flue gas temperature. S. Golovaty, A.V. Lesnykh, K.A. Shtym, Analysis of the operating modes of the chimney when switching the boiler to natural gas combustion. Flue gas temperature reduction
A beautiful enameled stove implies a beautiful enamelled chimney.
Is it possible to install stainless steel?
New Product
These enameled chimneys are coated with a special compound of high temperature and acid resistance. Enamel withstands very high flue gas temperatures.
For example, modular chimney systems "LOKKI" production of the Novosibirsk plant "SibUniversal" have the following data:
- The operating temperature of the chimney is 450°C, a short-term increase in temperature up to 900°C is allowed.
- Able to withstand the temperature of the "furnace fire" of 1160 ° C for 31 minutes. Although the standard is 15 minutes.
Flue gas temperature
In the table, we have collected the flue gas temperature indicators of various heating appliances.
After comparison, it becomes clear to us that operating temperature of enameled chimneys 450°С not suitable for Russian stoves and wood-burning fireplaces, wood-burning sauna stoves and coal-fired boilers, but for all other types of heating appliances, this chimney is quite suitable.
In the descriptions of the chimneys of the system "Locky" so it is directly stated that they are intended for connection to any type of heating devices with an operating temperature of exhaust gases from 80 ° C to 450 ° C.
Note. We love to fire up the sauna stove red-hot to the fullest. Yes, even for a long time. That is why the temperature of flue gases is so high, and that is why fires occur so often in baths.
In these cases, especially sauna ovens, you can use thick-walled steel or cast iron pipe as the first element after the furnace. The fact is that the main part of the hot gases is cooled to an acceptable temperature (less than 450 ° C) already on the first pipe element.
What is heat resistant enamel?
Steel is a durable material, but has a significant drawback - a tendency to corrosion. In order for metal pipes to withstand adverse conditions, they are coated with protective compounds. One of the options for the protective composition is enamel, and since we are talking about chimneys, the enamel must be heat-resistant.
Please note: enameled chimneys have a two-layer coating, the metal pipe is coated first with ground and then with topcoat enamel.
To give enamel the necessary properties, special additives are introduced into the molten charge during its preparation. The basis of the ground and top enamel is the same; for the manufacture of the charge, a melt is used from:
- quartz sand;
- Kaolin;
- Potash and a number of other minerals.
But additives for cover and ground enamel are used differently. Metal oxides (nickel, cobalt, etc.) are introduced into the soil composition. Thanks to these substances, reliable adhesion of the metal to the enamel layer is ensured.
Oxides of titanium, zirconium, as well as fluorides of some alkali metals are added to the composition of the cover enamel. These substances provide not only increased heat resistance, but also the strength of the coating. And to give coverage decorative properties in the process of preparing the cover enamel, colored pigments are introduced into the molten composition
Pipe material
Attention. The light weight of thin-walled metal and mineral wool makes it possible to dispense with the construction of a special foundation for the chimney system. Pipes are mounted on brackets on any wall.
Equipment
In a double-walled version, the space between the pipes is filled with mineral (basalt) wool, which is a non-combustible material with a melting point of more than 1000 degrees.
Manufacturers and suppliers of enameled chimney systems offer a wide range of accessories:
- Pipes double-circuit and single-circuit.
- Branches are double-circuit and single-circuit.
- Tees.
- (latches) rotary with fixation.
- Roof cuts - nodes for the passage of the roof.
- Ceiling cuts - nodes for the passage of the ceiling.
- Umbrellas.
- Headlines.
- Plugs.
- Flanges, including decorative ones.
- Protective screens.
- Fasteners: clamps, brackets, cleaning windows.
Mounting
In any case, we begin to mount the chimney “from the stove”, from the heater, that is, from the bottom up.
- The inner pipe of each next element goes inside the previous element. This prevents condensate or precipitation from entering the basalt insulation. BUT outer pipe, which is often called a shell, is put on the previous pipe.
- According to the requirements of fire safety standards, the pipe fit (nozzle depth) must be at least half the diameter of the outer pipe.
- Docking points are sealed with clamps or planted on a cone. This is determined by the design manufacturer. For reliable sealing, there are sealants with a working temperature of 1000 ° C.
- Joints of pipes with tees or bends must be fastened with clamps.
- Mounting brackets to the wall are installed at least 2 meters apart.
- Each tee is mounted on a separate support bracket.
- The chimney route should not have horizontal sections of more than one meter.
- In places where walls, ceilings and roofs pass, it is necessary to use elements that meet fire safety requirements.
- Chimney routes should not come into contact with gas, electricity and other communications.
In the course of the installation work reasonable care must be taken. It is recommended to use only a rubberized tool, this will avoid violation of the integrity of the pipe coating (chips, cracks). This is very important, since a corrosion process begins to develop at the site of damage to the enamel, destroying the pipe.
