Ministry of Education and Science of the Russian Federation

FGAOU VPO

Ural Federal University named after the first President of Russia B.N. Yeltsin

COURSE WORK

Verification thermal calculation of a hot water boiler

Head O.A. crayfish

Student P.A. Stadukhin

ENZ-320915s group

Yekaterinburg - 2015

Introduction

.Initial data

2.

.Thermal calculation of the boiler

3.1Estimated fuel characteristics

3.2Calculation of air volumes and combustion products

3

4Boiler thermal balance

5Thermal calculation of the furnace

6Calculation of convective beams

4.Estimated discrepancy heat balance

Conclusion

Bibliography

Introduction

This paper presents a verification thermal calculation of a hot water boiler designed to heat network water during gas combustion. A verification calculation is carried out to assess the savings and reliability of the boiler when operating on a given fuel, identify the necessary reconstructive measures, select auxiliary equipment and obtain raw materials for calculations: aerodynamic, hydraulic, metal temperature and pipe strength, pipe wear rate, corrosion, etc.

The specifics of the calculation of the boiler is the uncertainty of the intermediate temperatures of the gases and the working fluid - the heat carrier, including the temperature of the flue gases; therefore, the calculation is performed by the method of successive approximations, first setting a certain value of the temperature of the gases leaving the boiler, and then comparing it with the results of the calculation. Permissible deviations in the values ​​of this temperature should not exceed ± 5%.

1. Initial data

.Boiler brand: KV-GM-4.65-95P.

2.Fuel: Yarino-Perm gas pipeline.

.Boiler output Q to = 4.65 MW.

.Initial water temperature t 1=55about FROM.

.Maximum water temperature at the outlet of the boiler t 2=95about FROM.

.Water pressure at the boiler inlet: p 1= 12 bar.

.The boiler generates 60% of the nominal useful heat output.

2. Description of the design of the boiler and combustion device

Hot water boiler brand KV-GM-4.65-95P is designed for hot water temperature 95°C used in heating systems, hot water supply for industrial and domestic purposes.

The KV-GM type boiler is a device with no supporting frame. The pipe system has supports welded to the lower headers. The supports located at the junction of the combustion chamber and the convection shaft are fixed. Boilers of the KV-GM-4.65-95P type consist of a single pipe system.

The combustion chamber, which has a horizontal layout with direct-flow forced water movement, is shielded by pipes with a diameter of 51x4 mm, which are included in collectors with a diameter of 159x6 mm. The collectors are connected to radiative and convective heating surfaces having lightweight pipe insulation and gas-tight lining.

The convective heating surface is located in a vertical shaft and is assembled from U-shaped screens from pipes with a diameter of 28x3 mm.

The boiler is equipped with an RGMG type burner. The burner is installed on the air box of the boiler, which is attached to the shield on the front screen.

The movement of water and gas in the boiler is counter-current - network water is supplied to the convective heating surfaces and removed from the furnace screens. The movement of water is provided by a pump.

On the outlet collector of the boiler, up to the shut-off valves, there are installed: a pressure gauge, a temperature measuring device and a pipe with a shut-off device for removing air when filling the boiler. Equipped with safety valves.

The boiler is equipped with drain and air valves with shut-off valves, which ensure the possibility of removing water and sediments from the lower sections of all elements of the boiler and removing air from the upper ones.

KV-GM boilers are equipped with platform ladders for ease of maintenance.

