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 found that heat loss during 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 in environment q 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 it is translated into 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. When enough great importance 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 enough accurate experimental definition actual valueq 5 (in contrast to the definition adopted during testing of boilersq 5 as a residual member 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 separate sections of the brickwork. 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 proportion of these heat-releasing surfaces in total amount boiler heat lossunit 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, radiating heat Each other.
Designed guidelines determine both the method of measuring specific heat fluxes and the classification of all heat-releasing surfaces of the 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.
This scheme makes it possible to evaluate heat loss 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 boilergregate - a large number of points on each relatively small separate area - 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 labor intensity 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 are made characteristic areas brickwork and an inventory of the main heat-insulating structures (boxes, pipelines, etc.).
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 air ducts long distance it is recommended to make sketches on which the measurement points are marked.
B. Testing
8. Thermal testing Bricking is 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 sensors to measuring device extension cords are used that allow one measuring device to serve sensors located within a radius of approximately 10 m. Sensors are connected to the measuring device using an extension cord through plug connectors and, after reading the readings, are moved to a new location, which ensures measurement flow. 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 everyone a separate 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; in) average value 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 per average density heat flowq 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) short description boiler unit; 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; in) pivot tables 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 body. 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. . A source is installed to power the sensor heater 1 direct current 2 - 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 to make one measurement, determined by the inertia of the sensor body and stability external conditions heat transfer, when using a flat sensor is 3 - 8 min, when using a sensor with a rubber plate due to the relatively low thermal conductivity rubber - 20 - 30 min. AT last 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 device. 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 must not be subjected to any mechanical influences: pressure, friction, shock. Appendix 2
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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 ). « Power stations", 1965, No. 2.
This heat flux is described by the equation:
Q*= | T1−T2 | |||
ln(R02 | /R01) |
|||
2πλL | ||||
A convenient characteristic of the heat flux intensity for a pipe that does not depend on the radius cylindrical surface, is the linear (linear) heat flux density q l:
q l \u003d | T − T | |||||||||||||||
log(R 02 /R 01 ) | ||||||||||||||||
ln(R | /r) | - linear |
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thermal resistance of the pipe. |
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For multilayer pipe | ||||||||||||||||
q l \u003d | T 1 − T n +1 | |||||||||||||||
log(R 0,i +1 | / R 0, i ) | |||||||||||||||
i=1 | 2πλi | |||||||||||||||
For the heat transfer process, the heat flux density q l passing through a multilayer pipe is determined by the equation:
q l \u003d | T cf1 | − T av2 | ||||||||||||||||
+ ∑ | 0, i + 1 | |||||||||||||||||
2π R 01α 1i =1 | 2πλi | R0,i | 2πR 02 α2 |
|||||||||||||||
– external thermal resistances. |
||||||||||||||||||
2πRα | 2πR | |||||||||||||||||
If you enter the notation:
K l \u003d | ||||||||||
+ ∑ | 0,i | |||||||||
2π R 01α 1i =1 | 2πλi | R0,i | 2πR 02 α2 |
|||||||
then equation (5.6) takes the form:
q l \u003d K l (T cf. 1− T cf. 2) ,
where K l - linear coefficient heat transfer [W/(m K)]. Temperature difference between medium and contacting
surface is determined by the equations:
− T | |||||||||||
2πRα | |||||||||||
− T | |||||||||||
2πR 02 α1 | |||||||||||
EXAMPLES
1. The lining of the steam boiler furnace consists of two layers.
The inner layer is made of fireclay bricks: δ 1 \u003d 400 mm, λ 1 \u003d 1.4 W / (m K), and the outer layer is made of red brick: δ 2 \u003d 200 mm,
λ 2 =0.58 W/(m·K). The temperature of the internal and | outer surface |
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brickwork, respectively T 1 = | 900 ° C and T 3 \u003d 90 ° C. |
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Determine heat loss | through brickwork and the greatest |
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temperature T 2 red brick. | ||||||||||||||
Solution. | ||||||||||||||
For determining | heat q we use the equation |
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(5.1) for n = 2.0: | ||||||||||||||
T 1 - T 3 | 900 - 90 | 1292 W/m2. |
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400×10-3 | 200×10-3 | |||||||||||||
λ 1λ 2 | ||||||||||||||
To determine the temperature at the boundary of the outer and inner layers of the lining (T 2 ), we use equation (5.2):
T − T | |||||||
Hence T | T- | δ 1 q \u003d 900- | 400.10- 3 | × 1292= 530o C. |
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2. Determine the heat loss Q [W] through a wall of red |
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brick [λ = | length l = 5 m, height h = 4 m and |
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thickness δ = 510 mm, if the air temperature inside the room
T cf2 = - 30 ° C, heat transfer coefficient from the outer surface of the wall α 2 = 20 W / (m2 K). Calculate also the temperatures on the wall surfaces T p1 and T p2.
