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When making a fireplace or stove, much attention is paid to the chimney duct, since how efficiently the entire system will function depends on this element. In this case, one of the main tasks is to perform calculations and its cross-section. These parameters determine the conditions for optimal traction depending on the type of heating device and other parameters. Today we will talk about how such calculations are performed. At the same time, we do not encourage you to do them yourself, since without practice, no theory can guarantee a high-quality result - however, new knowledge will not harm anyone.
Reasons why we perform height calculations chimney, some.
Sandwich chimney prices
Sandwich chimney
How to calculate the pipe height yourself
Next, a method of independent calculation will be given - it is focused on SNiP 41-01-2003 “Heating, ventilation and air conditioning”. According to this document, we have the following information:
- the minimum height of the chimney, which is calculated from the top to the grate, is 5 m;
- the optimal height should be 6 m.
However, this data says absolutely nothing about which parameter to choose in a particular case, for specific equipment. Therefore, experts use the following formula.
Based on the presented formula, we will derive the main parameters that are necessary for accurate calculations.
- A– meteorological conditions of the surrounding area. This refers to a certain coefficient that has already been calculated by professionals and presented in descriptive documents. For example, for the northern regions the value of this parameter is set to 160.
- Mi is the mass of substances passing in a certain unit of time. This parameter can be found in the documentation supplied with the heater.
- F– the settling rate of particles that are formed during fuel combustion. This indicator can be found in regulatory documents for the type of fuel used. Let's take firewood and an electric heater as an example. In the first case, the calculated value is 25 units, and in the second - 1.
- (Spdki and Sfi)– the concentration of various substances in the gas that needs to be removed. Both indicators are also taken from the instructions supplied with the heating equipment.
- V– volume of exhaust gas.
- T– temperature difference between the incoming air and the gas leaving the pipe.
The height of the chimney above the roof is also a very important parameter. It is determined based on the shape of the roof - all data is also taken from the above SNiP.
If the roof of the building is flat, then the height of the pipe is determined as follows.
Table. Pipe height for flat roofing.
If you have a pitched roof, then the height of the chimney will be affected by its location relative to the ridge of the roof - this means the distance (distance between them).
Table. Pipe height for pitched roof.
Also, the height of the chimney above the ridge can be affected by third-party factors, such as buildings located in the immediate vicinity and tall trees. The presence of such interference forms a zone of wind pressure. In this zone it is almost impossible to create good draft, which will be sufficient for the normal operation of the heating device. To get out of this wind zone, you need to increase the height of the chimney by at least 50 cm.
A similar situation will occur if the heating equipment is located in a low utility room, attached to the house or located in close proximity. Both options are shown in the diagram above.
At independent device smoke exhaust structure, it is necessary to accurately calculate the parameters of the chimney cross-section, its overall height and the level of air circulation - draft, which will support the process of fuel combustion and remove toxic combustion products contained in the smoke into the atmosphere. Such calculations are quite complex, but important to ensure efficient and uninterrupted operation of the heating unit and the safety of residents.
For the normal functioning of heating devices that generate heat by burning fuel, a chimney is required.
The smoke exhaust design provides an influx of oxygen, without which neither gas nor solid or liquid fuel burns. In addition, smoke containing combustion products is removed through the chimney, which is a guarantee of safety. heating system– after all, indoor smoke is deadly to humans. This gas exchange is called draft.
Internal combustion boilers are equipped with coaxial chimneys, which create forced draft, exhausting smoke through one pipe and sucking in fresh air through another. Wood-burning stoves and most household boilers operate on natural draft, which is formed due to the difference in temperature and pressure in the heating device and at the chimney outlet.
The principle of operation of the chimney is simple:
- Gases released during fuel combustion have high temperature, low density and high blood pressure and they feel cramped inside the heating device;
- the smoke is directed to where there are no obstacles for it, that is, it moves in the direction where the pressure is lower, trying to fill a relatively free space; in addition, due to the low density, the gases tend upward;
- if the chimney is constructed correctly, then at the outlet of the pipe the cold air has low pressure and is not an obstacle to the exit of hot smoke;
- since the low pressure area is located above the boiler, the smoke goes along the most convenient path - up the chimney to the street.
