MINISTRY OF ENERGY AND ELECTRIFICATION OF THE USSR TECHNICAL DEPARTMENT FOR THE OPERATION OF POWER SYSTEMS
ALL-UNION STATE TRUST FOR THE ORGANIZATION AND
RATIONALIZATION OF DISTRICT POWER STATIONS AND NETWORKS
(ORGRES)
METHODOLOGICAL INSTRUCTIONS ON THERMAL
BILLING AND THERMAL TESTING
BOILER INSULATION
TECHNICAL INFORMATION BUREAU
MOSCOW 1967
Compiled by ORGRES Technical Information Bureau
Editor: eng. S.V.KHIZHNYAKOV
INTRODUCTION
It has been established that heat losses to the external environment from the surface of the lining of modern boilers should not exceed 300 kcal/m 2 ∙ h, and the maximum temperature on the outer surface of the brickwork should be no more than 55 °C at an ambient air temperature of about 30 °C on average along the height of the boiler [L. , , ].
At the same time, the total maximum allowable heat loss by the boiler unit to the environmentq 5 are determined by the "Thermal calculation of boiler units" [L. ], establishing the relationship between heat loss and steam output of boilers. According to thermal calculation for modern boilers with steam capacity D = 220 ÷ 640 t/hq 5 is 0.5 - 0.4% of the fuel consumption. This value, which is relatively small in the overall heat balance of the boiler, acquires a completely different scale when converted to absolute values, amounting to about10,000 kcal/h per 1 MW of installed capacity, and heat lossesq 5 exceed 50% of all heat losses through the thermal insulation of block power plants.
In some cases, due to deviations from design solutions, poor-quality installation, the use of inefficient materials and unsuccessful design solutions, partial destruction of the brickwork and thermal insulation of the boiler during repairs of process equipment, as well as as a result of aging during long-term operation, an excess of the valueq 5 above the standard values. With a sufficiently large value of heat losses from the boiler to the environmentQ 5 (kka l/h) even slightly exceeding the valueq 5 (%) is associated with very significant heat losses. So, for example, an increaseq 5 by 0.1% for modern boilers is equivalent to burning about 2.0 tons of standard fuel per year per 1 MW of installed capacity. In addition, the increaseq 5 significantly worsens the sanitary and technical condition of the boiler room.
Naturally, a sufficiently accurate experimental determination of the actual valueq 5 (in contrast to the definition adopted during testing of boilersq 5 as a residual member of the heat balance) and bringing it into line with existing standards should be put into practice in the same way as is customary for the rest of the thermal insulation of steam pipelines and equipment of power plants [L. ].
1. GENERAL PROVISIONS
When assessing the total heat losses of the boiler unit, the most difficult of the heat-shielding structures to be tested is its lining [L. , , ].
The linings of modern boilers are divided into two main types:
1. Pipe linings (stuffed and made of prefabricated slabs) mounted directly on screen pipes.
2. Shield brickwork mounted on the frame.
Old brick linings supported byI am on the foundation, currently left on small or obsolete boilers.
The design of modern brickworks provides for the presence of metal fasteners located in the thickness of the brickwork and partially extending to its outer surface (pins, brackets, etc.). These metal parts of brickworks are thermal bridges through which heat flows to individual areas of the surface. In some designs, the heat transfer is 30 - 40% of the total heat flow through individual sections of the lining. This circumstance provides for the need for an appropriate placement of measurement points on the surfaces of such brickworks, which ensures obtaining averaged heat transfer conditions.
According to the conditions of heat transfer, linings without metal sheathing and with metal sheathing differ significantly. A specific feature of the latter is the spreading of heat along the plane of the skin, which equalizes the temperature over its significant areas. Under various external conditions of heat transfer (air flows, local counter flow of radiant heat), such temperature equalization leads to a sharp fluctuation in the values of specific heat losses in adjacent sections of the skin. Another feature of brickwork with sheathing is the possibility of convective heat overflows along the height in the gap between the sheathing and brickwork.
These circumstances necessitate the measurement of heat losses along the skin at a sufficiently large number of points, especially along the height, despite the apparent uniformity of the temperature field.
The complexity of taking into account heat losses from the beams of the lining frame and the boiler is resolved in these guidelines by introducing some average measurement conditions. This decision is justified by the relatively small share of participation of these heat-releasing surfaces in the total amount of heat losses of the boiler.unit to the environment.
A feature of thermal tests of the insulation of pipelines and boiler ducts, which are in the sphere of intensive mutual heat exchange between themselves and the brickwork, is the need to carefully determine their really releasing, rather than absorbing, heat surface, i.e. surface not "closed" by a more intense oncoming heat flow coming from nearby objects.
The true direction of the heat flux is established in this case by control measurements of the specific heat flux from various surfaces that radiate heat to each other.
The developed guidelines define both the method for measuring specific heat fluxes and the classification of all heat-releasing surfaces of a boiler unit in terms of heat transfer conditions.
The measured specific heat fluxes, averaged for individual sections, refer to the areas of the heat-releasing surfaces of these sections, determined by direct measurement.
Such a scheme makes it possible to evaluate heat losses for individual elements of the lining and thermal insulation of the boiler, reveals the share of each element in the total amount of heat loss, and also characterizes the quality of the lining and thermal insulation.
The technical feasibility of thermal testing of the boiler lining was determined by the use of a fundamentally new device - a modeling heat meter ORGRES ITP-2. In difficult thermal conditions of operation of the boiler unit, the principle of operation and the design of the ITP-2 device allow, with sufficient accuracy and a small expenditure of time for a single measurement, to directly determine the specific heat fluxes withheat transfer surfaces (heat flux density) regardless of their shape, size, surface condition (insulation, metal) and heat transfer conditions.
The small inertia of the device, the small size of its sensors and their complete interchangeability allow mass measurements of heat flows with the simultaneous use of a large number of sensors from all heat-releasing surfaces of the boiler unit.
It should be noted that the use of other generally accepted methods for determining heat loss (1 - by the difference between the measured temperatures of the surface and the environment; 2 - by the thermal resistance of the heat-shielding layer, determined by the temperature difference in it; 3 - by direct measurement using heat flow meters such as a Schmidt heat meter ) in the conditions of the boiler unit cannot be recommended, as it often leads to distorted results [L. , ].
The reason for this limitation is related to the specifics of the heat transfer conditions on the boiler, which practically excludes the possibility of correctly determining the ambient air temperature and the heat transfer coefficient. a, as well as the presence of embedded metal parts and metal surfaces in the brickwork. Conditions for measuring specific heat fluxes in a boilerunit - a large number of points in each relatively small separate section - necessitates a number of additional devices for the ITP-2 heat meter. These devices (application) without changing the fundamental nature of the heat meter, facilitate the measurement technique and significantly reduce the complexity of the work.
The surface temperature of the lining and thermal insulation of the boiler (PTE Rules) during thermal tests is measured simultaneously with the measurement of heat flows with the ORGRES T-4 temperature probe (Appendix).
2. THERMAL TESTING OF BILLINGS
A. Preparatory work
1. Before the start of the test, a detailed acquaintance with the boiler diagram and the design of its lining and thermal insulation is made. At the same time, the design and materials of brickwork and thermal insulation, as well as all deviations from the project, are clarified..