In general, we can say that such chimneys have undoubted aesthetic advantages compared to stainless ones. But there are no technical, operational and installation advantages.
Flue gas temperature reduction can be achieved by:
Selection of the optimal dimensions and other characteristics of the equipment based on the required maximum power, taking into account the estimated safety margin;
Intensification of heat transfer to the technological process by increasing the specific heat flux (in particular, with the help of swirlers-turbulizers that increase the turbulence of the working fluid flows), increasing the area or improving the heat exchange surfaces;
Flue gas heat recovery using an additional technological process (for example, heating additional feed water using an economizer);
. installation of an air or water heater, or the organization of fuel preheating due to the heat of flue gases. It should be noted that air heating may be necessary if technological process requires high flame temperature (e.g. in glass or cement industry). Heated water can be used to feed the boiler or in hot water supply systems (including centralized heating);
Cleaning of heat exchange surfaces from accumulating ash and carbon particles in order to maintain high thermal conductivity. In particular, soot blowers can be used periodically in the convection zone. Cleaning of heat exchange surfaces in the combustion zone is usually carried out during the shutdown of equipment for inspection and maintenance, but in some cases cleaning without shutdown is used (for example, in refinery heaters);
Ensuring the level of heat production corresponding to existing needs (not exceeding them). The heat output of the boiler can be adjusted, for example, by selecting the optimal capacity of the nozzles for liquid fuel or the optimal pressure under which gaseous fuel is supplied.
Possible problems
Flue gas temperature reduction under certain conditions may conflict with air quality objectives, for example:
Preheating the combustion air leads to an increase in the temperature of the flame and, as a result, to a more intensive formation of NOx, which can lead to exceeding the established emission standards. Implementing air pre-heating in existing installations can be difficult or cost-effective due to lack of space, the need for additional fans, and NOx suppression systems (if there is a risk of exceeding regulations). It should be noted that the method of suppressing the formation of NOx by injecting ammonia or urea involves the risk of introducing ammonia into the flue gases. Preventing this may require the installation of expensive ammonia sensors and an injection control system, as well as, in the case of significant load variations, a complex injection system that allows the substance to be injected into an area with the correct temperature (for example, systems of two groups of injectors installed at different levels);
Gas cleaning systems, including NOx and SOx suppression or removal systems, operate only within a certain temperature range. If established emission standards require the use of such systems, the organization of their joint operation with recovery systems may be difficult and cost-inefficient;
In some cases, local authorities set a minimum flue gas temperature at the outlet of the pipe to ensure adequate flue gas dispersion and the absence of a flue flare. In addition, companies may, on their own initiative, apply such practices to improve their image. The general public may interpret the presence of a visible smoke plume as a sign of pollution environment, while the absence of a smoke plume can be seen as a sign of cleaner production. Therefore, under certain weather conditions, some enterprises (for example, waste incinerators) can specially heat the flue gases before being released into the atmosphere, using natural gas for this. This results in wasted energy.
energy efficiency
The lower the flue gas temperature, the higher the level of energy efficiency. However, reducing the temperature of gases below a certain level can be associated with some problems. In particular, if the temperature is below the acid dew point (the temperature at which water and sulfuric acid condense, typically 110-170°C depending on the sulfur content of the fuel), this can lead to corrosion of metal surfaces. This may require the use of materials that are resistant to corrosion (such materials exist and can be used in installations using oil, gas or waste as fuel), as well as the organization of the collection and processing of acid condensate.
The payback period can range from less than five years to fifty years, depending on many parameters, including plant size, flue gas temperature, etc.
The strategies listed above (with the exception of periodic cleaning) require additional investment. The optimal period for making a decision on their use is the period of design and construction new installation. At the same time, it is also possible to implement these solutions at an existing enterprise (provided that the necessary space for equipment installation is available).
Some applications of flue gas energy may be limited due to the difference between the temperature of the gases and the specific temperature requirement at the inlet of the energy consuming process. The acceptable value of this difference is determined by the balance between energy saving considerations and the cost of optional equipment necessary to use the energy of flue gases.
The practical possibility of recovery always depends on the availability of a possible application or consumer for the recovered energy. Measures to reduce the flue gas temperature can lead to an increase in the formation of some pollutants.
A modern chimney is not just a pipe for removing combustion products, but an engineering structure, on which the efficiency of the boiler, the efficiency and safety of the entire heating system directly depend. Smoke, back draft and, finally, a fire - all this can happen as a result of an ill-conceived and irresponsible attitude to the chimney. That is why you should take seriously the selection of material, components and installation of the chimney. The main purpose of the chimney is to remove the products of combustion of fuel into the atmosphere. The chimney creates draft, under the influence of which air is formed in the furnace, which is necessary for the combustion of fuel, and combustion products are removed from the furnace. The chimney must create conditions for complete combustion of fuel and excellent traction. And yet it must be reliable and durable, easy to install and durable. And therefore, choosing a good chimney is not as easy as we think.
