slopes 

Decreasing the exhaust gas temperature for a smoke exhauster. A good boiler means a good chimney. Brick chimneys and modern boilers

Temperature drop flue gases can be implemented through:

Selection optimal sizes 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). Thermal power 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. The introduction of air preheating on existing installations may be difficult or not 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, and - in the case of significant load variations - complex system injection, which 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), corrosion can occur. 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 additional equipment needed to use the flue gas energy.

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.

Flue gas and air temperature entering the smoke box should not be higher than 500 ° C. You cannot overestimate the volume of the smoke box (it is difficult to create the necessary heat stress in a large smoke box), but you cannot underestimate its size - it is difficult to create the necessary vacuum in a small smoke box: it cannot cope with a large amount of flue gases and air. Each fireplace has its own smoke box according to its size. The internal surfaces of the smoke collector must be smooth." At the level of the pass, a hermetically sealed cleaning door must be installed on either side.

As noted above, the combustion of fuel in fireplaces proceeds with a multiple excess of air. The fireplace does not have an entrance door, the path of smoke from the firebox to the room is blocked by a constant flow of air directed from the room to the hearth and then through the chimney to the atmosphere. In order to pass all this volume of flue gases and air, the chimney must be of sufficient section with an extremely smooth inner surface. The cross section of the chimney must correspond to the cross section of the fireplace inlet. It is known that the higher the chimney, the more draft is created in it. This should be taken into account, but on the basis of this, the section of the chimney should not be underestimated.

According to Swedish researchers, the area ratio cross section rectangular chimney to the area of ​​the fireplace inlet with a chimney height of 5 m should be 12 percent; with a chimney height of 10 m - 10 percent.

S.V. Golovaty, engineer;
A.V. Lesnykh, senior lecturer;
d.t.s. K.A. Shtym, Professor, Deputy Head of the Department for Research, Department of Thermal Power Engineering and Heat Engineering School of Engineering, Far Eastern Federal University, Vladivostok

Chimneys operate in difficult conditions: temperature, pressure, humidity changes, aggressive effects of flue gases, wind loads and loads from their own weight. As a result of mechanical (power and temperature), chemical and combined effects, damage to chimney structures occurs.

One of the problems of converting heat sources to burning natural gas is the possibility of condensation of flue gas water vapor in chimneys. In turn, the formation of condensate on the inner surface of the chimneys and the consequences of this negative process (such as wetting of supporting structures, an increase in the thermal conductivity of the walls, defrosting, etc.) lead to the following most common damage to structures:

1) destruction of the protective layer of reinforced concrete pipes, exposure and corrosion of reinforcement;

2) destruction of brick brick pipes;

3) intense sulfate corrosion of the inner surface of the concrete of the trunk of reinforced concrete pipes;

4) destruction of thermal insulation;

5) wasteland in the lining masonry, reduction of gas tightness and strength of the lining;

6) destruction of the brickwork of the lining of reinforced concrete and brick chimneys with flakes (surface destruction, peeling. - Approx. ed.);

7) reduced strength of the monolithic lining of reinforced concrete pipes.

Many years of experience in the operation of chimneys confirms the connection of the above-described damages with condensate formation: for example, in the process of visual inspection of the internal and external surfaces of the chimney shafts of various boiler houses, the following characteristic damages were revealed: deep erosion damages almost along the entire height of the chimney; in zones of active condensation of water vapor, destruction of bricks to a depth of 120 mm is observed, although the surface of the trunk is in working condition.

It should be noted that for different types fuel, the content of water vapor in the flue gases will be different. So, the largest number moisture is contained in the flue gases of natural gas, and the smallest amount of water vapor is contained in the combustion products of fuel oil and coal (table).

Table. Composition of flue gases during natural gas combustion.

