%.
Heat loss to the environment Q 5 depend on area outer surface unit and temperature difference between surface and ambient air (q 5» 0.5... 1.5 %).
Heat loss with slag Q 6 occur as a result of the removal of slag from the furnace, the temperature of which can be quite high. In pulverized coal furnaces with solid slag removal, the slag temperature is 600...700°C, and with liquid slag - 1500...1600°C.
These losses are calculated by the formula
where With shl is the heat capacity of the slag, depending on the temperature of the slag t line So, at 600°C With wl = 0.930 kJ/(kg×K), and at 1600°С With wl = 1.172 kJ/(kg×K).
Boiler efficiency and fuel consumption. The perfection of the thermal operation of a steam boiler is estimated by the gross efficiency coefficient h to br,%. Yes, in direct balance.
where Q to - heat usefully given to the boiler and expressed through the heat absorption of heating surfaces, kJ / s:
where Q st - heat content of water or air heated in the boiler and given to the side, kJ / s (the heat of blowing is taken into account only for D pr > 2% of D).
The efficiency of the boiler can also be calculated from the inverse balance:
The direct balance method is less accurate, mainly because of the difficulties in determining large masses of consumed fuel in operation. Heat losses are determined with greater accuracy, so the inverse balance method has found a predominant distribution in determining the efficiency.
In addition to the gross efficiency, the net efficiency is used, showing the operational excellence of the unit:
where q s.n - total heat consumption for auxiliary needs of the boiler, i.e. consumption electrical energy for the drive of auxiliary mechanisms (fans, pumps, etc.), steam consumption for blowing and fuel oil spraying, calculated as a percentage of the available heat.
From expression (7.13), the consumption of fuel supplied to the furnace is determined B kg/s,
Since part of the fuel is lost due to mechanical underburning, in all calculations of the volumes of air and combustion products, as well as enthalpies, estimated flow fuel B R , kg/s, taking into account the mechanical incompleteness of combustion:
When burning liquid and gaseous fuels in boilers Q 4 = 0
1. How are boiler units classified and what is their purpose?
2. Name the main types of boiler units and list their main elements.
3. Describe the evaporative surfaces of the boiler, list the types of superheaters and methods for controlling the temperature of superheated steam.
4. What types of water economizers and air heaters are used in boilers? Tell us about the principles of their device.
5. How are air supplied and flue gases removed in boiler units?
6. Tell us about the purpose of the chimney and the determination of its draft; indicate the types of smoke exhausters used in boiler installations.
7. What is the heat balance of the boiler unit? List the heat losses in the boiler and indicate their causes.
8. How is the efficiency of the boiler unit determined?
Auxiliary equipment of the boiler house includes various heaters, pumps, storage tanks (with open system heat supply), reduction and reduction-cooling units.
Basically, surface-type heat exchangers are used in boiler rooms. Depending on the location of the pipe system, heat exchangers are divided into vertical and horizontal.
vertical heat exchangers are used in large steam boilers for heating network water.
Horizontal heat exchangers are used to heat raw and chemically treated water.
Steam or hot water is used as a heat carrier in these heat exchangers.
Applied schemes for switching on deaerators are shown in Figure 4.4.
Vacuum deaerators are often installed in boiler rooms with hot water boilers. However, they require careful supervision during operation, therefore, in a number of boiler houses, atmospheric deaerators are preferred.
In figure 4.4, a shows a deaerator operating at an absolute pressure of 0.03 MPa. The vacuum in it is created by a water jet ejector. Make-up water after chemical water treatment is heated in a water-to-water hot water heater from a direct line with a temperature of 130 - 150 ° C. The temperature of the water after the deaerator is 70 ° C.
In figure 4.4, b the deaeration scheme is shown at a pressure of 0.12 MPa, i.e. above atmospheric. At this pressure, the temperature of the water in the deaerator is 104 ° C. Before being fed into the deaerator, chemically purified water is preheated in a water-to-water heat exchanger.
Figure 4.4 - Schemes for switching on deaerators:
a - vacuum; b - atmospheric; c - atmospheric with a deaerated water cooler.
When preparing water for the needs of hot water supply in boiler houses operating on closed system heat supply, various schemes for connecting local heat exchangers to the heat supply system are used. Currently, there are three schemes for connecting local heat exchangers, shown in Figure 4.5.
In figure 4.4, in The scheme of make-up water deaeration is shown, in which, after the deaeration column, water enters the deaerated water cooler, heating the chemically treated water. Then the chemically purified water is sent to a heat exchanger installed in front of the deaerator. The water temperature after the deaerated water cooler is about 70 o C.
The choice of the scheme for connecting local heat exchangers for hot water supply is made depending on the ratio of the maximum heat consumption for hot water supply Q G.V to maximum flow heat for heating Q O.
Boiler units are devices designed to produce steam or hot water of increased pressure due to the heat released during the combustion of fuel, or heat supplied from extraneous sources (usually with hot gases). They are divided into steam and hot water boilers. Boiler units that use (i.e., utilize) the heat of exhaust gases from furnaces or other main and by-products of various technological processes are called waste heat boilers.