Why are calculations needed?
Natural air exchange, in which combustion products are removed into the atmosphere, occurs only if the smoke exhaust structure has the correct shape and size.
If there are obstacles - turns, corners, sections of the smoke channel with low throughput - the smoke can go in a different direction, where nothing will interfere with its spread. If the chimney height is low, the temperature difference will be insufficient to generate draft, or wind pressure will appear, which will prevent the smoke from leaving the chimney, practically driving it back.
Pay attention! Defects in the chimney design will lead to the fact that the draft will either be insufficient for the normal operation of the heating device, or a reverse draft will appear and fuel combustion products will flow into the premises, which can cause poisoning or a fire.
However, about the size of the chimney it cannot be said that the larger the better. A chimney that is too long or wide will increase the cost of chimney construction. A stove or boiler with such a chimney will wear out under conditions of excess draft, burning fuel faster than necessary to warm the premises. The heat obtained during combustion will partially fly away into the chimney, causing heating costs to increase.
Therefore, in order for the heating unit to work efficiently and not break down, it is important to accurately calculate the main parameters of the chimney, which will ensure the optimal level of draft.
How to calculate parameters for a chimney
To erect a smoke exhaust structure, it is necessary to carry out the following calculations:
- total height and height relative to the roof,
- internal diameter of the chimney,
- generated traction.
All these parameters are interconnected. Geometric dimensions the chimney must be calculated to draw up its design, and the amount of draft must be calculated to determine the operability of the chimney in normal weather conditions for the area.
Calculation of internal diameter
The main regulatory requirement for the size of the chimney pipe is its compliance with the outlet pipe of the heating device.
Pay attention! For factory-made heat-generating units, it is enough to select a pipe that has the same cross-section as the outlet pipe, or a slightly larger size. Fulfillment of this condition will provide the necessary throughput chimney.
For wood stove or a domestic boiler that does not have a factory pipe, it is necessary to calculate the parameters of the internal cross-section of the chimney at which the chimney will have the required throughput.
To calculate the diameter of a round pipe or the width and length of a rectangular one, you need to find the area of its internal cross-section. The calculation is carried out in several stages.
First, calculate the volumetric output of flue gases per unit time (Vg):
Vg=Mt*Vt*(tg+273)÷273,
- where Mt is fuel consumption per unit of time, indicated in the accompanying documentation of the heating device,
- Vt – smoke emission coefficient of the fuel used,
- and tg is the temperature of the flue gases at the entrance to the chimney, usually indicated in the documentation for the heating device and equal to 120-150ºС.
The smoke emission coefficient varies for different fuels:
To calculate the cross-sectional area (S), it is enough to divide the resulting value (Vg) by the smoke removal velocity (v):
S=Vг÷v.
The optimal speed is 1-2 m/s. At this speed, soot and condensate do not have time to settle on the walls of the pipe, but the heat remains in the room and is not drawn out into the street.
Pay attention! For accurate calculations, it is important to bring all values to a single time parameter. If the speed is used for calculation in m/s, then the fuel consumption must be converted to kg/s.
For example, if the standard flow rate for a wood-burning stove is 8 kg/h, then when converting it is necessary to divide this value by 3600. Thus, the flow rate will be 0.0022 kg/s.
For a round pipe, the diameter (D) is calculated from the formula for the area of a circle:
D=2√S÷3.14.
For a square, the width (a) is calculated from the formula for the area of a square:
a≥√S.
For a rectangular section, the width (a) and length (b) are selected so that their product is greater than or equal to S:
S≥a*b.
Determining the height of the chimney pipe
To create a sufficient difference in pressure and temperature in the boiler and outside, the minimum required height of the chimney pipe should be 5 meters. But in addition to the minimum value, it is also necessary to calculate the height of the outlet above the roof.
These calculations take into account the type of roof, the location of the chimney in relation to the roof ridge, and the presence of nearby buildings or other tall objects.