Table 1

Technical characteristics of the boiler unit KV-GM-4.65-95P

Heat output, MW4.65 Operating pressure of water at the inlet to the boiler / at the outlet of the boiler, MPa 1.6 / 1.0 Water temperature at the inlet / outlet, ˚C70 / 150 Water flow through the boiler, t / h fuel for natural gas, m3/h501 Aerodynamic resistance, Pa, not more than 270 Excess air coefficient for natural gas according to GOST 5542, not more than 1.15 Exhaust gas temperature, ˚С130 Control range, %30 - 100 Natural gas boiler efficiency, %, not less than 94.4 Overall dimensions in lightweight insulation with metal sheathing, mm: - length along the protruding parts of the boiler block; - width along the protruding parts of the boiler block; - height from the floor level of the boiler room to the protruding parts of the boiler block 5720 2284 1985 Weight of the boiler without burner, kg, no more than 9700

3. Thermal calculation of the boiler

.1 Fuel Ratings

Fuel: Yarino-Perm gas pipeline.

CH 4 - 38

FROM 2H 6 - 25,1

FROM 3H 8 - 12,5

FROM 4H 10 - 3,3

FROM 5H 12 - 1,30

N 2 - 18,7

H 2S-1.1

Net calorific value Q n R = 46.890 MJ/m 3

Density at 0 º C and 101.3 kPa ρ = 1.196 kg/m 3

3.2 Calculation of air and combustion products volumes

The excess air coefficient increases as the combustion products move through the gas ducts of the boiler unit. This is due to the fact that the pressure in the gas ducts (for boilers operating under vacuum) is less than the ambient air pressure and suction occurs through leaks in the lining. atmospheric air into the gas path of the unit. Usually, in calculations, the temperature of the air sucked into the gas ducts is taken equal to 30 ° C.

For pressurized boilers, the excess air coefficient in the section of the duct from the furnace to the air heater is assumed to be constant.

We take the coefficient of air consumption in the furnace α t = 1.05 (2), airflow coefficient behind the convective surface α kp = α t + Δα, where Δα = 0.05 - air suction in the convective bundle (2): α wow = 1.1. Average value of the air flow coefficient α Wed = (α t + α kp )/2 = 1.075 (in the convective part).

Theoretical amount of air: V n about =12.37 m 3/h

Theoretical volumes of air and combustion products:

V n oRO2 =1.47 m 3/m 3

V n oN2 =9.96m 3/m 3

V n oH2O =2.47 m 3/m 3

V n oh g =13.9 m 3/m 3

Actual volume of water vapor:

Actual flue gas volume:

V n G = V n oRO2 +V n oN2 +V n H2O +(α i -1)V n about

Volume fraction of water vapor:

R H2O = V n H2O /V n G

Volume fraction of triatomic gases:

R RO2 = V n oRO2 /V n G

The total proportion of water vapor and triatomic gases:

R P = RH2O + R RO2

table 2

Calculation of air volumes and combustion products

No. p / p Name of the value Designation Dimension α t α Wed α wow 1. Actual volume of water vapor V H2O m 3/ m 32,4802,4852,4902. Actual volume of combustion products V G m 3/ m 314,52814,84315,1573. Volume fraction of water vapor in combustion products R H2O -0,1710,1670,1644. Volume fraction of triatomic gases in combustion products R RO2 -0,1010,0990,0975.Total fraction of water vapor and triatomic gasesR P -0,2720,2660,261

3.3 Calculation of enthalpies of air and combustion products

Table 3

Enthalpies of air and combustion products

t, оСIgo, kJ/m3Ivo, kJ/m3Ig= Igo+ Ivo (α t-1) Ig \u003d Igo + Ivo (α ух-1)30495,9100191816412000,052041,075200387633024041,14123,65400791967048254,2600122391026612752,3800167321396417430,21000211131778622002,31200262172169527301,751400310622567832345,91600360682972237554,11800411653379242854,62000463053792348201,15

3.4 Boiler thermal balance

During the operation of a hot water boiler, all the heat supplied to it is spent on generating useful heat contained in steam or hot water, and on covering various heat losses. The total amount of heat supplied to the boiler unit is called available heat and denoted by Q R . Between the heat that entered the boiler unit and left it, there is equality. The heat leaving the boiler unit is the sum of useful heat and heat losses associated with the technological process of generating steam or hot water. Therefore, the heat balance of the boiler for 1 m 3 gas under normal conditions has the form:

Q R = Q 1+Q 2+Q 3+Q 5, where

R - available heat, kJ/m 3;1- Useful heat contained in steam or hot water, kJ/m 3;2- heat loss with exhaust gases, kJ/m 3 ;3 - from chemical incompleteness of combustion, kJ/m3 ;5

Introduction

When calculating the heat balance of metallurgical furnaces, the problem often arises of determining heat losses through furnace barriers. Minimization of heat losses helps to save fuel and electricity, reduces the cost of production. In addition, for the correct choice of materials in the design of the furnace, it is necessary to know the temperature field in the wall in order to comply with the restrictions on the operating temperature of the materials. Therefore, when designing a furnace, an engineer must consider several wall design options and choose the best one from them. This article will consider a method for calculating heat losses through a flat multilayer wall of a thermal unit, described software to automate this calculation, as well as an analysis of the dependence of heat losses on various factors.

Theoretical basis

Bake- thermal technological equipment protected from the surrounding space, in which heat is generated from one or another primary type of energy and heat is transferred to the material subjected to heat treatment for technological purposes (melting, heating, drying, firing, etc.). At the same time, part of the released thermal energy is spent on the implementation of the technological process, and part is uselessly lost, heating environment. Reduction of heat losses makes it possible to increase the efficiency of furnaces and reduce energy consumption.

Part of the heat in furnaces is lost by transferring thermal conductivity through the refractory. Thermal conductivity is the process of heat transfer ( internal energy), occurring in direct contact of bodies (or parts of the body) with different temperature. Energy exchange is carried out by microparticles that make up substances: molecules, atoms, free electrons. The heat flux density of thermal conductivity depends on the temperature field and the thermal conductivity of the substance.

The set of temperature values ​​for all points of the body at a given time is called temperature field. In this case, if the temperature does not change in time, the field is considered stationary, and if it changes, it is considered non-stationary. The simplest is the case of a one-dimensional stationary temperature field.

Heat is transferred by thermal conduction from the more heated layers of the body to the less heated ones, i.e. in the direction of decreasing temperature. The amount of heat transferred through any surface per unit time is called the heat flux Q. The heat flux per unit surface characterizes the heat flux density q. According to the Fourier law, the heat flux density is proportional to the temperature gradient:

q = -λgrad t     (1.1)

where q is the heat flux density, W/m2
λ - coefficient of thermal conductivity of the material, W / (m * K)
grad t – temperature gradient, K/m

The proportionality factor λ in equation (1.1) is the thermal conductivity of the material and characterizes its ability to conduct heat. Smallest values thermal conductivity coefficients have gases, the largest - metals. In the construction of furnaces, materials are used that have a relatively low coefficient of thermal conductivity: refractory and heat-insulating materials.

Refractory called non-metallic materials intended for use at high temperatures in thermal units and having a fire resistance of at least 1580 ° C. Refractories perform the function of retaining heat in a limited volume of the working space of the furnace, and therefore they must have low thermal conductivity and the ability to withstand high temperatures. The variety of service conditions necessitated the creation of a large assortment of refractories with various properties. The most common refractories are chamotte, dinas, magnesite, chromomagnesite.

To reduce the heat flux of thermal conductivity through the laying of furnaces, heat-insulating materials, i.e. materials with low thermal conductivity. Examples of heat-insulating materials are asbestos, diatomaceous earth, slag wool, refractory lightweights. In this case, the masonry is made of several layers: the inner layers are made of materials with high thermal resistance (refractories), and the outer layers are made of less resistant materials with lower thermal conductivity (thermal insulation). When designing a furnace, it is necessary to choose the design of the furnace walls so that the amount of heat loss is minimal and the restrictions on the thermal resistance of materials are observed.