Brick chimneys and modern boilers
Local resistances in a rectangular chimney
Few people know that the only correct shape of the chimney is a cylinder. This is due to the fact that the swirls formed in right angles prevent the removal of smoke and lead to the formation of soot. All home-made chimneys of square, rectangular and even triangular shapes not only turn out to be more expensive than even a steel round chimney, but also create a lot of problems, and most importantly, they can reduce the efficiency of the best boiler from 95 to 60%
Round section of the chimney
Old boilers operated without automatic control and with high flue gas temperatures. As a result of this, the chimneys almost never cooled down, and the gases did not cool below the dew point and, as a result, did not spoil the chimneys, but at the same time a lot of heat was wasted for other purposes. In addition, this type of chimney has a relatively low draft due to the porous and rough surface.
Modern boilers are economical, their power is regulated depending on the needs of the heated premises, and therefore, they do not work all the time, but only during periods when the temperature in the room drops below the set one. Thus, there are periods of time when the boiler does not work, and the chimney cools down. The walls of the chimney, working with a modern boiler, almost never heat up to a temperature above the dew point, which leads to a constant accumulation of water vapor. And this, in turn, leads to damage to the chimney. An old brick chimney can collapse under new working conditions. Since the exhaust gases contain: CO, CO2, SO2, NOx, the temperature of the exhaust gases of wall-mounted gas boilers is quite low - 70 - 130 °C. Passing through a brick chimney, the exhaust gases cool down and when the dew point reaches ~ 55 - 60 ° C, condensate falls. Water, settling on the walls in the upper part of the chimney, will cause them to get wet, in addition, when connected
SO2 + H2O = H2SO4
formed sulphuric acid, which can lead to the destruction of the brick channel. To avoid condensation, it is advisable to use an insulated chimney or install a stainless steel pipe into an existing brick channel.
Condensation
At optimal conditions operation of the boiler (flue gas temperature at the inlet 120-130°C, at the exit from the mouth of the pipe - 100-110°C) and a heated chimney, water vapor is carried away together with the flue gases to the outside. When the temperature on the inner surface of the chimney is below the dew point temperature of gases, water vapor cools and settles on the walls in the form of tiny droplets. If this is repeated frequently, the brickwork of the flue and chimney walls will become soaked with moisture and collapse, and black tarry deposits will appear on the outer surfaces of the chimney. In the presence of condensate, the draft sharply weakens, the smell of burning is felt in the rooms.
Outgoing flue gases, as they cool in the chimneys, decrease in volume, and water vapor, without changing in mass, gradually saturates the outgoing gases with moisture. The temperature at which water vapor completely saturates the volume of exhaust gases, that is, when their relative humidity is equal to 100%, is the dew point temperature: the water vapor contained in the combustion products begins to turn into a liquid state. Dew point temperature of combustion products various gases- 44 -61 ° С.
Condensation
If the gases passing through smoke channels, are strongly cooled and lower their temperature to 40 - 50 ° C, then water vapor, formed as a result of evaporation of water from the fuel and combustion of hydrogen, settles on the walls of the channels and the chimney. The amount of condensate depends on the flue gas temperature.
Cracks and holes in the pipe, through which cold air enters, also contribute to the cooling of gases and the formation of condensate. When the cross-section of the pipe or chimney channel is higher than required, flue gases rise slowly and coldly through it. outdoor air cools them in a pipe. The surface of the walls of the chimneys also has a great influence on the traction force, the smoother they are, the stronger the draft. Roughness in the pipe helps to reduce traction and trap soot on itself. The formation of condensate also depends on the wall thickness of the chimney. Thick walls warm up slowly and retain heat well. Thinner walls heat up faster, but retain heat poorly, which leads to their cooling. The thickness of the masonry brick walls of the chimneys passing through internal walls building, must be at least 120 mm (half a brick), and the thickness of the walls of smoke and ventilation ducts located in the outer walls of the building must be 380 mm (one and a half bricks).
Outside air temperature has a great influence on the condensation of water vapor contained in gases. IN summer time years, when the temperature is relatively high, condensation on the inner surfaces of the chimneys is too small, since their walls cool for a long time, therefore, moisture evaporates instantly from the well-heated surfaces of the chimney and no condensate forms. In the winter season, when the outside temperature is negative, the walls of the chimney are very cool and the condensation of water vapor increases. If the chimney is not insulated and becomes very cold, increased condensation of water vapor occurs on the inner surfaces of the chimney walls. Moisture is absorbed into the walls of the pipe, which causes dampness of the masonry. This is especially dangerous in winter, when ice plugs form in the upper sections (at the mouth) under the influence of frost.