Pay attention! When designing a chimney, it is necessary to take into account the location of fasteners. Every 2 m in height, the pipe is fixed to the walls and ceilings; if the chimney rises above the roof by more than 1.2 m, braces are used for additional fixation.
Thrust calculation
To check the functionality of the chimney, which will be built in accordance with the calculations of the height and internal cross-section of the pipe, a draft calculation is additionally carried out.
Draft, that is, the difference in pressure at the inlet and outlet of the chimney (P), is calculated by the formula:
ΔP=hд*(ρв-ρг),
where hd is the height of the chimney above the heating device,
c – air density on the street,
g – smoke density.
The height of the chimney has already been calculated, but the density of air and flue gases will need to be calculated. Depending on weather conditions, the air density indicator changes.
ρв=ρн*273÷tв,
where n=1.29 kg/cub.m – air density under normal conditions,
and tв is the ambient temperature.
This is important! To calculate thrust, the most unfavorable climatic conditions in the region are taken - warm time year. For calculations, temperature in Kelvin is used, so we add 273 to the temperature in degrees Celsius.
The flue gas density is calculated using a similar formula, based on the average temperature of the flue gases in the pipe (tg):
ρg=ρn*273÷tg.
Δtg=(tg+td),
where tg is the temperature of the gases at the outlet of the heating device, indicated in the accompanying documentation,
and td is the temperature of the smoke at the outlet of the pipe.
But the temperature of the flue gases emitted by the chimney into the atmosphere requires additional calculations and depends on the power of the heating device (Q) and the height of the chimney (hd):
td=tg-hd*V*√1000÷Q,
where B is the heat transfer coefficient of the chimney, depending on the material from which the chimney is made and the level of its thermal insulation.
If, after all the calculations, the draft level is in the range of 10-20 Pa, then the designed chimney will cope with its task and ensure uninterrupted operation of the heating unit. Otherwise, you will need to increase the height of the chimney or equip the head with a deflector or smoke exhauster to create artificial draft.
Pay attention! Calculation of thrust requires accuracy, so all intermediate calculations must be recorded so that they can be checked. To eliminate possible errors in calculations, you can use an online calculator or contact a specialist.
Features of calculating chimney parameters for domestic boilers
Factory-made heating appliances usually do not require serious calculations of the chimney - the main parameters are indicated in the accompanying documentation. Distinctive feature Carrying out calculations for household boilers is their simplicity.
- The height is determined by general rules, taking into account the specific location of the smoke exhaust structure relative to the roof of the house and nearby buildings.
- The diameter of the internal cross-section of the chimney is selected in accordance with the power of the heat-generating unit, without taking into account either the type of fuel or the volume of gases emitted. Since factory devices are produced according to manufacturing standards, all parameters have long been calculated and collected in a table.
I welcome my dear reader and bring to your attention an article on how to correctly calculate the chimney in your home.
The main component of comfort in the home is warmth. The owner of the house has to solve heating problems in a private home. For heating, which can be installed independently (except for gas), including chimneys.
To ensure complete removal of flue gases from the room, good draft and prevent draft overturning, it is necessary to comply with the requirements of regulatory documentation.
It is the chimney that ensures the safety of residents. An improperly designed chimney is unable to completely remove toxic combustion products (carbon dioxide and carbon monoxide, sulfur oxides), smoke and soot. There can even be a fire if hot smoke enters the room. Particularly dangerous is the overturning of the draft - turning the movement of smoke in the opposite direction.
Therefore, when installing a chimney, it is necessary to correctly calculate its parameters - they ensure the creation of ideal draft and complete removal harmful substances.
What parameters require calculation?
The main parameters of any vertical chimney are the length and internal (working) cross-section of the smoke exhaust channel. All the information below is equally true for heating appliances using any type of fuel, but the parameters of chimneys for gas boilers are calculated by specialists from a licensed design organization.
Length
The length of the chimney is determined by SP 7.13130.2013. Its length (or height) is measured from the head at the top of the pipe to the grate of the heating boiler. The standard provides for the minimum height of the chimney and the height of the head relative to the roof ridge.