2. Sketches of the characteristic areas of brickwork and an inventory of the main heat-insulating structures (ducts, pipelines, etc.) are drawn up.
3. An external inspection of the brickwork is carried out, during which deviations from the project are clarified and external defects are fixed: lack of insulation, cracks, finishing defects, etc.
B. Measurement of areas of heat-releasing surfaces
4. Determination of the area of heat-releasing surfaces is carried out by direct measurement. On the boilerunits with a symmetrical arrangement, the measurement is carried out on one half of the combustion chamber and the convection shaft.
5. When measuring the area, only those surfaces that give off heat to the environment are taken into account. In case of closing the brickwork by others, I give off heatthe projection of these elements onto the lining is subtracted from its area by the closing elements, and the heat-releasing surface of the closing elements themselves is calculated by their protruding part.
6. For beams of different profiles and different locations, a conditional scheme for determining the area of heat-releasing surfaces and surfaces covering the lining on which they are located can be adopted. In this case, the measurement of the heat flux density is carried out only withfrontal side (side "b" in the diagram), and the area is determined in accordance with the diagram (Fig.).
7. When determining the area, I give off heatsurfaces that are difficult to access for measuring pipelines and air ducts, their length can be taken according to the dimensions indicated in the drawings and diagrams, specifying the insulation perimeter by selective measurement.
For long air ducts, it is recommended to make sketches on which the measurement points are marked.
B. Testing
8. Thermal tests of the brickwork are carried out with the possible constant operation of the boiler. Therefore, when the boiler is stopped during the testing period, the latter can be continued after its start-up only when the stationary mode of heat transfer from the external surfaces of the boiler to the environment is restored.
Approximately, this requires about 36 hours after the boiler is stopped for10 - 12 hours and about 12 hours after the boiler shutdown for 4 - 6 hours.
Rice. 1. Scheme for determining the conditional areas of beams of various profiles:
I , II - horizontal and vertical beams
Square those yielding surface (m 2) is determined: for horizontal beams 1, 2, 3, 4 - (a + b), 5- a; for vertical beams 1, 2 - (a + b). 3, 4 - (2a + b). Closing surface area (m 2) for all beams in all cases - b
9. During the testing period, according to operational data, the average values of steamperformance and fuel consumption, as well as the maximum deviations of these values from the average (with a time stamp).
The brand and calorie content of the fuel are also fixed.
10. Measurements of specific heat losses (heat flux density) from heat-releasing surfaces are carried out in separate sections within each mark (site) on each side of the boiler with a set measurement frequency (item and table):
Table 1
Map No. ______ Measurement site name
(for example: combustion chamber front __ 16.34 ÷ 19.7)
|
a) bricking; b) brick frame beams; c) boiler frame beams; d) downpipes in the area of the combustion chamber and the cold funnel; e) pipelines within the convective part; f) drum and pipelines within the combustion chamber; g) main steam pipeline to the first GPP; h) air ducts; i) sites; j) other (hatches, blowers, manholes, etc.) a) 6 cm 2 of the brickwork area, downpipes and main steam pipeline; b) 15 m 2 of the area of pipelines, air ducts, boiler drum and platforms; c) 10 m 2 of the area of \u200b\u200bthe beams of the frames of the lining and the boiler. Taking into account that the heat losses from the beams of the lining frames and the boiler in the overall balance of heat losses are small, in relation to specific conditions, measurements on individual inconveniently and far located beams can be neglected. 13. Measurements of specific heat losses (heat flux density) are made by the ORGRES ITP-2 heat meter (see Appendix). Flat heat meter sensors are mounted on special telescopic handles, which allow you to install sensors at different heights. Search sensors used to measure the density of heat fluxes from pipelines are mounted directly on the latter. At least 10 sensors are installed on each measuring device. To connect the sensors to the measuring device, extension cords are used, which allow one measuring device to serve sensors located within a radius of approximately 10 m. measurement flow is ensured. 14. The procedure for measuring the density of heat fluxes with the ITP-2 heat meter is given in the appendix. 15. Measurements of surface temperatures with a temperature probe T-4 (Appendix) are made in the same places as the measurements of thermal causes, on the basis of - one change in temperature per 5 -10 heat flux measurements. The ambient temperature is also measured by the temperature sensor.pom T-4 within each mark of the boiler at a distance of 1 m from the heat-releasing surface. 16. In the presence of heat-releasing non-insulated surfaces with a temperature of more than 100 - 120 ° C, the heat flux is calculated conditionally from the temperature of the surface and ambient air using traffic (Appendix). In the graph, the dotted curve for determining heat loss from 1 m 2 refers to a flat surface, but can also be applied to pipelines with a diameter of 318 mm and above. To determine heat loss from 1 p o g. m of pipeline of any diameter more than 318 mm, the value of heat loss found from the dotted curve must be multiplied by π d n. The surface temperature is determined by direct measurement or is assumed to be equal to the coolant temperature. 3. RECORDING THE RESULTS OF THERMAL TESTS17. For each individual section, a primary measurement document is compiled - a map in the form indicated in Table. . The map includes: a) the name of the individual heat-releasing elements of this section; b) area (m 2 ) heat-releasing surface of each element of this section; c) the average value of the heat flux density (q, kcal / m 2 ∙ h) for each element, calculated as the arithmetic mean of all measurements on this element within the site; d) total heat flow ( Q, kcal /h) from each heat-releasing element, defined as the product of the area of the heat-releasing elementSm 2 on the average heat flux densityq kcal / m 2 ∙ h ( Q = S ∙ q kcal/h); e) average surface temperaturet n°C of each element,calculated as the arithmetic mean value for all measurements on a given element within the site; f) ambient temperaturet in° C, measured in this area; g) the number of measurements of heat flux density carried out for each element. Total values are calculatedS m 2, Qkcal/h and the number of measurements. The serial number, mark and name of the measurement site are put on the map. On the observation log, according to which the map was compiled, a mark is made: “To the map№ ...» table 2 Results of thermal tests of the boiler lining (for example: combustion chamber)
Table 4 The results of thermal tests of the lining on the enlarged elements of the boiler unit (summary)
4. PROCESSING OF TEST RESULTSa) a brief description of the boiler; b) basic information on the brickwork and thermal insulation project, including sketches of the brickwork details characteristic of this design, information on the main heat-insulating structures and data on the inspection of the condition of the brickwork and thermal insulation of the boiler unit; c) summary tables of test results in the form of table. , and . Rice. 2. Heat meter sensor circuit The ITP-2 heat meter consists of a sensor and a secondary device. The sensors are interchangeable, since the scale of the secondary device is graduated according to the electrical resistance of the sensors and their geometric dimensions. Sensor circuit The heat meter sensor (Fig. ) consists of a highly thermally conductive (aluminum) housing 4, in which a heater 3 made of manganin wire and a trim battery are placed on a heat-insulating gasket 5.thermal thermocouples, the junctions of which 2 and 6 are located on both sides of the heat-insulating gasket. The heater 3 and the junctions of the differential thermocouple 2 are covered with a heat-conducting copper plate 1, which is the actual heated element of the heat meter. The junctions of the differential thermocouple b are located under the heat-insulating gasket on the sensor housing. Thus, the battery of differential thermocouples indicates the presence or absence of a temperature difference between the sensor housing and the heated element. The heat meter kit includes two sensors (Fig. ): a) sensor in the form of a disk with bevelled edges 1 is used to measure the density of heat fluxes from flat surfaces. It is connected using a spring device ("viluki”), inserted into special grooves, with a handle of the holder and through a plug connector with a wire with a secondary device; b) a sensor in the form of a disk with a certain radius of curvature on the lower plane 2, inserted into a rubber plate, is used to measure the density of heat fluxes from cylindrical surfaces. The rubber plate has lugs at the edges for attaching the sensor