Chimney icing
It is not recommended to attach hinged gas boilers to chimneys of large cross-sections and heights: draft weakens, increased condensate forms on internal surfaces. The formation of condensate is also observed when boilers are connected to very high chimneys, since a significant part of the flue gas temperature is spent on heating a large heat absorption surface.
Chimney insulation
To avoid supercooling of flue gases and condensation on the internal surfaces of smoke and ventilation ducts, it is necessary to maintain the optimal thickness of the outer walls or insulate them from the outside: plaster, cover with reinforced concrete or cinder-concrete slabs, shields or clay bricks.
Steel pipes must be pre-insulated or insulated. The type and thickness of the insulation will help you choose any manufacturer.
Table. B.2
| t, C | , kg/m3 | , J/(kgK) | , [W/(m K)] | , m2 /from | Pr |
| 100 | 0,950 | 1068 | 0,0313 | 21,54 | 0,690 |
| 200 | 0,748 | 1097 | 0,0401 | 32,80 | 0,670 |
| 300 | 0,617 | 1122 | 0,0484 | 45,81 | 0,650 |
| 400 | 0,525 | 1151 | 0,0570 | 60,38 | 0,640 |
| 500 | 0,457 | 1185 | 0,0656 | 76,30 | 0,630 |
| 600 | 0,505 | 1214 | 0,0742 | 93,61 | 0,620 |
| 700 | 0,363 | 1239 | 0,0827 | 112,1 | 0,610 |
| 800 | 0,330 | 1264 | 0,0915 | 131,8 | 0,600 |
| 900 | 0,301 | 1290 | 0,0100 | 152,5 | 0,590 |
| 1000 | 0,275 | 1306 | 0,0109 | 174,3 | 0,580 |
| 1100 | 0,257 | 1323 | 0,01175 | 197,1 | 0,570 |
| 1200 | 0,240 | 1340 | 0,01262 | 221,0 | 0,560 |
Task number 5. Heat transfer by radiation
Pipe wall diameter d= …[mm] heated to temperature t1 =…[°C] and has a coefficient of thermal radiation. The pipeline is placed in a channel with a cross section bXh[mm] whose surface has a temperature t2 =…[°C] and emissivity c2 = … [W/(m2 K4 )] .Calculate the reduced emissivity and heat loss Q pipeline due to radiant heat transfer.
The conditions of the task are given in Table 5.
The values of the thermal emissivity of materials are given in Table B.1 of Appendix B.
Task options
Table. five
| №tasks | d, [mm] | t1 , [°С] | t2 , [°С] | c2 ,[W/(m2 K4 )]. | bXh, [mm] | Pipe material |
| 1 | 400 | 527 | 127 | 5,22 | 600x800 | oxidized steel |
| 2 | 350 | 560 | 120 | 4,75 | 480x580 | aluminumrough |
| 3 | 300 | 520 | 150 | 3,75 | 360x500 | concrete |
| 4 | 420 | 423 | 130 | 5,25 | 400x600 | cast iron |
| 5 | 380 | 637 | 200 | 3,65 | 550x500 | brass oxidized |
| 6 | 360 | 325 | 125 | 4,50 | 500x700 | oxidized copper |
| 7 | 410 | 420 | 120 | 5,35 | 650x850 | polished steel |
| 8 | 400 | 350 | 150 | 5,00 | 450x650 | oxidized aluminum |
| 9 | 450 | 587 | 110 | 5,30 | 680x580 | polished brass |
| 10 | 460 | 547 | 105 | 5,35 | 480x600 | polished copper |
| 11 | 350 | 523 | 103 | 5,20 | 620x820 | rough steel |
| 12 | 370 | 557 | 125 | 5,10 | 650x850 | turned cast iron |
| 13 | 360 | 560 | 130 | 4,95 | 630x830 | polished aluminum |
Table continuation. five
| 14 | 250 | 520 | 120 | 4,80 | 450x550 | brass rolling |
| 15 | 200 | 530 | 130 | 4,90 | 460x470 | polished steel |
| 16 | 280 | 540 | 140 | 5,00 | 480x500 | rough cast iron |
| 17 | 320 | 550 | 150 | 5,10 | 500x500 | oxidized aluminum |
| 18 | 380 | 637 | 200 | 3,65 | 550x500 | polished brass |
| 19 | 360 | 325 | 125 | 4,50 | 500x700 | polished copper |
| 20 | 410 | 420 | 120 | 5,35 | 650x850 | rough steel |
| 21 | 400 | 350 | 150 | 5,00 | 450x650 | turned cast iron |
| 22 | 450 | 587 | 110 | 5,30 | 680x580 | polished aluminum |
| 23 | 460 | 547 | 105 | 5,35 | 480x600 | brass rolling |
| 24 | 350 | 523 | 103 | 5,20 | 620x820 | oxidized steel |
| 25 | 370 | 557 | 125 | 5,10 | 650x850 | aluminumrough |
| 26 | 450 | 587 | 110 | 5,30 | 450x650 | concrete |
| 27 | 460 | 547 | 105 | 5,35 | 680x580 | cast iron |
| 28 | 350 | 523 | 103 | 5,20 | 480x600 | brass oxidized |
| 29 | 370 | 557 | 125 | 5,10 | 620x820 | oxidized copper |
| 30 | 280 | 540 | 140 | 5,00 | 480x500 | polished steel |
Neighbor files in item [UNSORTED]
Source: https://StudFiles.net/preview/5566488/page:8/
7. Gas-air path, chimneys, flue gas cleaning
Gasman - industrial gas equipment Directory GOST, SNiP, PB SNiP II-35-76 Boiler plants
7.1. When designing boiler rooms, draft installations (smoke exhausters and blowers) should be adopted in accordance with the specifications of manufacturers. As a rule, draft units should be provided individually for each boiler unit.