Solution. | |||||||||||||||
Using the equation | (5.3) for n = | 1, find the density |
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heat flux: | |||||||||||||||
T av1− T av2 | 18 - (- 30) | 58.5 W/m2. |
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510×10-3 | |||||||||||||||
α1 λ α2 | |||||||||||||||
Therefore, the heat loss through the wall will be equal to:
Q \u003d q S \u003d 58.5 5 4 \u003d 1170 W.
To determine the temperatures of the wall surfaces, we use equations (5.4). Of these follows:
q=18- | × 58.5 \u003d 10.4 ° C |
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q = -30 - | × 58.5 \u003d - 27.1 ° C. |
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3. Determine heat consumption q l through the pipe wall (d 1 / d 2 =
= 20/30 mm) made of heat-resistant steel, thermal conductivity
which λ \u003d 17.4 W / (m K), and the temperatures of the outer and inner surfaces T 1 \u003d 600 ° C, T 2 \u003d 450 ° C.
Solution.
To determine the heat flow through the pipe wall, we use equation (5.5) for n = 1:
T1−T2 | 600 - 450 | 40750 W/m. |
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log(R 02 /R 01 ) | × 10-2 | ||||||||||||
× 3.14 | × 17.4 | × 10 | |||||||||||
4. Calculate the heat loss from 1 m of uninsulated pipe |
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diameter d 1 / d 2 = 300/330 mm, laid on an open |
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air, if water flows inside the pipe with an average temperature T cp1 = 90 ° C. Ambient air temperature T cp2 = - 15 ° C. The coefficient of thermal conductivity of the pipe materialλ = 50 W / (m K), the coefficient of heat transfer from water to the pipe wall α 1 = 1000 W/(m2 K) and from pipe to ambient air α 2 = 12 W/m2 K. Determine also the temperatures on the inner and outer surfaces of the pipe.
Solution. | ||||||||||||||||||||||
Heat loss from 1.0 m | pipeline | find using |
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using equation (5.6) for n = 1: | ||||||||||||||||||||||
q l \u003d | T av1− T av2 | |||||||||||||||||||||
2πRα | 2πRα | |||||||||||||||||||||
90 - (- 15) | ||||||||||||||||||||||
16.5×10-2 | ||||||||||||||||||||||
2×3.14×15×10−2×103 | 2×3.14×50 | 15×10-2 | 2×3.14×16.5×10- 2×12 | |||||||||||||||||||
652 W/m.
×652 | 89.8o C, |
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cf1 2π R 01 α 1 | 2π × 15 × 10- 2 × 103 | |||||||||||||||||||
and from (5.5) we find: | ||||||||||||||||||||
ln(R | / R) = 89.8 - | |||||||||||||||||||
16.5×10-2 | × 652 \u003d 89.6o C. | |||||||||||||||||||
2π × 50 | 15×10-2 | |||||||||||||||||||
TASKS | ||||||||||||||||||||
Determine the coefficient of thermal conductivity | brick |
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wall thickness | δ = 390 mm if the temperature is at | internal |
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wall surface T 1 = 300 ° C and on the outer T 2 = 60 ° C. |
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Heat loss through the wall | q = 178 W/m2. | |||||||||||||||||||
5.2. Through the flat metal wall of the boiler furnace
with a thickness δ = 14 mm, a specific heat flux q = 25000 W/m2 passes from gases to boiling water. Thermal conductivity coefficient of steel λ = 50 W/(m K).
Determine the temperature difference across the wall surfaces.
5.3. Determine the specific heat flux through a concrete wall with a thickness of δ = 300 mm, if the temperatures on the inner and outer surfaces of the wall, respectively, are T 1 = 15 ° C and
T 2 \u003d - 15 ° C.
Thermal conductivity coefficient of concrete λ = 1.0 W/(m K).