to the object under test. The sensor is connected by a wire to the secondary device via a plug connector. Scheme of the secondary device The scheme of the secondary device is shown in fig. . To power the sensor heater 1, a direct current source 2 is installed - three batteries of the Saturn type. To measure the strength of the current passing through the heater, a milliammeter 3 is included in the circuit of the latter, rheostats 4 are included to adjust the current strength. The battery of differential thermocouples is connected directly to zerolionometer 5. The sensor is connected to the secondary device with a plug connector 10. Based on the selected measurement limits 0 - 100 and 0 - 500 kcal/m 2 ∙ h, the area of the heated element is 6 cm 2 and the resistance of the heater is 25 Ohm, the measurement limits of the milliammeter are respectively 52.9 and 118.2 mA. To ensure these limits, additional resistances 6 and shunt resistance 7 were selected, taking into account the characteristics of the milliammeter. Rice. 4. Scheme of the secondary device For energizing and shorting the nulga frameswitch 8 is installed on the lionometer and switch 9 is used to change the measurement limits. Measurement of heat flux density To measure the heat flux density, the heat meter sensor is connected to the secondary device using a plug connector. When the switch 8 is in the “off” position, the position of the null galvanometer pointer is checked, and, if necessary, is set to “0” by the corrector. Switch 9 is set to the measurement limit corresponding to the expected heat flux. On flat surfaces or surfaces with a large (more than 2 m) radius of curvature, the measurement is made with a flat sensor. To do this, the sensor with the help of the holder is pressed by the lower flat part to the measured surface and the switch 8 is set to the "on" position. On surfaces with a small radius of curvature (pipeline), the measurement is made by a sensor with a rubber plate. To do this, the sensor is superimposed on the measured surface so that the curvature of the lower part of the sensor coincides with the curvature of the measured surface, and the rubber plate is tightly attached (attached) to the measured object using the ears it has. When applying the sensor to the tested heated surface, the highly thermally conductive sensor housing takes its temperature; due to the temperature difference between the sensor housing and the heated element, emf appears at the output of the battery of differential thermocouples. and the null galvanometer pointer deviates from the "0" position. Gradually, the rheostats “roughly” and “finely” increase the current strength in the sensor heater. With an increase in the temperature of the heater, and, consequently, the junctions of the battery of differential thermocouples located under the heated element, the null galvanometer needle begins to approach the value "0". When pwhen the arrow passes through “0”, the current in the heater decreases with the help of rheostats until the zero-galvanometer needle takes a stable zero position. The stable position of the zero-galvanometer needle is achieved more easily when it is slowly brought to "0". To do this, the following technique is used: when the sensor is applied to a hot surface, before turning on the current supply to the heater, the null galvanometer needle deviates to the left position. A deliberately overestimated current is given to the heater (the extreme right position of the milliammeter needle), while the null galvanometer needle begins to quickly approach "0". To reduce the current strength should begin until the pointer passes through "0" - for 2 - 3 divisions. In practice, the cycle of setting the arrow to "0" (more ↔ less) is repeated several times with a gradual decrease in the adjustment range. With a stable (at least 1 min) zero position of the zero galvanometer pointer, the value of the heat flux density is read using a milliammeter. The equality of the density of heat fluxes from the heated element of the sensor and from the surface under test is ensured by the fact that with a high thermal conductivity of the sensor body, the temperature field inside it is equalized and at the moment of balancing the temperature of the body (equal to the temperature of the surface being tested) and the temperature of the heated element, the insulating gasket of the sensor will be surrounded by an isothermal surface so the same as the whole sensor. The time required for one measurement, determined by the inertia of the sensor body and the stability of the external conditions of heat transfer, when using a flat sensor is 3 - 8 minutes, when using a sensor with a rubber plate due to the relatively low thermal conductivity of rubber - 20 - 30 minutes. In the latter case, the actual measurement should be started 15–20 minutes after the sensor is installed on the measurement object. The high sensitivity of the measuring circuit makes it possible to take for the zero position of the null galvanometer the fluctuations of the needle within 1 - 2 divisions around zero. The painted sensors supplied with the heat meter are suitable for measuring the heat flux density on both insulating and painted metal surfaces. For measurements on shiny metal surfaces, probes with a shiny metal surface must also be used. The need to change the batteries can be judged by the drop in current. If the arrow of the milliammeter is not set to 500 kcal/ m 2 ∙ h, the Saturn batteries should be changed. Heat meter accessories 1. To mount the heat meter sensors on flat surfaces, telescopic handles-holders are used. The height of the installation (mounting) of the sensor is regulated by changing the length of the handle and its angle of inclination (Fig. ). 2. Search sensors are fastened to surfaces with a small radius of curvature by pinning to it by special belt lugs (Fig. ). In the presence of a metal or asbestos-cement coating, the sensor is attached by tying to the same ears with a cord or wire. Rice. 5. Installation of heat meter sensors on a flat surface: 1 - sensors; 2 - handles-holders 3. Connections e sensors to the measuring device is carried out using an extension cord, which has connectors at the ends that correspond to the connectors of the sensor and the secondary device (Fig. ). When installing at a high altitude, the cord is connected to the sensor in advance. Therefore, at least 3 extension cords should be provided for each measuring instrument. Rice. 6. Installation of the search sensor on the pipeline: 1 - pipeline; 2 - sensor; 3 - mounts Rice. 7. Extension cord with connectors 4. To measure heat flux densities greater than 500 kcal/m 2 ∙ h observed on individual elements of the boiler unit, an additional measurement range of 0 - 1000 kcal / m 2 ∙ h is built into the heat meter and a separate power supply unit of 4 elements is used " Zs-ut- 30" (Fig. and). The measurement limit of the milliammeter in this case should be equal to 167 mA. When measuring the value of the specific heat flux, a scale of 0 - 100 kcal / m 2 ∙ h is used with a coefficient of 10. Instrument check During operation, the heat meter is subjected to a mandatory periodic check of electrical indicators within the time limits determined by the operating conditions, but at least once every two years. Storage rules The heat meter should be stored indoors at a temperature of 5 to 35°С and relative air humidity not higher than 80%. In the air of the room where the heat meter is stored, there should be no harmful impurities that cause corrosion. The surface of the heated elements of the sensors should not be subjected to any mechanical influences: pressure, friction, impacts. Appendix 2
|
54,81 |
55,03 |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
55,26 |
55,48 |
55,71 |
55,94 |
56,16 |
56,39 |
56,61 |
56,84 |
57,06 |
57,29 |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
57,52 |
57,74 |
37,97 |
58,19 |
58,42 |
58,65 |
58,87 |
59,10 |
59,32 |
59,55 |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
59,77 |
60,00 |
60,23 |
60,45 |
60,68 |
60,90 |
61,13 |
61,35 |
61,58 |
61,81 |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
62,03 |
62,26 |
62,48 |
62,71 |
62,93 |
63,16 |
63,39 |
63,61 |
63,84 |
64,06 |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
64,29 |
64,52 |
64,74 |
64,97 |
65,19 |
65,42 |
65,64 |
65,87 |
66,10 |
66,32 |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
66,55 |
66,77 |
67,00 |
67,22 |
67,45 |
67,68 |
67,90 |
68,13 |
68,35 |
68,58 |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
68,81 |
69,03 |
69,26 |
69,48 |
69,71 |
69,93 |
70,16 |
70,39 |
70,61 |
70,84 |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
71,06 |
71,29 |
71,51 |
71,74 |
71,97 |
72,19 |
72,42 |
72,64 |
72,87 |
73,09 |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
73,32 |
73,55 |
73,77 |
74,00 |
74,22 |
74,45 |
74,68 |
74,90 |
75,13 |
75,35 |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
75,58 |
75,80 |
76,03 |
76,26 |
76,48 |
76,71 |
76,93 |
77,15 |
77,38 |
77,61 |
2. After measurementR100the thermometer is placed in a melting ice thermostat and the resistance of the thermometer is determined at 0 ° C (R 0 ). This resistance must not deviate from the nominal value of 53 ohms by more than by ±0.1%.