7.2. Group (for individual groups boilers) or common (for the entire boiler house) forced draft installations may be used when designing new boiler houses with boilers with a capacity of up to 1 Gcal / h and when designing reconstructed boiler houses.
7.3. Group or common draft installations should be designed with two smoke exhausters and two draft fans. The design capacity of the boilers, for which these installations are provided, is ensured by the parallel operation of two smoke exhausters and two blowers.
7.4. The choice of draft units should be made taking into account the safety factors for pressure and performance in accordance with App. 3 to these rules and regulations.
7.5. When designing draft installations to control their performance, it is necessary to provide guide vanes, induction couplings and other devices that provide economical methods of regulation and are supplied complete with equipment.
7.6.*
The design of the gas-air path of boiler houses is carried out in accordance with the standard method of aerodynamic calculation of boiler plants of the TsKTI im. I. I. Polzunova.
For built-in, attached and roof boilers, openings for supplying combustion air should be provided in the walls, usually located in the upper zone of the room. The dimensions of the open section of the openings are determined based on ensuring the air velocity in them is not more than 1.0 m/s.
7.7. The gas resistance of mass-produced boilers should be taken according to the manufacturer's data.
7.8. Depending on the hydrogeological conditions and layout solutions of boiler units, external gas ducts should be provided underground or aboveground. Gas ducts should be made of brick or reinforced concrete. The use of above-ground metal gas ducts is allowed as an exception, subject to an appropriate feasibility study.
7.9. Gas and air pipelines inside the boiler room can be designed as steel, round section. Rectangular gas ducts may be provided at the junction with rectangular equipment elements.
7.10. For sections of gas ducts where ash accumulation is possible, devices for cleaning should be provided.
7.11. For boilers operating on sour fuel, if there is a possibility of condensate formation in the gas ducts, corrosion protection of the internal surfaces of the gas ducts should be provided in accordance with building codes and rules for the protection of building structures from corrosion.
CHIMNEY
7.12. Chimneys of boiler rooms should be built according to standard projects. When developing individual projects of chimneys, it is necessary to be guided by technical solutions adopted in standard projects.
7.13. For the boiler room, it is necessary to provide for the construction of one chimney. It is allowed to provide two or more pipes with appropriate justification.
7.14.* The height of the chimneys with artificial draft is determined in accordance with the Guidelines for the calculation of dispersion in the atmosphere harmful substances contained in the emissions of enterprises and Sanitary standards for the design of industrial enterprises. The height of the chimneys under natural draft is determined on the basis of the results of the aerodynamic calculation of the gas-air duct and is checked according to the conditions of dispersion of harmful substances in the atmosphere.
When calculating the dispersion of harmful substances in the atmosphere, the maximum allowable concentrations of ash, sulfur oxides, nitrogen dioxide and carbon monoxide should be taken. In this case, the amount of emitted harmful emissions is usually taken according to the data of boiler manufacturers, in the absence of these data, it is determined by calculation.
The height of the mouth of the chimneys for built-in, attached and roof boilers must be above the boundary of the wind backwater, but not less than 0.5 m above the roof, and also not less than 2 m above the roof of the higher part of the building or the tallest building within a radius of 10 m.
7.15.* The diameters of the outlet openings of steel chimneys are determined from the condition of optimal gas velocities based on technical and economic calculations. The diameters of the outlets of brick and reinforced concrete pipes are determined on the basis of the requirements of clause 7.16 of these rules and regulations.