The composition of the boiler includes: a furnace, a superheater, an economizer, an air heater, a frame, a lining, thermal insulation, and a lining.
Auxiliary equipment is considered to be: draft blowers, devices for cleaning heating surfaces, equipment for fuel preparation and fuel supply, slag and ash removal, ash collecting and other gas cleaning devices, gas and air pipelines, water, steam and fuel pipelines, fittings, headset, automation, control and protection devices and devices, water treatment equipment and chimney.
Valves include control and shut-off devices, safety and water test valves, pressure gauges, water-indicating devices.
The headset includes manholes, peepers, hatches, gates, dampers.
The building in which the boilers are located is called the boiler room.
A complex of devices, including a boiler unit and auxiliary equipment, is called a boiler plant. Depending on the type of fuel burned and other conditions, some of the specified items of auxiliary equipment may not be available.
Boiler plants supplying steam to the turbines of thermal power plants are called power plants. For the supply of steam to industrial consumers and heating of buildings, special production and heating boiler plants are created. As sources of heat for boiler plants, natural and artificial fuels, exhaust gases from industrial furnaces and other devices, etc. are most often used.
The technological scheme of the boiler plant with a drum steam boiler operating on pulverized coal is shown in Fig. 7.1.
The fuel from the coal storage after crushing is fed by a conveyor to the raw coal bunker 7, from which it is sent to the pulverized preparation system, which has a coal-pulverizing mill 2. The pulverized fuel is transported through pipes in the air flow to the burners 4 of the furnace of the boiler 5 located in the boiler room 14 using a special fan. Secondary air is also supplied to the burners by a blow fan 13 (usually through the air heater of the boiler 10). Water for feeding the boiler is supplied to its drum 7 by a feed pump 12 from the feed water tank 11, which has a deaeration device. Before water is supplied to the drum, it is heated in the water economizer 9 of the boiler. Evaporation of water occurs in the pipe system 6. Dry saturated steam from the drum enters the superheater 8, then goes to the consumer.
Fig.1. Technological scheme of the boiler plant
The fuel-air mixture supplied by the burners to the combustion chamber (furnace) of the steam boiler burns out, forming a high-temperature (1500 ° C) torch that radiates heat to pipes 6 located on the inner surface of the furnace walls. These are evaporative heating surfaces called screens. Having given some of the heat to the screens, flue gases with a temperature of about 1000 ° C pass through the upper part of the rear screen, the pipes of which are located here at large intervals (this part is called a festoon), and wash the superheater. Then the combustion products move through the water economizer, air heater and leave the boiler with a temperature slightly higher than 100 °C. The gases leaving the boiler are cleaned of ash in the ash collector 15 and are emitted into the atmosphere through the chimney 17 by a smoke exhauster 16. The pulverized ash captured from the flue gases and the slag that has fallen into the lower part of the furnace are removed, as a rule, in the water flow through the channels, and then the resulting pulp is pumped out by special bager pumps 18 and removed through pipelines.
The drum boiler unit consists of a combustion chamber and gas ducts, a drum, heating surfaces under the pressure of the working medium (water, steam-water mixture, steam), an air heater, connecting pipelines and air ducts (Fig. 7.1). The pressurized heating surfaces include the water economizer, the evaporative elements, formed mainly by the firebox screens and festoon, and the superheater. All heating surfaces of the boiler, including the air heater, are usually tubular. Only some powerful steam boilers have air heaters of a different design. The evaporating surfaces are connected to the drum and together with the downcomers connecting the drum to the bottom collectors of the screens form a circulation circuit. In the drum, steam and water are separated, in addition, a large supply of water in it increases the reliability of the boiler.
The lower trapezoidal part of the furnace of the boiler unit (Fig. 7.1) is called a cold funnel - it cools the partially sintered ash residue falling out of the torch, which falls into a special receiving device in the form of slag. Oil-fired boilers do not have a cold funnel. The gas duct, in which the water economizer and the air heater are located, is called convective (convective shaft), in which heat is transferred to water and air mainly by convection. The heating surfaces built into this gas duct and called tail ones allow reducing the temperature of combustion products from 500-700°C after the superheater to almost 100°C, i.e. more fully use the heat of the burned fuel.
The entire piping system and the boiler drum are supported by a frame consisting of columns and cross beams. The furnace and gas ducts are protected from external heat losses by lining - a layer of refractory and insulating materials. On the outer side of the lining, the boiler walls are gas-tight sheathed with steel sheet in order to prevent excess air from sucking into the furnace and knocking out dusty hot combustion products containing toxic components.
separation devices. Wet saturated steam produced in the drum of low and medium pressure boilers can carry away drops of boiler water containing salts dissolved in it. In high and ultra-high pressure boilers, steam pollution is also caused by additional entrainment of silicic acid salts and sodium compounds, which dissolve in steam.