Attitude must be within 1.426 ÷ 0.002 * .
_____________
* The specified method for checking resistance thermometers is provided for by GOST 6651-59 and is described in detail in Instruction 157-62 of the Committee for Standards, Measures and Measuring Instruments under the Council of Ministers of the USSR.
3. The secondary device of the temperature probe is verified using a resistance box with an accuracy class of at least 0.02, which has a decade with hundredths of an ohm. When checking, it is necessary to take into account that the device is calibrated with the resistance of the supply wiresR ext, equal to 1 ohm. The calibration table for copper resistance thermometers with graduation 23 is given inTemperature difference between pipe metal and air, deg
0,91
0,91
0,91
0,91
0,95
0,95
0,96
0,96
1,00
1,00
1,00
8. Vasilyeva G.N. [and etc.] . Determination of heat losses of boiler units to the environment ( q 5 ). "Electric Stations", 1965, No. 2.
The operation of a heat generating plant is accompanied by heat losses, usually expressed in fractions,%:
q i= (Q i/ Q p p) ⋅ 100.
1. Losses of heat with the outgoing flue gases of the heat generator
q 2 = (Q 2 / Q p p) ⋅ 100, %.
In a heat generator, this is most often the largest part of the heat loss. Heat loss with flue gases can be reduced by:
Reducing the volume of flue gases by maintaining the required coefficient of excess air in the furnace α t and reducing air suction;
Reducing the temperature of flue gases, for which tail heating surfaces are used: a water economizer, an air heater, a contact heat exchanger.
The temperature of flue gases (140…180 °C) is considered profitable and largely depends on the condition of the internal and external heating surfaces of the boiler tubes and economizer. The deposition of scale on the inner surface of the walls of the boiler pipes, as well as soot (fly ash) on the outer heating surface, significantly worsen the heat transfer coefficient from flue gases to water and steam. Increasing the surface of the economizer, air heater for deeper cooling of flue gases is not advisable, since this reduces the temperature difference Δ T and the intensity of metal increases.
An increase in the temperature of the outgoing flue gases can occur as a result of improper operation and fuel combustion: high thrust (the fuel burns out in the boiler bundle); the presence of leaks in the gas partitions (gases go directly through the gas ducts of the boiler unit, without giving off heat to the pipes - heating surfaces), as well as with high hydraulic resistance inside the pipes (due to the deposition of scale and sludge).
2. Chemical underburning
q 3 = (Q 3 / Q p p) ⋅ 100, %.
Heat losses from chemical incompleteness of fuel combustion are determined by the results of the analysis of volatile combustible substances H 2 , CO, CH 4 in the outgoing flue gases. Causes of chemical incompleteness of combustion: poor mixture formation, lack of air, low temperature in the furnace.
3. Mechanical underburning
q 4 = (Q 4 / Q p p) ⋅ 100, %.
Heat losses from mechanical incompleteness of fuel combustion are typical for solid fuel and depend on the share of fuel failure through the grate into the ash removal system, entrainment of particles of unburned fuel with flue gases and slag, which can melt a particle of solid fuel and prevent it from completely burning out.
4. Heat loss from external cooling of enclosing structures
q 5 = (Q 5 / Q p p) ⋅ 100, %.
Occur due to the temperature difference between the outer surface of the heat generator and the surrounding outdoor air. They depend on the quality of the insulating materials, their thickness. For supporting q 5 within the specified limits, it is necessary that the temperature of the outer surface of the heat generator - its lining - does not exceed 50 °C.
Heat loss q 5 decrease in the direction of movement of the flue gases along the gas path, therefore, for the heat generator, the concept of the heat conservation coefficient is introduced
φ = 1 − 0.01 q 5 .
5. Losses with physical heat of slag
q 6 = (Q 6 / Q p p) ⋅ 100, %.
They arise due to the high temperature of slags of the order of 650 ° C, and are characteristic only during the combustion of solid fuels.
Tables for calculating heat losses, gross efficiency, natural, estimated and conditional fuel consumption of the heat generator are given in the reference literature.
Lecture 4
Furnace and burner devices
Furnace devices
Firebox- a device designed to burn fuel in order to obtain heat. The firebox performs the function of combustion and a heat exchanger - heat is simultaneously transferred from the combustion torch by radiation and from combustion products by convection to the screen surfaces through which water circulates. The share of radiant heat exchange in the furnace, where the temperature of the flue gases is about 1000 ° C, is greater than the convective one, therefore, most often, the heating surfaces in the furnace are called radiation.
For combustion of natural gas, fuel oil and pulverized solid fuel, chamber furnaces are used, in the design of which three main elements can be distinguished: a combustion chamber, a screen surface, a burner device.
1. A combustion chamber or a furnace volume is a space separated by a lining from the environment.
brickwork are called fences separating the combustion chamber and gas ducts of the heat generator from the external environment. The lining in the boiler unit is made of red or diatomaceous bricks, refractory material or metal shields with refractories.
The inner part of the lining of the firebox - lining, from the side of flue gases and slags, is made of refractory materials: fireclay bricks, fireclay concrete and other refractory masses. Brickwork and lining should be sufficiently dense, especially highly refractory, resistant to the chemical attack of slags and have a low thermal conductivity.
The lining can be supported directly on the foundation, on metal structures (framework) or mounted on the pipes of the screens of the combustion chamber and gas ducts. Therefore, there are three brickwork designs: massive - has its own foundation; on-frame (lightweight) - it has no foundation, it is attached to a metal frame; on-pipe - attached to the screen surfaces.
Rice. 6.1. Frontal and lateral section of a water-heating boiler with a firebox and lining made of fireclay bricks
The frame serves to fasten and support all elements of the boiler unit (drums, heating surfaces, pipelines, lining, stairs and platforms) and is a metal structure, usually of a frame type, connected by welding or bolted to the foundation.