7.16. In order to prevent the penetration of flue gases into the thickness of the structures of brick and reinforced concrete pipes, positive static pressure on the walls of the exhaust shaft is not allowed. To do this, the condition R1 must be met: increase the diameter of the pipe or use a pipe of a special design (with an internal gas-tight gas outlet shaft, with backpressure between the shaft and the lining).
7.17. The formation of condensate in the trunks of brick and reinforced concrete pipes that discharge products of combustion of gaseous fuels is allowed under all operating modes.
7.18.*
For boilers operating on gaseous fuels, the use of steel chimneys is allowed if it is not economically feasible to increase the flue gas temperature.
For autonomous boiler rooms, chimneys must be gas-tight, made of metal or non-combustible materials. The pipes must have, as a rule, external thermal insulation to prevent the formation of condensate and manholes for inspection and cleaning.
7.19.
Openings for gas ducts in one horizontal section of the pipe shaft or foundation sleeve must be evenly spaced around the circumference.
The total weakening area in one horizontal section should not exceed 40% of the total sectional area for a reinforced concrete shaft or foundation glass and 30% for a brick pipe shaft.
7.20. The supply gas ducts at the junction with the chimney must be designed in a rectangular shape.
7.21. In conjugation of gas ducts with a chimney, it is necessary to provide temperature-settlement seams or compensators.
7.22. The need to use lining and thermal insulation to reduce thermal stresses in the trunks of brick and reinforced concrete pipes is determined by heat engineering calculation.
7.23. In pipes designed to remove flue gases from the combustion of sour fuel, in the event of condensate formation (regardless of the percentage of sulfur content), a lining of acid-resistant materials should be provided along the entire height of the shaft. In the absence of condensate on the inner surface of the flue gas outlet pipe, under all operating modes, it is allowed to use lining made of clay brick for chimneys or ordinary clay brick of plastic pressing of a grade of at least 100 with a water absorption of not more than 15% on a clay-cement or complex mortar of a grade of at least 50.
7.24. The calculation of the height of the chimney and the choice of design for protecting the inner surface of its shaft from the aggressive effects of the environment should be carried out based on the conditions of combustion of the main and reserve fuel.
7.25. The height and location of the chimney must be agreed with the local Office of the Ministry of Civil Aviation. Light protection of chimneys and external marking coloring must comply with the requirements of the Manual on the Aerodrome Service in Civil Aviation of the USSR.
7.26. The projects should provide for corrosion protection of the external steel structures of brick and reinforced concrete chimneys, as well as the surfaces of steel pipes.
7.27. In the lower part of the chimney or foundation, manholes should be provided for inspecting the chimney, and, if necessary, devices that ensure the removal of condensate.
FLUE GAS CLEANING
7.28. Boilers designed to operate on solid fuels (coal, peat, oil shale and wood waste) must be equipped with flue gas cleaning units from ash in cases where
Note. When applied solid fuel as an emergency installation of ash collectors is not required.
7.29.
The choice of the type of ash collectors is made depending on the volume of gases to be cleaned, the required degree of purification and layout possibilities based on the technical and economic comparison of options for installing ash collectors various types.
As ash collecting devices should be taken:
- blocks of cyclones TsKTI or NIIOGAZ - with a volume of flue gases from 6000 to 20000 m3 / h.
- battery cyclones - with a volume of flue gases from 15,000 to 150,000 m3 / h,
- battery cyclones with recirculation and electrostatic precipitators - with a volume of flue gases over 100,000 m3 / h.
"Wet" ash collectors with low-calorie Venturi pipes with drop eliminators can be used in the presence of a hydro-ash and slag removal system and devices that exclude the discharge of harmful substances contained in ash and slag pulp into water bodies.
Volumes of gases are taken at their operating temperature.
7.30. The coefficients for cleaning ash collecting devices are taken by calculation and must be within the limits established by App. 4 to these rules and regulations.
7.31. The installation of ash collectors must be provided on the suction side of smoke exhausters, as a rule, in open areas. With appropriate justification, it is allowed to install ash collectors indoors.
7.32. Ash collectors are provided individually for each boiler unit. In some cases, it is allowed to provide a group of ash collectors or one sectioned apparatus for several boilers.
7.33. When operating a solid fuel boiler house, individual ash collectors should not have bypass gas ducts.
7.34.
The shape and inner surface of the ash catcher bunker must ensure complete ash discharge by gravity, while the angle of inclination of the bunker walls to the horizon is assumed to be 600 and, in justified cases, not less than 550 is allowed.
The ash catchers must have hermetic seals.
7.35. The speed of gases in the inlet gas duct of ash collecting installations should be taken at least 12 m/s.
7.36. "Wet" spark arresters should be used in boiler houses designed to work on wood waste, in cases where ApB≤5000. After the ash collectors, spark arresters are not installed.