5.4. Determine the heat loss q through the roof of the fiery furnace,
5.5. Determine the heat consumption Q [W] through a brick wall with a thickness of δ \u003d 250 mm on an area of 3 × 5 m2, if the temperatures
wall surfaces | T1= | and T 2 | and coefficient |
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thermal conductivity of a brick λ = 1.16 BT / (m K). | ||||||
5.6. Calculate the heat flux density q | through the flat |
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uniform machine tool, thickness | much less wide |
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us and heights, if | completed: | |||||
a) from steel λ st \u003d 40 W / (m K); from | λ b = 1.1 W / (m K); c) from |
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diatomite brick λ k \u003d 0.11 W / (m K). In all cases, the thickness
The inner layer is made of refractory bricks with a thickness of δ 1 = 350 mm, and the outer layer is made of red brick with a thickness of δ 2 = 250 mm.
Determine the temperature on the inner surface of the wall T 1 and on the inner side of the red brick T 2, if on the outer side the wall temperature T 3 \u003d 90 ° C, and the heat loss through 1 m2 of the wall surface is 1 kW. The thermal conductivity coefficients of refractory and red bricks are respectively equal to:
bricks and diatomite filling between them. The diatomite filling has a thickness of δ 2 = 50 mm and λ 2 = 0.14 W/(m·K), and the red brick has δ 3 = 250 mm and λ 3 = 0.7 W/(m·K).
By how many times is it necessary to increase the thickness of the red brick so that the lining of the furnace without diatomite backfill has the same internal thermal resistance as with backfill?
5.9. Determine the heat flux q through the surface of the steel wall of the boiler [δ 1 \u003d 20 mm, λ 1 \u003d 58 W / (m K)], covered with a layer of scale
[δ 2 \u003d 2 mm, λ 2 \u003d 1.16 W / (m K)]. The highest temperature of the wall surface is 250°C, and lowest temperature scale 100 ° C. Also determine the highest temperature of the scale.
5.10. Calculate the heat flow through 1 m2 of the clean heating surface of the steam boiler and the temperature on the wall surfaces, if the following values are given: flue gas temperature T cp1 = = 1000 ° C, boiling water temperature T cp2 = 200 ° C, heat transfer coefficients from gases to the wall α 1 = 100 W / (m2 K) and from the wall to boiling water α 2 = 5000 W / (m2 K). The thermal conductivity coefficient of the wall materialλ = 50 W/(m K) and the wall thickness δ = 12 mm.
5.11. Solve problem 10 under the condition that during operation the heating surface of the steam boiler from the side of the flue gases was covered with a layer of soot with a thickness of δ c = 1 mm
[ λ s = 0.08 W/(m K)], and from the water side - a layer of scale with a thickness of δ n = 2 mm [λ n = 0.8 W/(m K)]. Calculate the heat flow through 1 m2
contaminated heating surface and temperature on the surfaces of the respective layers T p1 , T p2 , T p3 and T p4 .
Compare the calculation results with the answer to problem 10 and determine the decrease in the heat load q (in %).
5.12. Determine the heat flux density q [W / m2] through a brick wall 510 mm thick with a coefficient of thermal conductivity λ k \u003d 0.8 W / (m K), covered on the outside with a layer of thermal insulation
heat transfer from the outer surface α 2 \u003d 20 W / (m2 K). Calculate also the temperatures on the surfaces of the wall T p1, T p2 and on the surface of the layer T p3.
5.13. The steam heater coils are made of heat-resistant steel pipes with a diameter of d 1 / d 2 = 32/42 mm with a coefficient
Calculate the specific heat flux through the wall per unit length of the pipe q l.
5.14. Reinforced concrete chimney covered with inside refractory lining layer λ1 = 0.5 W/(m K).
Determine the thickness of the lining δ 1 and the temperature of the outer surface of the pipe T 3, provided that the heat loss does not exceed q l = 2000 W / m, and the highest temperatures of the lining and concrete do not exceed T 1 = 421 ° C and T 2 = 200 ° C.
5.15. Steel steam pipeline covered with two layers of thermal insulation same thickness[δ = 50 mm, λ2 = 0.07 W/(m K), λ3 = 0.14 W/(m K)].
Determine the heat loss q l [W / m] and the temperature T 3 at the interface between these layers. Repeat these calculations, provided that the insulation of the first layer is installed in place of the second.