2. Screen radiation heating surface is made of steel pipes with a diameter of 51…76 mm, installed with a step of 1.05…1.1. Screens perceive heat due to radiation and convection and transfer it to water or steam-water mixture circulating through pipes. Screens protect brickwork from powerful heat flows.
In vertical water-tube boilers (Fig. 6.2a), the heating surface consists of a developed bundle of boiler pipes 2, rolled into the upper 1 and lower 3 drums, furnace screens 6, fed with water from the boiler drums through the downpipes 7 and connecting 4 from the chambers (collectors 5). The evaporative heating surfaces of screen-type boiler units (Fig. 6.2b) consist of a drum 1, a system of screen pipes 6 with bottom 8 and 9 and top 5 screen collectors, systems of downcomer 7 and connecting 10 pipes.
Rice. 6.2. Screen heating surfaces of boilers:
a - vertical water tube, b - screen type
1 and 3 - upper and lower drums, 2 and 7 - boiler and lower pipes, 4 and 10 - connecting pipes, 5, 8 and 9 - collectors, 6 - furnace screens
3. Burners are installed on one or two opposite (opposite) heating surfaces, on the hearth, or in the corners of the furnace. An embrasure is arranged on the walls of the boiler furnace - a hole in the lining lined with refractory material, where an air register and a burner are installed.
With any type of fuel (gaseous, liquid or pulverized), air is mainly (except for injection burners) blown into the furnace by a blower fan through air registers or air guides, which ensures intensive swirl and exit (supply) of the fuel-air mixture in the narrowest section of the furnace embrasure at a speed 25…30 m/s.
The air guide is an axial-type vane swirler with movable blades that rotate around their axis. It is also possible to install fixed profile blades at an angle of 45…50° to the air flow. The swirling of the air flow intensifies the processes of mixture formation and combustion, but at the same time, the resistance along the air path increases. Guide vanes are convenient for automatic control of the performance of fans and smoke exhausters.
Burner devices
Depending on the type of fuel burned, there are many designs of burners.
1. When burning solid pulverized fuel, mixing type burners are used. A snail is installed in the embrasure of the combustion chamber, in which the dust-air mixture (pulverized fuel with primary air) is twisted and transported through the annular channel to the burner outlet, from where it enters the furnace in the form of a swirling short torch. Secondary air, through another similar snail, is fed into the furnace at a speed of 18 ... 30 m / s, in the form of a powerful swirling flow, where it is intensively mixed with a dust-air mixture. The productivity of the burners is 2…9 t/h of coal dust.
2. When burning fuel oil, nozzles and oil burners are used: mechanical, rotary and steam-air (steam-mechanical).
Mechanical nozzle. Fuel oil heated up to approximately 100 °C under pressure of 2…4 MPa enters the channel, moves to the nozzle (spray head), where the swirler-sprayer is installed.
Mechanical centrifugal nozzles are divided into unregulated and adjustable drain. It should be noted that this division is very conditional: you can change the flow of both nozzles. Unregulated nozzles include nozzles with a small depth of regulation and those in which the change in supply is associated with their shutdown, removal from the combustion device and replacement of the spray element.
Mechanical centrifugal atomizers, which differ in the layout of the spraying elements, are additionally sometimes subdivided into nozzles with replaceable atomizers constantly operating in all modes, which is mainly due to the operating conditions of the boiler.
Rice. 6.3. Mechanical non-adjustable centrifugal nozzle
The mechanical adjustable centrifugal nozzle of domestic auxiliary boilers (Fig. 6.3) consists of a body 6 with a handle 7, a barrel 5, which is a thick-walled pipe with a fitting at the end, a locking sleeve 4, a distributor (nozzle) 3, a spray washer 2 and a head 1. Fuel from the fuel injector pump through the holes in the housing and the barrel bore through the drillings in the locking sleeve and the distributor, it enters the spray washer. The spray washer of this design has four channels 8 located tangentially to the circumference of the vortex chamber. Through them, the fuel rushes to the center and into the vortex chamber 9, where it is intensively untwisted. From it, the fuel enters the furnace through the central hole 10 in the form of a rotating cone of finely dispersed particles.
The contact surfaces of the spray washer 2 and the distributor 3 are carefully processed, polished and, when assembling the head, they are pressed one against the other with a locking sleeve 4.
Spray washers are made of high-alloy chromium-nickel or chromium-tungsten steels. Depending on the nozzle feed, the number of tangential channels can be from two to seven.
The shape of the nozzle jet depends on the ratio f k /f o , in which f k is the total area of all tangential channels, f o is the cross-sectional area of the central hole. The smaller this ratio, the greater the angle of the spray cone, and the shorter the length of the torch.
Washers are usually made under numbers. Each number corresponds to a specific feed, which is indicated in the technical documentation. Sometimes numbers are indicated on washers corresponding to the values of the diameter of the central hole and the ratio f k / f o, while foreign firms apply symbols in the form of indices (Fig. 6.4). For example: the letter X indicates that the front end wall of the washer is made flat, the letter W - spherical; the figure on the left is the conditional number of the drill for making the central hole, the number on the right is the ratio f k /f o , increased 10 times.
Rice. 6.4. Spray washer
Rotary nozzle. Fuel is fed through the channel and nozzle to the rotating bowl, crushed and discharged into the combustion chamber.
Rice. 6.5. Device for rotary oil and gas
burners RGMG-10 (-20, -30):
1 – gas pipeline; 2 - air box; 3 – frame ring; 4 - gas pipe;
5 , 6 - a pipe for installing an ignition protective device (EPD) and a photo sensor; 7 - gas chamber; 8 – a forward ring of the air directing device; 9 – conical ceramic tunnel (embrasure); 10 – swirlers of the air guide device; 11 – rotary nozzle;
12 – gas outlets; 13 – a frame for centering the secondary air swirler; 14 - support pipe; 15 – guide frame bearing; 16 - guide frame 17 - air damper; 18 – a window for air supply to the swirler; 19 – burner cover
The fuel pressure - fuel oil is 0.15 ... 1 MPa, and the bowl rotates at a speed of 1500 ... 4500 rpm. Air enters around the bowl through the cone, envelops the rotating flow of droplets and mixes with it. Advantages: powerful oil pumps and fine purification of fuel oil from impurities are not required; wide control range (15…100%). Disadvantages: complex design and increased noise level.
Steam-air or steam-mechanical nozzle. The fuel is fed into the channel, along the outer surface of which the atomizing medium enters - steam or compressed air (with a pressure of 0.5 ... 2.5 MPa).
The steam exits the channel at a speed of up to 1000 m/s and atomizes the fuel (fuel oil) into tiny particles.
Air is blown by a fan through an embrasure.
Rice. 6.6. Steam-mechanical nozzle
Rice. 6.7. Atomizing washer of the steam-mechanical nozzle
In a steam-mechanical (Fig. 6.6), as in a mechanical nozzle, fuel under pressure is supplied to the annular channel 3, from where it enters the vortex chamber 4 through six tangential channels 9 of the atomizer 2, twists in it and through the central hole 5 in the form of a conical film exits into furnace. In the steam part 1 of the atomizer there is also an annular chamber 6, where steam is supplied through tangential channels 7, twists in it and enters the furnace through the annular gap 8 at the very root of the conical fuel film, which thus receives additional energy and is sprayed into small drops. Further, these drops undergo secondary crushing due to resistance forces.