Source: https://gazovik-gas.ru/directory/add/snip_2_35_76/trakt.html
Chimney Condensation and Dew Point
14.02.2013
A. Batsulin
To understand the formation of condensate in furnace chimneys, it is important to understand the concept of dew point. The dew point is the temperature at which water vapor in the air condenses into water.
At each temperature, no more than a certain amount of water vapor can be dissolved in the air. This quantity is called the saturation vapor density for a given temperature and is expressed in kilograms per cubic meter.
On fig. 1 shows a plot of saturated vapor density versus temperature. The partial pressures corresponding to these values are marked on the right. Based on the data in this table. On fig. 2 shows the initial section of the same graph.
Rice. one.
Saturated water vapor pressure.
Rice. 2.
Saturated water vapor pressure, temperature range 10 - 120 * C
Let's explain how to use the graph with a simple example. Take a pot of water and cover with a lid. After some time, under the lid, an equilibrium will be established between water and saturated water vapor. Let the temperature of the pan be 40*C, then the vapor density under the lid will be about 50 g/m3. The partial pressure of water vapor under the cover according to the table (and graph) will be 0.07 atm, the remaining 0.93 atm will be air pressure.
(1 bar = 0.98692 atm). We begin to slowly heat the pan, and at 60 * C the density of saturated steam under the lid will already be 0.13 kg / m3, and its partial pressure will be 0.2 atm. At 100 * C, the partial pressure of saturated steam under the lid will reach one atmosphere (i.e., external pressure), which means that there will no longer be air under the lid. Water will begin to boil, and steam will escape from under the lid.
In this case, the density of saturated steam under the cover will be 0.59 kg/m3. Now we close the lid hermetically (i.e., turn it into an autoclave) and insert a safety valve into it, for example, at 16 atm, and continue to heat the pan itself. The water will stop boiling, and the pressure and density of the steam under the lid will increase, and when 200*C is reached, the pressure will reach 16 atm (see graph). In this case, the water will boil again, and steam will come out from under the valve.
Now the steam density under the cover will be 8 kg/m3.
In the case of consideration of condensate precipitation from flue gases (FG), only part of the graph up to a pressure of 1 atm is of interest, since the furnace communicates with the atmosphere and the pressure in it is equal to atmospheric to within a few Pa. It is also obvious that the dew point of the DG is below 100*C.
water vapor in flue gases
To determine the dew point of flue gases (i.e. the temperature at which condensate falls out of the DG), it is necessary to know the density of water vapor in the DG, which depends on the composition of the fuel, its moisture content, excess air coefficient and temperature. The vapor density is equal to the mass of water vapor contained in 1 m3 of flue gases at a given temperature.
The formulas for the DW volume were derived in this work, section 6.1, formulas P1.3 - P1.8. After the transformations, we obtain an expression for the vapor density in the flue gases depending on the moisture content of the wood, the coefficient of excess air and temperature. The humidity of the source air makes a small correction, and is not taken into account in this expression.
The formula has a simple physical meaning. If we multiply the numerator of the big fraction by 1/(1+w), we get the mass of water in the DW, in kg per kg of wood. And if we multiply the denominator by 1/(1+w), we get the specific volume of DG in nm3/kg. The multiplier with temperatures serves to convert normal cubic meters into real ones at a temperature T. After substituting the numbers, we get the expression:
It is now possible to determine the flue gas dew point graphically. Let's superimpose the graph of the vapor density in the DW on the graph of the density of saturated water vapor. The intersection of the graphs will correspond to the dew point of the DG at the appropriate humidity and excess air. On fig. 3 and 4 show the result.
Rice. 3.
The dew point of flue gases with an excess of air is one and different moisture content of the wood.
From fig. 3 it follows that in the most unfavorable case, when wood is burned with a moisture content of 100% (half of the mass of the samples is water) without excess air, condensation of water vapor will begin at about 70 * C.
Under typical conditions for batch kilns (wood moisture 25% and excess air about 2%), condensation will begin when the flue gases cool down to 46*C. (see fig. 4)
Rice. 4.
Flue gas dew point at wood moisture content of 25% and various air excesses.
From fig. 4 also clearly shows that excess air significantly lowers the temperature of condensation. Adding excess air to the chimney is one way to eliminate condensation in pipes.
Correction for fuel composition variability
All of the above considerations are valid if the composition of the fuel remains unchanged over time, for example, gas is burned in the tolivnik or pellets are fed continuously. In the case of burning firewood in a batch oven, the composition of the flue gases changes with time. First, volatiles burn out and moisture evaporates, and then the coal residue burns out. Obviously, in the initial period, the content of water vapor in the DG will be significantly higher than calculated, and at the stage of combustion of the coal residue, it will be lower. Let's try to roughly estimate the dew point temperature in the initial period.