Temperature T 4 on the outside | surfaces are the same in both cases. |
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kova and is equal to 50 ° C. | |||||||
Determine the temperature at the boundaries of the layers of a three-layer |
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pipe insulation. The inner diameter of the pipe d = 245 mm. | |||||||
layers and thermal conductivity coefficients of insulating |
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materials | respectively | are equal: δ1 = 100 mm, δ2 = 20 mm, δ3 = 30 |
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mm, λ1 = | 0.03 W/(m K), | 0.06 W/(m K) | and λ3 = 0.12 W/(m K). |
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Temperature | internal | pipeline surface 250° С, | |||||
outer surface of the insulation 65°C. | |||||||
Define | heat flow | through the surface | |||||
steam pipeline (d 1 / d 2 \u003d 140/150), insulated with two layers of thermal
and on the outer surface of the insulation T 4 \u003d 55 ° C. | ||||
How will heat loss through an insulated wall change, | ||||
swap insulating layers? | ||||
5.18. Pipeline diameter d 1 /d 2 | 44/51 mm, on which |
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flowing oil, covered | thickness δ2 = 80 | |||
Thermal conductivity coefficients of pipeline material and concrete
oil to the wall α1 = 100 W/(m2 K) and from the concrete surface to air
α2 = 10 W/(m2 K).
Determine the heat loss from 1 m of the pipeline covered with concrete. 5.19. Flat aluminum sheet 0.8mm thick plates-
wall water content λ = 203.5 W/(m K). Determine the specific heat flux transferred through the wall.
5.20. Estimate heat losses from 1.0 m of a pipeline with a diameter of d 1 / d 2 = 150/165 mm, covered with an insulation layer with a thickness of δ1 = 60 mm, if the pipeline is laid in air with T cp2 = - 15 ° C and water flows through it with an average temperature T cp1 = 90° C. The thermal conductivity coefficients of the pipe material and insulation are respectively λ1 = 50 W/(m K), λ2 = 0.15 W/(m K), and the heat transfer coefficients from the insulation surface to the ambient air are α2 = 8 W/(m2 K), and from water to the pipe wall α1 = 1000 W/(m2 K). Calculate also
temperature on the outer surface of the pipe and the outer surface of the insulation.
5.21. Determine the required capacity of the auditorium heating radiators if the masonry of its outer wall (8× 4.5 m, δ = 500 mm) is made of red brick (λ = 0.7 W / m K), and surface temperatures T] = 12 ° C and T 2 = −15 ° C. (Windows are conditionally absent). What is the depth of freezing of the wall.
5.22. The window in the auditorium has double frames with a gap between panes of 60 mm. Calculate heat loss through window opening 5× 3 m, if the glass thickness is δ = 4 mm, and their temperatures correspond to
corresponding surfaces T 1 \u003d 10 ° C and T 4 \u003d -18 ° C. λ st \u003d 0.74 and
λ air = 0.0244 W / m K.
5.23 Calculate the linear density of the heat flux through the wall of the coil from pipes (d 1 / d 2 \u003d 40 / 47 mm) of heat-resistant steel
(λ \u003d 16.5 W / (m K)), if the temperatures of its internal and outer surfaces are 400°C and 600°C, respectively. At what value of the pipe radius is the temperature in the wall equal to 500 ° C.
5.24. The steel steam pipeline (d 2 = 100 and δ = 5 mm) is laid in the open air T cp2 = 20 ° С. = 0.11 W/m K).
Calculate the heat loss per linear meter of the steam pipeline and the temperature at its boundaries, if the steam temperature is T cp1 = 300°C, and the heat transfer coefficients from steam to the inner surface of the steam pipeline and from the outer surface of the second insulation layer to air are 90 and 15 W/(m2, respectively) TO).
In boilers, as well as other heating installations, not all the heat that is released during the combustion of fuel is used. Enough most of heat leaves together with combustion products into the atmosphere, part is lost through the boiler body and a small part is lost due to chemical or mechanical underburning. Mechanical underburning refers to heat loss due to the failure or entrainment of ash elements with unburned particles.
The heat balance of the boiler is the distribution of heat that is released during the combustion of fuel to useful heat used for its intended purpose, and to heat losses that occur during the operation of thermal equipment.
Scheme of the main sources of heat loss.
As the reference value of the heat input, the value that could be released at the lowest calorific value of all fuel is taken.
If the boiler uses solid or liquid fuel, then the heat balance is in kilojoules relative to each kilogram of fuel consumed, and when gas is used, relative to each cubic meter. In both cases, the heat balance can be expressed as percentage.
Heat balance equation
The equation for the heat balance of the boiler when burning gas can be expressed by the following formula:
Optimal load parameters provide high performance heating system.