Any fuel oil injector must have a device for good mixing of fuel with air, which is achieved by using various types of swirling devices - registers. A set of injectors with a register and other accessories is called oil burner.
3. Gas burners.
Rice. 6.8. Gas burner GG-1
(designed for natural gas combustion in furnaces of steam and hot water boilers of types E or KV-GM):
1-air box; 2-gas manifold; 3- swirler; 4- confuser; 5-gate; 6-sector; 7-electromagnet; 8-adjusting screw; 9-fitting; 10-nipple
Gas-burning devices (burners) are designed to supply a gas-air mixture or separately gas and air to the place of combustion (into the furnace), stable combustion and regulation of the combustion process. The main characteristic of the burner is its heat output, i.e. the amount of heat released during the complete combustion of the gas supplied through the burner is determined by the product of the gas consumption by its lower calorific value.
The main parameters of the burners are: rated thermal power, rated gas (air) pressure in front of the burner, nominal relative length of the flame, coefficients for limiting and operating control of the burner in terms of thermal power, specific metal content, pressure in the combustion chamber, noise characteristic.
There are three main methods of burning gas:
1) diffusion– gas and air in the required quantities are supplied separately to the furnace, and mixing takes place in the furnace.
2) Mixed- a well-prepared mixture of gas and air is supplied to the burner, containing only a part (30 ... 70%) of the air necessary for combustion. This air is called primary. The rest (secondary) air enters the torch (burner mouth) by diffusion. The same group includes burners, in which the gas-air mixture contains all the air necessary for combustion, and mixing occurs both in the burner and in the torch itself.
3) Kinetic- a fully prepared gas-air mixture with an excess amount of air is fed into the burner. Air is mixed with gas in the mixers, and the mixture quickly burns out in a short, faint flame, with the obligatory presence of a combustion stabilizer.
The presence of a stable flame is the most important condition for the reliable and safe operation of the unit. In case of unstable combustion, the flame can slip inside the burner or break away from it, which will lead to the gas contamination of the furnace and gas ducts and the explosion of the gas-air mixture during subsequent re-ignition. The speed of flame propagation for different gases is not the same: the highest is 2.1 m / s
- for a mixture of hydrogen with air, and the smallest 0.37 m / s - a mixture of methane with air. If the speed of the gas-air flow is less than the speed of flame propagation, there is a flashover of the flame in the burner, and if it is more, the flame is detached.
According to the method of supplying combustion air, the following designs of burners are distinguished:
1. Burners with air supply to the place of combustion due to rarefaction in the furnace created by a chimney or smoke exhauster, or convection. The mixing of gas with air occurs not in the burner, but behind it, in the loophole or furnace, simultaneously with the combustion process. These burners are called diffusion, they evenly heat the entire furnace, have a simple design, operate silently, the torch is resistant to separation, flashover is impossible.
2. Burners with gas injection, or injection. A jet of gas coming from a gas pipeline under pressure is ejected from one or more nozzles at high speed, as a result, a vacuum is created in the mixer injector, and air is sucked (injected) into the burner and mixed with gas while moving along the mixer. The gas-air mixture passes through the mixer throat (the narrowest part), which equalizes the mixture jet, and enters its expanding part - the diffuser, where the mixture speed decreases and the pressure increases. Further, the gas-air mixture enters either into the confuser (where the speed increases to the calculated one) and through the mouth - to the place of combustion, or into the collector with fire holes, where it burns out in the form of small bluish-violet torches.
3. Burners with gas injection by air. They use the energy of jets of compressed air created by a fan to suck gas, and the gas pressure in front of the burner is maintained constant with the help of a special regulator. Advantages: gas supply to the mixer is possible at a speed close to the speed of air; the possibility of using cold or heated air with variable pressure. Disadvantage: use of regulators.
4. Burners with forced air supply without preliminary preparation of the gas-air environment. The mixing of gas with air occurs during combustion (i.e. outside the burner), and the length of the torch determines the path on which this mixing ends. To shorten the torch, gas is supplied in the form of jets directed at an angle to the air flow, the air flow is swirled, the difference in gas and air pressures is increased, etc. According to the mixture preparation method, these burners are diffusion burners (flame flashback is impossible), they are used as backup when transferring one fuel to another in DKVR boilers, in the form of hearth and vertical slot burners.
5. Burners with forced air supply and preliminary preparation of the gas-air mixture, or oil-gas burners. They are the most common and provide a predetermined amount of mixture before entering the furnace. Gas is supplied through a series of slots or holes, the axes of which are directed at an angle to the air flow. To intensify the process of mixture formation and combustion of fuel, air is supplied to the place of mixing with gas in a swirling flow, for which the following are used: vane units with a constant or adjustable blade angle, snail shape of the burner body, tangential feed or tangential blade swirlers.
In the process of modernization (reconstruction), when replacing some materials in the lining of boilers with others, it is necessary to check how the replacement will affect heat loss (q 2) through unshielded enclosing structures and whether the temperatures for the materials used will be acceptable. Heat loss through the brickwork (q 2), the temperature of the outer surface and the temperature in the plane of contact between the layers of the brickwork can be determined from the diagram shown in fig. Pr-2 for stationary heat flow. The diagram gives the value of heat loss through the brickwork and the temperature of the outer surface of the unshielded brickwork, depending on the thermal resistance of the brickwork.
where: S 1, S 2, S 3 - the thickness of the individual layers of the lining;
λ 1 , λ 2 , λ 3 - thermal conductivity of the material of these layers at their average temperature, which
taken according to the reference data of section 10 with a coefficient of 1.2,
masonry gas permeability.
The temperature in the plane of contact between the layers is determined by the formula:
where: t 1 is the surface temperature of the layer with a higher temperature;
t 2 is the temperature of the second surface in the plane of contact between the layers;
The ratio of the thickness of the respective layer in m to its thermal conductivity in W/(m⋅K) or
kcal/(m⋅hour⋅deg).
Example. Determine the heat loss through 1m 2 of unshielded lining with a thickness of: lightweight fireclay γ = 1000 kg / m 3 - 280 mm and mineral wool γ = 150 kg / m 3 - 50 mm at an internal surface temperature t 1 \u003d 1000 0 С.
We set the temperature in the plane of contact between the fireclay and mineral wool layers t 2 \u003d 110 0 C and the temperature of the outer surface of the wall t 3 \u003d 70 0 C.
Average temperature of fireclay layer:
Average temperature of the mineral wool layer:
The coefficient of thermal conductivity of the fireclay layer, taking into account the coefficient of gas permeability at t sr.sh:
λ w.r. =λ w.555 ⋅ k gas.pr. =0.5⋅1.2=0.6 W/(m⋅K) or 0.43⋅1.2=0.516 kcal/(m⋅h⋅g),
λ w - see the nomogram in fig. 10.5.