The object of the study is a brick chimney with a height of H = 80 m, designed to remove flue gases from 5 steam boilers DE-16-14. For this chimney, measurements were taken at an outdoor temperature of -5 ° C and a wind speed of 5 m/s. At the time of the measurements, two boilers were in operation, DE-16-14: st. No. 4 with a load of 8.6 t / h (53.7% of the nominal) and st. No. 5 with a load of 9.5 t/h (59.3% of the nominal), the operating parameters of which were used to set the boundary conditions. The flue gas temperature was 124 °C at the boiler st. No. 4 and 135 O C - on the boiler st. No. 5. The temperature of the exhaust gases at the inlet to the chimney was 130 ° C. The coefficient of excess air at the inlet to the chimney was α=1.31 (O 2 =5%). The total consumption of flue gases is 14.95 thousand m 3 / h.

Based on the measurement results, various modes of operation of the chimney were simulated. The measured composition and temperature of the flue gases were taken into account when calculating the characteristics of the flue gas flow. The calculation took into account meteorological and climatological conditions at the time of measurements (outside air temperature, wind speed). In the process of modeling for analysis, the operating modes of the heat source were calculated under loads and climatic conditions at the time of measurements. As is known, the temperature of condensation of water vapor in exhaust gases in chimneys begins at temperatures of the inner surface of 65-70 ° C.

According to the results of the calculation for the formation of condensate during the operation of the heat source, at the time of measurements, the temperature of the flue gases on the inner surface of the pipe was 35-70 ° C. Under these conditions, water vapor condensate may form on the entire surface of the pipe. To prevent the formation of water vapor condensate on the inner surface of the chimney, the operating mode of the boiler room equipment was selected, which will ensure sufficient flue gas flow and a temperature on the inner surface of the chimney of at least 70 ° C. To prevent the formation of condensate on the inner surface of the chimney, it is necessary to work with three boilers at rated load D nom at -20 ° C and two boilers at +5 ° C.

The figure shows the dependence of the flow of exhaust gases (with a temperature of 140 ° C) through the chimney on the outside air temperature.

Literature

1. The use of secondary energy resources / O. L. Danilov, V. A. Munts; USTU-UPI. - Ekaterinburg: USTU-UPI, 2008. - 153 p.

2. Work processes and issues of improving the convective surfaces of boiler units / N.V. Kuznetsov; Gosenergoizdat, 1958. - 17 p.

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 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 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 sulfuric 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 the gases, water vapor cools and settles on the walls in the form of tiny droplets. If this happens frequently, brickwork the walls of the smoke channels and the pipe are saturated with moisture and collapse, and black resinous deposits appear on the outer surfaces of the pipe. 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. The temperature of the dew point of combustion products of various gases is 44 -61°C.


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, also contribute to the cooling of gases and the formation of condensate. When the section of the channel of the pipe or chimney is higher than required, the flue gases rise slowly through it and the cold outside air cools them in the 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. AT 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. AT winter time years, when the outside temperature is negative, the walls of the chimney cool strongly 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 flue and ventilation ducts, it is necessary to withstand optimal thickness external walls or insulate them from the outside: plaster, close 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.

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 temperatures flue gases.

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 directly it is said that they are designed to be connected to any type of heating appliances with operating temperature exhaust gases from 80°С to 450°С.

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 a 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, metal pipe covered first with ground, and then with cover 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 the coating decorative properties in the process of preparing the cover enamel, colored pigments are introduced into the melted composition.

Pipe material

Attention. Lightweight thin-walled metal and mineral wool allows you to do without the device of a special foundation of 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 non-combustible material with a melting point greater than 1000 degrees.

Manufacturers and suppliers of enameled chimney systems offer 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 heater i.e. from bottom to top.

  1. 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.
  2. According to the requirements of fire safety standards, the pipe fit (nozzle depth) must be at least half the diameter of the outer pipe.
  3. 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.
  4. Joints of pipes with tees or bends must be fastened with clamps.
  5. Mounting brackets to the wall are installed at least 2 meters apart.
  6. Each tee is mounted on a separate support bracket.
  7. The chimney route should not have horizontal sections of more than one meter.
  8. In places where walls, ceilings and roofs pass, it is necessary to use elements that meet fire safety requirements.
  9. 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 damage to 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.