- QT=Q1+Q2+Q3+Q4+Q5+Q6;
- where QT is total thermal heat that entered the boiler furnace;
- Q1 - useful heat, which is used to heat the coolant or produce steam;
- Q2 is the loss of heat that escapes into the atmosphere with the products of combustion;
- Q3 - heat loss associated with incomplete chemical combustion;
- Q4 - heat loss due to mechanical underburning;
- Q5 - heat loss through the walls of the boiler and pipes;
- Q6 - heat loss due to the removal of ash and slag from the furnace.
As can be seen from the heat balance equation, when burning gaseous or liquid fuel there are no Q4 and Q6 values, which are typical only for solid fuels.
If the heat balance is expressed as a percentage of the total heat (QT=100%), then given equation takes the form:
- 100=q1+q2+q3+q4+q5+q6.
If we divide each term of the heat balance equation from the left and right sides by QT and multiply it by 100, we get the heat balance as a percentage of the total heat input:
- q1=Q1*100/QT;
- q2=Q2*100/QT and so on.
If liquid or gaseous fuel is used in the boiler, then there are no losses q4 and q6, the boiler heat balance equation in percent takes the form:
- 100=q1+q2+q3+q5.
Each type of heat and equations should be considered in more detail.
Heat that was used for its intended purpose (q1)
Scheme of the principle of operation of a stationary heat generator.
The heat that is used for its direct purpose is considered to be that which is spent on heating the coolant, or obtaining steam with a given pressure and temperature, which is calculated from the temperature of the water entering the economizer of the boiler. The presence of an economizer significantly increases the amount of useful heat, as it allows more use the heat contained in the products of combustion.
During the operation of the boiler, the elasticity and pressure of the steam inside it increases. The boiling point of water also depends on this process. If in normal conditions The boiling point of water is 100°C, and as the vapor pressure increases, this figure increases. In this case, the steam that is in the same boiler together with boiling water is called saturated, and the boiling point of water at a given pressure saturated steam is called the saturation temperature.
If there are no water droplets in the steam, then it is called dry saturated steam. Mass fraction dry saturated steam in wet steam is the degree of dryness of the steam, expressed as a percentage. In steam boilers, steam humidity ranges from 0 to 0.1%. If the humidity exceeds these indicators, the boiler does not work in the optimal mode.
Useful heat, which is spent on heating 1 liter of water from zero temperature up to the boiling point at constant pressure, is called the enthalpy of the liquid. The heat required to convert 1 liter of a boiling liquid into a vapor state is called latent heat vaporization. The sum of these two indicators is the total heat content of saturated steam.
Heat loss with combustion products escaping into the atmosphere (q2)
This type of loss in percentage terms shows the difference between the enthalpy of the flue gases and the cold air entering the boiler. The formulas for determining these losses differ when using different types fuel substances.
The combustion of fuel oil leads to heat loss due to chemical underburning.
When using solid fuel, the losses q2 are:
- q2=(Ig-αg*Ic)(100-q4)/QT;
- where Ig is the enthalpy of gases leaving the atmosphere (kJ/kg), αg is the coefficient of excess air, Iv is the enthalpy of the air required for combustion at the temperature of its entry into the boiler (kJ/kg).
The indicator q4 is introduced into the formula because the heat released during the physical combustion of 1 kg of fuel should be taken into account, and not for 1 kg of fuel entering the furnace.
When using gaseous or liquid fuels, the same formula has the form:
- q2=((Ig-αg*Ic)/QT)*100%.
Heat losses with flue gases depend on the state of the heating boiler itself and the mode of operation. For example, when manually loading fuel into the furnace, heat losses of this type increase significantly due to the periodic influx of fresh air.
Losses of thermal energy with flue gases leaving the atmosphere increase with an increase in their temperature and the amount of air consumed. For example, the temperature of gases leaving the atmosphere in the absence of an economizer and an air heater is 250-350°C, and in their presence it is only 120-160°C, which increases the amount of useful heat several times.
Boiler wiring diagram.
On the other hand, an insufficient temperature of the outgoing combustion products can lead to the formation of water vapor condensate on the heating surfaces, which also affects the formation of ice buildup on chimneys in winter.