The coefficient of thermal conductivity of the mineral wool layer at t sr.m.v. :
λ m.w.r. = λ m.w.90 = 0.128 W/(m⋅K) or 0.11 kcal/(m⋅h⋅g),
λ m.v. – see nomogram in fig. 10.8.
Thermal resistance of brickwork:
(m 2 ⋅K) / W or
(m 2 ⋅h⋅g) / kcal.
According to the nomogram in Fig. Pr-2, the temperature of the outer wall at R \u003d 1.02 (m 2 ⋅K) / W or 1.19 (m 2 ⋅h⋅g) / kcal and t 1 \u003d 1000 0 С will be t 3 \u003d 85 0 С and the flow heat through the lining q 2 \u003d 890 W / m 2 or 765 kcal / m 2 ⋅ h. The temperature in the plane of contact between the layers will be equal to:
The obtained value of t 2 does not correspond significantly (not close) to the accepted one. We set the temperature in the plane of contact between the fireclay and mineral wool layers
t 2 \u003d 440 0 С, the temperature of the outer surface of the wall t 3 \u003d 88 0 С and recalculate. ;
λ w.r. =λ w.720 ⋅ k gas.pr. =0.547⋅1.2=0.656 W/(m⋅K) or 0.47⋅1.2=0.564 kcal/(m⋅h⋅g);
λ m.w.r. = λ m.w.264 = 0.14 W/(m⋅K) or 0.12 kcal/(m⋅h⋅g);
(m 2 ⋅K) / W or
(m 2 ⋅h⋅g) / kcal.
According to the nomogram in Fig. Pr-2, the temperature of the outer wall at R \u003d 0.936 (m 2 ⋅K) / W or 1.09 (m 2 ⋅h⋅g) / kcal and t 1 \u003d 1000 0 С will be t 3 \u003d 90 0 С and q 2 \u003d 965 W / m 2 or 830 kcal / (m 2 ⋅ h) (heat loss through unshielded lining). We specify the temperature in the plane of contact between the layers:
The results obtained are close to the accepted values, therefore, the calculation is correct.
The maximum temperature for the use of mineral wool is 600 0 C (see Table 10.46), i.e. the use of these materials when laying the boiler in this case is advisable.
The temperature of the outer surface of the lining t 3 \u003d 90 0 C does not meet the requirements of the Sanitary Norms. Therefore, the thermal resistance of the lining - R exchange should be increased to ~4 (m 2 ·h ·g) / kcal (see the nomogram in Fig. Pr-2). Thermal resistance can be increased by arranging an additional layer of heat-insulating material with t max of application not higher than 110 0 C.
Frame. The boiler frame is a metal structure that supports the drum, heating surfaces, lining, stairs and platforms, as well as auxiliary elements of the unit and transfers their weight to the foundation. Low-pressure and low-capacity boilers are installed on a frame fixed directly on the foundation, or brick lining, and then the main purpose of the frame is to give the steam generator lining greater stability and strength. The frame of a modern boiler is a complex metal structure, and a large amount of metal is spent on its manufacture. In high-pressure boilers, the mass of the frame is 20-25% of the total mass of the boiler metal, or 0.8-1.2 tons per ton of its hourly output. The frame is a frame structure made of standard metal profiles made of mild steel grade St.3, and consists of a number of main and auxiliary columns and horizontal beams connecting them, receiving the load from the drums, the pipe system of heating surfaces, as well as horizontal and diagonal beams that serve to give strength and rigidity to the frame system.
On fig. 67 shows a frame diagram of a high pressure drum boiler.
Columns are usually made of two steel channels or I-beams, rigidly connected to each other by plates of sheet steel; columns transfer significant concentrated loads to the foundation - hundreds of tons. To avoid excessive specific pressures on the foundation, the columns are equipped with shoes (Fig. 68) made of sheet steel and squares. The support plane of the shoes is calculated for the compressive stress allowed for the foundation material and is fixed in the foundation with bolts or embedded in it. The main horizontal beams are welded to the columns and together form a frame system. Bearing and spacer horizontal beams are made of steel channels, I-beams or squares.
When the assortment of rolled profiles does not provide the required strength of columns and beams, they are made in the form of a welded structure made up of a number of profiles and sheet steel. Part of the frame are the platforms necessary for servicing the boiler, which work like horizontal trusses and increase the rigidity of the frame. Scaffolds are made of frames of rolled profiles and sheets of corrugated steel welded to them. Stairs between the platforms are made of steel strips, between which steps are welded. The angle of inclination of stairs should not exceed 50° to the horizontal, and their width should be at least 600 mm.
Rice. 67. Diagram of the boiler frame:
1 - columns; 2 - load-bearing ceiling beams; 3 - farm;
4 - crossbar; 5 - racks
The frame is calculated as a frame structure operating under static load from the weight of the steam generator elements and additional thermal stresses arising under the influence of uneven heating of frame parts and structures welded to them. In order to prevent overheating of the frame elements, its columns, horizontal beams and trusses are usually located outside the brickwork. When installing the steam generator outside the building, the wind load on the surface, which limits the steam generator and is transferred to the frame, must also be taken into account. Boiler drums, collectors of screens of superheaters and water economizers lengthen when heated, and in order to prevent the occurrence of large thermal stresses in them and in the frame elements on which they are fixed, it is necessary to provide for the possibility of their free expansion. For this purpose, the drums are installed on special movable supports fixed on the horizontal beams of the frame, or suspended from these beams. The drums of medium and large capacity boilers are usually mounted on two movable supports. The design of such a support is shown in Fig. 69.
With a large length of the drum, when, when installed on two supports, its deflection is more than 10 mm, the drum is suspended from the frame at several statically most advantageous points. The collectors of screens, superheaters and water economizers are attached to the frame with hinged hangers, and if they are short, they freely rest on sliding supports fixed on the frame.
Purpose and requirements for brickwork. The brickwork of the boiler is a system of fences that separate the combustion chamber and gas ducts from the environment. The main purpose of the lining is to direct the flow of combustion products, as well as its thermal and hydraulic isolation from the environment. Thermal insulation is necessary to reduce heat losses to the environment and to ensure the permissible temperature of the outer surface of the brickwork, which, according to the conditions of safe work of personnel, should not exceed 55 °C. Hydraulic isolation is necessary to prevent cold air from sucking into the gas ducts or knocking out combustion products due to the pressure difference in the gas ducts and outside, which occurs when the boiler is operated with a vacuum or with pressure in the gas path.
Boiler lining elements operate in various conditions. The outer surface of the lining has a low and relatively constant temperature, its inner surface is in the region of high and variable temperature, decreasing along the gas flow. In the direction of gas flow, the vacuum in the gas ducts increases, and the pressure decreases when the steam generator is operating under pressurization. The loads on the lining elements are also different from its weight and internal stresses arising from unequal temperature elongations of its parts.
The most severe conditions are found in the inner part of the lining of the furnace, which is exposed to high temperatures of more than 1600 ° C, and when burning solid fuel, also to the chemical and mechanical effects of slag and ash. As a result of the interaction of the lining material with slag, as well as mechanical wear by slag and ash, the lining is destroyed.
The construction of the lining. According to the purpose and working conditions, the following basic requirements are imposed on the brickwork: low thermal conductivity, tightness, mechanical strength and thermal stability. In addition, the brickwork design should be simple and not require large labor and time costs for its manufacture and installation.