Impurities carried away with steam are deposited in the superheater, which is extremely undesirable, as it can lead to burnout of the superheater tubes. Therefore, the steam before leaving the boiler drum is subjected to separation, during which drops of boiler water are separated and remain in the drum. Steam separation is carried out in special separation devices, in which conditions are created for natural or mechanical separation of water and steam.
Natural separation occurs due to the large difference in the densities of water and steam. The mechanical inertial separation principle is based on the difference in the inertial properties of water droplets and steam with a sharp increase in speed and a simultaneous change in direction or swirl of the wet steam flow.
Figure 14.4 shows circuit diagrams separating devices.
Thrust devices. For the normal operation of the boiler unit, a continuous supply of air for the combustion of fuel and the continuous removal of combustion products are necessary.
In modern boiler plants, a scheme with a vacuum through the gas ducts is widespread. The disadvantages of this scheme include the presence of air suction into the gas waste through leaks in the fences and the operation of smoke exhausters on dusty gases. The advantage of such a scheme is the absence of knocking out and leakage of flue gases into the boiler room, since the air is pumped into the furnace by a fan, and the flue gases are removed by a smoke exhauster. Recently, a pressurized scheme has been widely used in powerful power boiler plants. The furnace and the entire gas path are under pressure of 3-5 kPa. Pressure builds up powerful fans; smoke exhauster is missing. The main disadvantage of this scheme is the difficulties associated with ensuring proper tightness of the furnace and gas ducts of the boiler unit.
To obtain thrust, it is necessary to increase the height of the pipe or the temperature of the flue gases. However, when using any of these methods, it must be borne in mind that the height of the pipe is limited by its cost and strength, and the temperature of the gases - optimal value boiler plant efficiency. Therefore, most modern boiler plants are equipped with artificial traction, to create which a smoke exhauster is used to overcome the resistance of the gas path. In this case, the height of the pipe is chosen in accordance with sanitary requirements.
The air pressure generated by the fan should also be determined based on the aerodynamic calculation of the air path (air ducts, air heater, burner, etc.). boiler unit.
Fundamentals of water treatment. One of the main tasks safe operation boiler plants is the organization of a rational water regime, in which scale does not form on the walls of the evaporative heating surfaces, there is no corrosion and high quality generated steam. The steam generated in the boiler plant is returned from the consumer in a condensed state; in this case, the amount of condensate returned is usually less than the amount of steam generated.
Losses of condensate and water during blowdown are replenished by adding water from any source. This water must be suitably treated before entering the boiler unit. Water that has passed preliminary training, is called additional, mixture of returned condensate and make-up water – nutritional and the water that circulates in the boiler circuit, boiler room.
The normal operation of boiler units depends on the quality of the feed water. The physical and chemical properties of water are characterized by the following indicators: transparency, suspended solids content, dry residue, salinity, oxidizability, hardness, alkalinity, concentration of dissolved gases (CO 2 and O 2).
Transparency is characterized by the presence of suspended mechanical and colloidal impurities, and the content of suspended solids determines the degree of water pollution by solid insoluble impurities.
Fuel supply. For normal and uninterrupted operation of boiler plants, it is required that fuel is supplied to them continuously. The fuel supply process consists of two main stages: 1) fuel supply from the place of its extraction to warehouses located near the boiler house; 2) fuel supply from warehouses directly to boiler rooms.
Flue gas cleaning and ash and slag removal. When solid fuels are burned, a lot of ash is produced. During the layered combustion process, the main part of the mineral fuel impurities (60-70%) turns into slag and falls through the grates into the ash pan. In pulverized coal furnaces, most of the ash (75-85%) is carried away from the boilers with flue gases.
Currently, the following types of ash collectors are used in boiler houses: 1) inertial mechanical; 2) wet; 3) electrostatic precipitators; 4) combined.
Inertial (mechanical) ash collectors operate on the principle of separating ash particles from a gas stream under the influence of inertia forces.
Currently, wet type ash collectors are widely used. Figure 14.5 shows a diagram of a wet ash collector (scrubber) with a lower tangential supply of dusty gas.
The principle of operation of electrostatic precipitators is that dusty gases pass through an electric field formed between a steel cylinder (positive pole) and a wire passing along the axis of the cylinder (negative pole). The main mass of ash particles receives a negative charge and is attracted to the walls of the cylinder, an insignificant part of the ash particles receives a positive charge and is attracted to the wire. With periodic shaking of the electrostatic precipitator, the electrodes are freed from ash. Electrostatic precipitators are used in boiler rooms with a flue gas flow rate of more than 70,000 m 3 / h, classified as normal conditions.
Combined ash collectors are two-stage, with the operation of each stage based on different principles. Most often, a combined ash collector consists of a battery cyclone (first stage) and an electrostatic precipitator (second stage).
The ash and slag removal process can be divided into two main operations: cleaning of slag and ash bins and transportation of ash and slag to ash dumps or slag concrete products.