Method of calculation

The mathematical model of the problem is based on the methodology for calculating heat losses through the enclosures of thermal installations, described in the work “Calculation of heat losses through furnace enclosures” (V. B. Kutyin, S. N. Gushchin, B. A. Fetisov).

The essence of the calculation is to determine the heat flux through the wall in the stationary mode with boundary conditions III kind. It is assumed that heat transfer through the wall is carried out by thermal conductivity, and heat transfer from the outer wall to the environment is carried out by radiation and natural convection. The calculation takes into account the dependence of the coefficient of thermal conductivity of the material of the layers on temperature.

The initial data for the calculation are given in Table 1.

Table 1 - Initial data

The calculation is carried out by the method of successive approximations. Initially, an arbitrary temperature field is set. Then the thermal resistances of the layers are determined by the formula:

The heat transfer coefficient from the outer surface is determined by the formula:

The total heat flux density is calculated by the formula:

The density of the heat flux transmitted through the wall by thermal conductivity is determined by the formula:

The density of the heat flux given off by the outer surface to the environment is determined by the formula:

The refined temperature field is determined by the formula:

The iterative process continues until relative error doesn't get smaller set value. Finally, the amount of heat loss per unit time is calculated:

Heat Loss Calculation Software

To automate the calculation of heat losses through a flat multilayer furnace wall was developed. The program has a convenient graphical interface that allows you to interactively set the required design of the refractory wall and save its data in a file for later use. The calculation results are presented in the form of tables, graphs and heat maps. The program takes data on the coefficients of thermal conductivity of materials from a database that can be replenished by the user.

Heat Loss Study

With the help of convenient means of the graphical interface of the program, it is possible to analyze the influence of various factors on heat losses in the unit.

The dependence of heat losses on the thickness of the lining layer

To study the dependence of heat losses on the thickness of the lining layer, several variants of the initial data were prepared, differing only in the thickness of the lining layer. The lining material is high-alumina refractory, the material of the thermal insulation layer is lightweight chamotte. Other parameters are given in Table 2.

Study wall design

Table 2 - Variant of initial data

The study here and further was carried out using the built-in program to compare the results of the calculation. The comparison results are shown in Figure 1. It can be seen that heat losses decrease with increasing lining thickness, but only slightly.

Picture 1 - The dependence of heat losses on the thickness of the lining

Dependence of heat losses on the thickness of the thermal insulation layer

To study the dependence of heat losses on the thickness of the thermal insulation layer, several variants of the initial data were prepared, differing only in the thickness of the thermal insulation layer. The wall structure is shown in Figure 2, other parameters are the same as in the previous study (Table 2).

Picture 2 - Wall design for research

The results of the study are shown in Figure 3. It can be seen that heat losses decrease sharply with an increase in the thickness of the thermal insulation layer.

Picture 3 - Dependence of heat losses on the thickness of thermal insulation

Dependence of heat losses on the material of thermal insulation

To study the influence of the thermal insulation material, we consider several variants of the wall design, which differ only in the material of the thermal insulation. The design of the test wall is shown in Figure 4, and other parameters are shown in Table 2.

Figure 4 - Wall design for research

The results of the study are shown in Figure 5. From the diagram, we can conclude that heat losses can vary significantly depending on the material of thermal insulation, therefore right choice The latter is very important when designing furnaces. Of the selected materials, mineral wool has the best heat-insulating properties.

Figure 5 - Dependence of heat losses on the material of thermal insulation

Figures 6, 7 show more detailed results for two calculation options. It can be seen that when using more advanced thermal insulation, not only heat losses are reduced, but also the temperature of the outer surface of the wall, which improves the working conditions of the furnace staff.

Figure 6 - Calculation results for one variant of the initial data

Figure 7 - Calculation results for the second version of the initial data

Dependence of heat losses on the emissivity of the outer surface of the wall

In most cases, the outer surface of the furnace wall is represented by a casing made of mild steel, with varying degrees of corrosion. The influence of the casing on heat transfer by thermal conduction is small, but heat transfer by radiation can be influenced by applying coatings with varying degrees blackness. To study this effect, we consider several variants of the initial data, which differ only in the degree of blackness of the outer surface. The design of the wall under study is shown in Figure 8, see Table 2 for other parameters.