Previously, the lining of steam generators was carried out only from red and refractory bricks, from which its walls and vaults were laid out, fastened with steel beams and tie bolts. The lining of modern steam generators is a combined system made of bricks, refractory boards, insulating materials, metal fasteners, sealing coatings, metal sheathing and other elements. The design of the lining is changed and improved as the steam generator structure develops and the production of refractory products and insulating materials develops.
Bricks, depending on the design and method of fastening, can be divided into the following types (Fig. 70):
a) wall brick lining, based directly on the foundation;
b) lightweight lining made of refractory and diatomaceous bricks, insulating boards and steel sheathing, fixed to the steam generator frame using metal structures;
c) light lining, made of fireclay or heat-resistant concrete slabs, heat-insulating slabs and metal sheathing or sealing coating.
The indicators of these types of brickwork are characterized by the following data:
Wall lining it is used for low power steam generators with a wall height of not more than 12 m. At a higher height, the lining becomes mechanically unreliable. In this case, it is made in the form of an outer lining of red brick with a thickness of 1-1.5 bricks and an inner lining of refractory bricks, which in the area of an unshielded firebox should have a thickness of 1-1.5 bricks, and in gas ducts with a temperature of 600-700 ° C - at least 0.5 bricks (Fig. 70a ).
With a relatively large size of the combustion chamber and a high temperature of its walls, in order to prevent a breakdown in the connection between the layers of refractory and red brick, the masonry is divided into sections and the lining is unloaded in height (Fig. 70b ).
To reduce heat loss through the lining, channels are sometimes left between the lining and the lining, which are filled with loose insulating material - diatomaceous earth, ground slag, etc. To prevent the occurrence of internal temperature stresses that destroy the masonry, arising under conditions of its uneven heating, expansion joints filled with asbestos cord are provided in the walls of the masonry, which provide the possibility of its free expansion.
Lightweight brickwork were previously used in medium power steam generators. The design of the lightweight brickwork is shown in Fig. 70v . Brickwork is carried out from two or three layers of various materials with a total thickness of up to 500 mm. The inner refractory layer - lining - has a thickness of 113 mm, and with a low degree of shielding 230 mm, the middle insulating layer of diatomite bricks is 113 mm, the facing layer of covelite plates is 65-150 mm. The middle insulating layer is often made of 100 mm thick covelite boards replacing diatomite bricks. Reducing the thickness and weight of the lining made it possible to rest it directly on the frame, as a result of which it became possible to make it of any height, setting unloading belts every 1-1.5 m. In this case, the entire wall is divided into a number of tiers, each of which rests on cast iron or steel brackets mounted on the frame of the steam generator. To ensure the possibility of free expansion between the bracket and the masonry, horizontal expansion joints filled with asbestos cord are provided.
In some designs, to prevent lining collapses, special fastenings of vertical tiers to the frame using cast-iron hooks are used. Outside, the lining is sheathed with steel sheets or protected with gas-tight plaster (Fig. 70 G).
Rice. 70. Constructions of linings of vertical walls:
a, b – massive, free-standing: 1 - unloading belts;
2 - lining; c - lightweight on-frame: 1 - steel or
cast iron brackets; 2 - shaped fireclay brick;
3 - horizontal expansion joint; 4 - shaped fireclay
brick; 5 - fireclay brick; 6 - shaped fireclay brick;
7 - cast iron hook; 8 - horizontal pipes fixed on
frame; 9 - lightweight heat-insulating brick or
heat-insulating plate; 10 - outer metal sheathing;
11 - unloading and attracting belts; g - shield brickwork:
1 - the first layer of a shield made of refractory concrete; 2 - steel mesh;
3, 4 - heat-insulating plates; 5 - gas-tight coating
Light brickwork frame type is made of shields consisting of two layers of heat-insulating materials, protected from the side of the gases washing them with a layer of refractory concrete. The metal frame of the shields of such brickwork is attached to the frame of the steam generator. Slabs of 1000x500 mm and 1000x1000 m in size are also used from lime-silica materials, covered from the gas side with refractory fireclay concrete. Plates intended for installation in places unprotected by pipes with a higher temperature have a greater thickness and mass. To transfer their mass to the frame, additional embedded cast-iron brackets are provided. Frame brickwork is mainly used in the field of superheaters, gas turning chambers and the convection shaft of high power steam generators. In fireboxes, frame lining is used on straight walls. The advantages of the on-frame construction of the brickwork are its low weight and significant facilitation of installation work. However, with such a brickwork, its repair and maintenance of density are difficult.
Pipe lining (Fig. 71) is made in the form of separate layers, sequentially applied in a plastic state to pipes of screens and other heating surfaces, or in the form of slab-panels with refractory and heat-insulating layers, installed on stiffening beams fixed on pipes.
In this case, the panels are manufactured at the factory, and the heat-resistant layer can be applied in a plastic state to the screen pipes by hand. For the pipe lining of the combustion chamber, the bearing elements are the pipes of the screens, and as a result of thermal elongations, the lining moves with them.
A variety of pipe lining are those used in the furnace incendiary belts.
Rice. 71. Pipe lining:
1 - layer of chromite mass; 2 - steel mesh;
3,4 - heat-insulating plates; 5 - gas-tight coating
HARD-BLOW MACHINES
The task of draft machines is to exhaust flue gases and supply air to ensure the normal operation of the boiler at all loads. Ensuring the reliability of their operation is of great importance, because the blades of smoke exhausters are subject to wear by fly ash. The economical operation of draft machines is also of great importance. So, the efficiency (50 - 90%) depends on the rational aerodynamics of the rotor, and, consequently, the consumption for the own needs of the boiler plant.
The following machines are used in draft installations: centrifugal (radial) fans with forward-curved blades (Fig. 72a), or with backward-curved blades (Fig. 72b), and axial fans (Fig. 73).
Fans and smoke exhausters with shoulder blades curved forward, have found wide application due to the fact that even at moderate peripheral speeds, they allow you to create sufficiently high pressures. However, these machines have low efficiency (65-70%). Such forced draft machines are common in boiler plants of relatively low power.
Centrifugal draft machines with shoulder blades curved back, are the most perfect - efficiency = 85÷90%. However, the pressure increase is 2-2.5 times less than in machines with forward-curved blades.
Since the developed pressure is proportional to the square of the flow rate at the outlet of the impeller, a higher circumferential speed must be applied, which requires very careful balancing of the rotor. The dust content of the gas stream adversely affects the operation of the impeller.
Rice. 72. Centrifugal (radial) fan:
a - shoulder blades bent forward; b - shoulder blades, curved back
For boilers for power units with a capacity of 300 MW and more, as smoke exhausters, axle machines. In them, the gas moves along the axis.
Rice. 73. Axial draft machine
Axial draft machines have fairly high efficiency (about 65%). The pressure increase coefficient per stage is low, therefore, several stages are used. Power plants operate two-stage axial smoke exhausters. Due to the increased circumferential speed, axle machines have a high noise level. A large proportion of dynamic pressure creates certain difficulties in its transformation into static pressure. The small radial clearance between the blades and the casing creates additional requirements for installation and operation.