Let the volatiles burn out from the bookmark in the first third of the heating process, and all the moisture contained in the bookmark evaporates during this time. Then the concentration of water vapor in the first third of the process will be three times higher than the average. At 25% wood moisture and a 2-fold excess of air, the vapor density will be 0.075 * 3 = 0.225 kg/m3. (see FIG. blue graph). The condensation temperature will then be 70-75*C. This is an approximate estimate, since it is not known how the composition of the DG changes in reality as the bookmark burns out.
In addition, unburned volatiles condense from the flue gases together with water, which, apparently, will slightly increase the dew point of the DW.
Condensation in chimneys
Flue gases rising up chimney gradually cool down. When cooled below the dew point, condensation begins to form on the walls of the chimney. The cooling rate of the DG in the chimney depends on the flow area of the pipe (the area of its inner surface), the material of the pipe and its planting, as well as the intensity of combustion. The higher the burning rate, the greater the flow of flue gases, which means that, all other things being equal, the gases will cool more slowly.
The formation of condensate in the chimneys of stoves or intermittent fireplace stoves is cyclical. At the initial moment, while the pipe has not yet warmed up, condensate falls on its walls, and as the pipe warms up, the condensate evaporates. If the water from the condensate has time to evaporate completely, then it gradually impregnates brickwork chimney, and black tar deposits appear on the outer walls. If this happens on the outer section of the chimney (on the street or in a cold attic room), then the constant wetting of the masonry in winter will lead to the destruction of the stove brick.
The temperature drop in the chimney depends on its design and the amount of DG flow (fuel combustion intensity). In brick chimneys, the drop in T can reach 25 * C per linear meter. This justifies the requirement to have a DG temperature at the outlet of the furnace (“on the view”) of 200-250*C, in order to make it 100-120*C at the pipe head, which is obviously higher than the dew point. The temperature drop in insulated sandwich chimneys is only a few degrees per meter, and the temperature at the outlet of the furnace can be reduced.
Condensate, formed on the walls of a brick chimney, is absorbed into the masonry (due to the porosity of the brick), and then evaporates. In stainless steel (sandwich) chimneys, even a small amount of condensate formed in the initial period immediately begins to flow down. "for condensate".
Knowing the rate of burning wood in the stove and the cross section of the chimney, it is possible to estimate the decrease in temperature in the chimney per linear meter using the formula:
q - coefficient of heat absorption of the brick chimney walls, 1740 W/m2 S - area of the heat-receiving surface of 1 m of the chimney, m2s - heat capacity of flue gases, 1450 J/nm3*СF - flue gas flow, nm3/hV - specific volume of diesel generator, at 25% humidity wood and 2 times excess of air, 8 Nm3/kgBh - hourly fuel consumption, kg/h
The coefficient of heat absorption of the walls of the chimney is conditionally taken as 1500 kcal / m2 h, because for the last flue of the furnace, the literature gives a value of 2300 kcal/m2h. The calculation is indicative and is intended to show general patterns. On fig. 5 shows a graph of the dependence of the temperature drop in chimneys with a section of 13 x 26 cm (five) and 13 x 13 cm (four) depending on the speed of burning wood in the firebox of the stove.
Rice. five.The temperature drop in a brick chimney per linear meter, depending on the rate of burning wood in the stove (flue gas flow). The coefficient of excess air is taken equal to two.
The numbers at the beginning and at the end of the graphs indicate the speed of the DG in the chimney, calculated based on the DG flow, reduced to 150 * C, and the cross section of the chimney. As can be seen, for recommended GOST 2127-47 speeds of about 2 m/s, the DG temperature drop is 20-25*C. It is also clear that the use of chimneys with a section larger than necessary can lead to strong cooling of the DG and, as a result, condensation.
As follows from Fig. 5, a decrease in the hourly consumption of firewood leads to a decrease in the flow of exhaust gases, and, as a result, to a significant drop in temperature in the chimney. In other words, the temperature of the exhaust gases, for example, at 150 * C for a brick oven of periodic action, where firewood is actively burning, and for a slow-burning (smoldering) stove are not at all the same thing. Somehow I had to observe such a picture, fig. 6.
Rice. 6.
Condensation in a brick chimney from a stove long burning.
Here, a smoldering furnace was connected to a brick pipe with a brick section. The burning rate in such a furnace is very low - one bookmark can burn for 5-6 hours, i.e. the burning rate will be about 2 kg/h. Of course, the gases in the pipe cooled below the dew point and condensate began to form in the chimney, which soaked the pipe through and dripped onto the floor when the stove was fired. Thus, long-burning stoves can only be connected to insulated sandwich chimneys.