The amount of air consumed depends on the type of burner and the mode of operation. If it is increased from optimal value, then this leads to high content air in the exhaust gases, which additionally carries away part of the heat. This is an inevitable process that cannot be stopped, but can be brought to minimum values. AT modern realities The air flow coefficient shall not exceed 1.08 for full injection burners, 0.6 for partial air injection burners, 1.1 for forced air-mixed burners and 1.15 for externally mixed diffusion burners. The presence of additional air leaks in the furnace and boiler pipes leads to an increase in heat losses with outgoing air. Maintaining the air flow at an optimal level allows you to reduce the value of q2 to a minimum.
To minimize the value of q2, it is necessary to clean the external and inner surface boiler, ensure that there is no scale, which reduces the transfer of heat from the combusted fuel to the heating medium, comply with the requirements for the water used in the boiler, monitor the boiler and pipe connections for damage in order to prevent air inflow. The use of additional electric heating surfaces in the gas path consumes electricity. However, the savings from optimal fuel consumption will be much higher than the cost of electricity consumed.
Heat loss from chemical underburning of fuel (q3)
This type of circuit protects the heating system from overheating.
The main indicator of incomplete chemical combustion of fuel is the presence in the exhaust gases of carbon monoxide (when using solid fuel) or carbon monoxide and methane (when burning gaseous fuel). The heat loss from chemical underburning is equal to the heat that could be released during the combustion of these residues.
Incomplete combustion of fuel depends on the lack of air, poor mixing of fuel with air, a decrease in temperature inside the boiler, or when the flame of burning fuel comes into contact with the walls of the boiler. However, an excessive increase in the amount of incoming oxygen not only does not guarantee complete combustion of the fuel, but can disrupt the operation of the boiler.
The optimal content of carbon monoxide at the outlet of the furnace at a temperature of 1400°C should be no more than 0.05% (in terms of dry gases). At such values, heat loss from underburning will be from 3 to 7%, depending on the fuel. Lack of oxygen can bring this value up to 25%.
But it is necessary to achieve such conditions that there is no chemical underburning of the fuel. It is necessary to ensure optimal air supply to the furnace, maintain a constant temperature inside the boiler, and achieve thorough mixing of the fuel mixture with air. The most economical operation of the boiler is achieved when the content carbon dioxide in combustion products escaping into the atmosphere, at the level of 13-15%, depending on the type of fuel. With an excess of air intake, the content of carbon dioxide in the outgoing smoke can decrease by 3-5%, but heat loss will increase. At normal operation heating equipment losses q3 are 0-0.5% for pulverized coal and 1% for layered furnaces.
Heat loss from physical underburning (q4)
This type of loss occurs due to the fact that unburned fuel particles fall through the grate into the ash pan or are carried along with the combustion products through the pipe into the atmosphere. Heat loss from physical underburning directly depends on the design of the boiler, the location and shape of the grate, traction force, the state of the fuel and its sintering.
The most significant losses are from mechanical underburning during layered combustion of solid fuel and excessively strong traction. In this case, a large number of small unburned particles are carried away with the smoke. This is especially well manifested when using heterogeneous fuel, when small and large pieces of fuel alternate in it. The combustion of each layer turns out to be non-uniform, since small pieces burn out faster and are carried away with smoke. Air enters the resulting gaps, which cools large pieces of fuel. At the same time, they are covered with a slag crust and do not burn out completely.
Heat losses during mechanical underburning are usually about 1% for pulverized coal furnaces and up to 7.5% for layered furnaces.
Heat loss directly through the boiler walls (q5)
This type of loss depends on the shape and design of the boiler, the thickness and quality of the lining of both the boiler and the chimney pipes, and the presence of a heat-insulating screen. Besides, big influence The design of the furnace itself, as well as the presence of additional heating surfaces and electric heaters in the smoke path, have an impact on losses. These heat losses increase in the presence of drafts in the room where the heating equipment, as well as on the number and duration of the opening of the furnace and hatches of the system. Reducing the number of losses depends on the correct lining of the boiler and the presence of an economizer. Favorably, the thermal insulation of pipes through which exhaust gases are discharged into the atmosphere affects the reduction of heat losses.
Heat loss due to ash and slag removal (q6)
This type of loss is typical only for solid fuel in lumpy and pulverized state. When it is not burned, particles of uncooled fuel fall into the ash pan, from where they are removed, taking with them part of the heat. These losses depend on the ash content of the fuel and the ash removal system.
The heat balance of the boiler is a value that shows the optimal and economical operation of your boiler. By the magnitude of the heat balance, it is possible to determine measures that will help save fuel burned and increase the efficiency of heating equipment.