Figure 8 - Wall design for research

Figure 9 and Table 3 present the results of the study. The legend indicates the material of the casing and in parentheses - its degree of blackness. It can be seen that heat losses decrease with a decrease in the degree of emissivity of the outer surface to an insignificant degree. However, given that the cost of painting the furnace casing is less than the introduction of additional thermal insulation, coating the casing with light aluminum paint can be recommended to reduce heat losses.

Table 3 - Dependence of heat losses on the degree of emissivity of the outer surface

Figure 9 - Dependence of heat losses on the degree of emissivity of the outer surface

Negative effect of thermal insulation

Let us consider the effect of thermal insulation on the temperature field in the wall of a high-temperature furnace. To do this, consider two options for the design of the wall. In the first, the wall consists of a layer of magnesite, and in the second, a layer of magnesite and a layer of slag wool as thermal insulation. The temperature fields for these cases are shown in Figures 10, 11.

Figure 10 - Temperature field in the absence of thermal insulation

Figure 11 - Temperature field in the presence of thermal insulation

In the absence of thermal insulation, the temperature in the working layer of the lining changes from 472 to 1675 degrees, and in the presence of a thermal insulation layer, from 1519 to 1698. It follows that the introduction of thermal insulation leads to an increase in temperature in the lining layer, which should adversely affect its durability .

The negative effect of thermal insulation on the lining service is especially pronounced for high-temperature furnaces: arc steel-smelting, ferroalloy, etc. In the book "Electrothermal Processes and Installations" (Aliferov A.I.) ) was not widely used. Typically, such insulation leads to an increase in temperatures in the working layer of the lining and a sharp drop in its durability, especially on large EAF. Losses due to EAF downtime for lining repairs far exceed the savings from reducing power consumption due to a decrease in heat flow through the wall. Therefore, thermal insulation of walls and vaults of chipboard, as a rule, is economically unprofitable. (This provision does not apply to the design of the bottom of the chipboard, for which thermal insulation is applied).

Due to the unsatisfactory durability of refractories on large, powerful EAFs, the lining is replaced with water-cooled panels. Despite the increase in the density of the heat flux removed from the water-cooled surfaces, in comparison with the density of the heat flux through the lined surfaces, the power consumption increases significantly only in furnaces of small capacity. The use of water-cooled panels allows to increase the service life of the refractory lining.

conclusions

Based on the study, it can be concluded that the main measures to reduce heat losses through masonry will be the following:

Increasing the thickness of the thermal insulation layer
- Use of heat-insulating materials with low thermal conductivity
- Painting the housing with light aluminum paint (or coating with another material with a low degree of blackness)

For high-temperature furnaces, instead of using thermal insulation, it is advisable to use water-cooled body panels, which allow you to extend the life of the lining and save on reducing downtime for its repair.

Sources

1. Markin V.P. Calculations for heat transfer / V. P. Markin, S. N. Gushchin, M. D. Kazyaev. - Ekaterinburg: USTU-UPI, 1998. - 46 p.
2. Voronov G. V., Startsev V. A. Refractory materials and products in industrial furnaces and auxiliary facilities / G. V. Voronov, V. A. Startsev. - Yekaterinburg: USTU-UPI, 2006. - 303 p.
3. Kut'in V.B. Calculation of heat losses through furnace enclosures / V. B. Kut'in, S. N. Gushchin, B. A. Fetisov. - Yekaterinburg: USTU-UPI, 1996. - 17p.
4. Refractory materials. Structure, properties, tests. Reference book / J. Allenstein and others; ed. G. Rouchka, H. Wutnau. – M.: Intermet Engineering, 2010. – 392 p.
5. Zobnin V. F., Heat engineering calculations of metallurgical furnaces / V. F. Zobnin, M. D. Kazyaev, B. I. Kitaev et al. - M.: Metallurgy, 1982. - 360 p.
6. Aliferov A. I. Electrothermal processes and installations: Tutorial/ A. I. Aliferov and others; ed. V.N. Timofeeva, E.A. Golovenko, E.V. Kuznetsova - Krasnoyarsk: Siberian federal university, 2007. - 360 p.

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 sustainability 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 necessary 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.

At great length 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 by hinged hangers, and if they are short, they freely rest on sliding supports fixed to 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 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, while its inner surface is in a region of high and variable temperature, which decreases along the gas flow. In the direction of the gas flow, the vacuum in the gas ducts increases, and the pressure during the operation of the steam generator under pressurization decreases. The loads on the lining elements are also different from its weight and internal stresses arising from unequal temperature elongations of its parts.

In the most difficult conditions inner part lining of the furnace, exposed to high temperatures of more than 1600 ° C, and when burning solid fuels, also chemical and mechanical impact 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 diatomite bricks, insulating plates and steel cladding, fixed to the steam generator frame with metal structures;

c) light lining, made of fireclay or heat-resistant concrete slabs, heat-insulating slabs and metal sheathing or sealing coating.

Indicators specified types brickworks are characterized by the following data:

Wall lining is used for low power steam generators with a wall height of not more than 12 m. higher altitude brickwork 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 relatively large sizes of the combustion chamber and the 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 carry it out at 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 more than high temperature, have a large thickness and mass. To transfer their mass to the frame, additional embedded cast-iron brackets are provided. Frame lining is mainly used in the field of superheaters, gas turning chambers and the convective shaft of high-power steam generators. In fireboxes, frame lining is used on straight walls. The advantages of the frame construction of the brickwork are its small mass and significant relief. 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, successively 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 refractory 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. Great importance ensures the reliability of their work, 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 enough 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 enough 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. Big share dynamic pressure creates certain difficulties in its transformation into static. A small radial clearance between the blades and the casing creates Additional requirements for installation and operation.

B.Ya. Kamenetsky, host Researcher, GNU VIESKh, Moscow

In layered furnaces with cyclic fuel loading, bricking, in addition to the main function of reducing heat loss, also plays another function. special role. Due to its thermal inertia, the brickwork is sufficient for a long time retains its temperature, which contributes to the heating and ignition of fuel fractions. When loading a fresh portion, the fuel covers almost the entire surface of the layer, as a result of which the surface temperature of the layer decreases sharply, as can be seen from Fig. 1. The temperature of the gases in the furnace also decreases, and during this time interval in the furnace heat exchange system, the surface temperature of the lining is the highest. Radiation from the brick surface to the layer at these moments contributes to heating and upper ignition of the fuel.

In order to study the thermal regimes, determine the heat fluxes on the inside and heat losses, measurements were taken temperature conditions furnace linings. The work was carried out on a heating boiler with a manual layered furnace, in which the lining of fireclay bricks with a thickness of 380 mm is simultaneously a pedestal for two packages of boiler sections. The height of the pedestal is 1.2 m, including 0.5 m above the grate.

Temperature measurements were carried out using a probe - a quartz glass tube with a diameter of 8.5 mm with XA thermocouples, moved in a through hole in the side wall of the brickwork. Kuznetsk coal of grade 2SS was burned in the boiler, the furnace cycle (the time between adjacent loads) was 10 min.

The results of measurements of the non-stationary temperature of the brickwork at a thermal load of the grate of 0.55 MW/m 2 (fuel consumption - 72 kg/h) are shown in Fig. 2. 2.

The temperature on the outer surface of the lining at a height of 0.4 m from the level of the grate was 60 ° C, and on the inner surface - 800 ° C. The temperature decreases disproportionately towards the outer surface across the thickness of the brickwork, which indicates a decrease in heat flow through the brickwork as a result of leakage (flows) of heat in the vertical direction. Heat leaks occur due to uneven heating of the lining in height: the temperature of the brick in the ash pan is lower than the temperature of the grate and is 60-70 ° C, and at the upper end of the masonry in contact with the boiler sections - 80-100 ° C.

On the outer surface of the brickwork, the heat flux calculated both according to the conditions convective heat transfer with natural air convection q \u003d α ek (t n -t c), and according to the thermal conductivity of the lining q \u003d α * dt / dx gives a value of 0.5 kW / m 2, and on the inner surface - q \u003d 2.7 kW / m 2. Heat losses from the side and bottom surface of the lining are significant - 4% of the boiler power of 220 kW even with a lining thickness of 380 mm.

More larger achieve heat loss to the environment while reducing the thickness of the lining. For example, in the furnace of a heat generator with a 2 MW chimney without heat-receiving screens, an unshielded brick lining 2 m high has a thickness of only 250 mm. To ensure its reliable operation, it was necessary to increase the excess air in the furnace to a value of α=2.6. However, the temperature of the inner surface of the lining was 1100 °C at the level of 1.8 m from the grate and 900 °C at the level of 0.4 m (Fig. 3). The average heat fluxes through the brickwork increased to 2.2 kW / m 2 at the level of 0.4 m, and up to 2.6 kW / m 2 at the level of 1.8 m. In this case, the temperature difference along the height of the brickwork reaches 200 ° C on the inner surface and decreases in thickness, which leads to heat transfers from the upper layers to the lower ones.

Interesting results were recorded when this heat generator was stopped. When the fuel supply is stopped and the fan continues to operate, the heat release in the furnace decreases, which leads to a rapid cooling of the lining from the inner surface and a monotonous decrease in its temperature (Fig. 4). After 25 minutes, the heat flux directed from the furnace to the brickwork surface decreases to 0 and then changes its direction. With further cooling of the furnace and a decrease in the temperature of the inner surface of the lining, a maximum occurs in the temperature distribution over the thickness of the lining. The temperature of the layers inside the brickwork even rises, and the temperature maximum moves inward. The reason for such a deformation of the temperature field of the brickwork is associated with a more intense cooling of the inner surface, especially the lower layers, leading to large heat transfers from the upper central layers. After 45 minutes they are still heated to 300°C.

conclusions

1. In boilers with layered furnaces, the thermal inertia of the lining contributes to the heating and ignition of the loaded fuel.

2. Heat loss from the side and bottom surface of the lining (fireclay bricks) make up a significant amount - 4% of the boiler power of 220 kW, even with a lining thickness of 380 mm.

3. Due to the uneven heating of the lining along the height, heat leaks occur. If the fuel supply is interrupted while the fan is running, this leads to the fact that the maximum temperature moves inside the brickwork.

Literature

1. Kamenetsky B.Ya. On the applicability of the Normative method for calculating furnace heat transfer to layered furnaces. Teploenergetika. 2006. No. 2. S. 58-60.

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 escapes 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 the total amount of 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. The mass fraction of 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 expended to convert 1 liter of boiling liquid into a vapor state is called the latent heat of 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 of fuels.

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 exhaust 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. In modern realities, the air flow coefficient should not exceed 1.08 for burners with full injection, 0.6 for burners with partial air injection, 1.1 for burners with forced air supply and mixing, and 1.15 for diffusion burners with external mixing. An increase in heat loss with outgoing air is caused by the presence of additional air leaks in the furnace and boiler pipes. 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 of carbon dioxide in the combustion products escaping into the atmosphere is 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 losses are affected by the design of the furnace itself, as well as the presence of additional heating surfaces and electric heaters in the smoke path. These heat losses increase in the presence of drafts in the room where the heating equipment is located, as well as on the number and duration of opening of the furnace and system hatches. 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.