It should be borne in mind that the height determined by calculations is nominal. In fact, the chimney of a modern boiler is located on the side, the boiler outlet is located in the upper part of the unit at a certain height relative to the bottom of the firebox. Therefore, the actual length of the vertical part of the chimney will be slightly less than the calculated data - the distance from the bottom of the firebox to the outlet pipe. This distance can be determined from the unit drawing or measured.
Don't think that extending the chimney is a good thing. The draft is based on the rise of hot exhaust gases, and if the pipe length is too long, the gases cool and the draft decreases. For the same reason, the chimney must be insulated. Even traditional ones need insulation stove chimneys made of red brick. The situation with draft is further aggravated by the fact that the exhaust gases from modern boilers with high efficiency have a low temperature.
With a moderately elongated chimney, the draft increases, and more heat escapes into the atmosphere. Too strong a current of gases will extinguish gas heating units. .
Installing a low chimney in a residential building is completely unacceptable - there is not enough vacuum in it, gases rise at a lower speed, which is why the draft decreases. It is allowed to install low chimneys on free-standing stoves, for example in a barbecue garden, etc.).
Chimney diameter
The diameter of the chimney is no less important for creating draft. When installing modern heating units, the minimum diameter is determined simply - according to the unit’s passport (and the diameter of the outlet pipe). In addition, the regulatory documents provide the minimum cross-sectional dimensions of chimneys corresponding to a unit of a certain power. These data are given in the unit passport.
If for some reason the internal cross-section of the chimney is chosen to be square, then its area should be no less than the circular structure.
When using an existing traditional stove, the chimney is left the same. When installing brick oven and a chimney with your own hands, you need to use the project (preferably not downloaded on the Internet!) and lay out the pipe in exact accordance with the drawing. The same applies to free-standing brick structures.
Calculation methods
Professionals use several methods for calculating chimney parameters. When installing large industrial pipes designers are tasked with very complex calculations. They take into account:
- quantity and composition of flue gases;
- concentration of harmful substances in combustion products;
- pipe material;
- roughness of the internal surface of the hog;
- meteorological situation in the region.
The calculation of large pipes often serves as a topic for a thesis at the institute. Fortunately, the size of chimneys in a private house is smaller, and the power of heating units is also inferior to industrial facilities.
When designing a small pipe for a private house, simpler, although quite accurate, calculations are used. The chimney parameters obtained as a result of calculations ensure complete evacuation of combustion products from the firebox and ensure the safety of residents. Such calculations allow you to avoid mistakes in choosing parameters and when installing the chimney, and to avoid subsequent reworking of structures. Such alterations in an already built and finished house are much more difficult than during the construction process.
When calculating the parameters of chimneys of a private house, exact and approximate (Swedish) calculation methods are used.
Exact method
For heating, ready-made heating or heating and cooking units of industrial production with a certain chimney diameter are often installed. With sauna heaters or oil stoves for garages, ready-made sandwich chimneys of small diameter (100 mm) are usually also installed.
For more accurate calculations, it is necessary to know the characteristics of the fuel and the heating boiler, the rate of rise of flue gases, the amount of fuel burned, the shape of the chimney (sectional shape, number of bends). The calculations are quite complex and require the use of some special data, so let’s consider the calculation for a wood-burning heating unit (fireplace) - they are often made original, relying only on the experience of the stove maker, so calculation of the cross-section is necessary.
Let the chimney have a round cross-section, without turns. Data for calculation:
- the average speed of movement of flue gases along the hog v is taken equal to 2 m/s;
- mass of firewood burned within an hour in the firebox – B=5 kg/h (humidity 25%);
- temperature of exhaust gases – t = 160°С.
Formula for determining the volume of exhaust gases:
where V is the volume of air required for complete combustion of 1 kg of fuel. For firewood with a humidity of 25%, combustion of 1 kg requires 10 m3 of air.
The value obtained as a result of calculations is rounded up to the nearest standard size round pipes(for example 125 mm). A square or rectangular brick chimney is not made smaller than 140x140 mm. The cross-section of the pipe for a traditional stove should be one and a half times larger than the size of the blower.
The volume of combustion products under normal conditions and the temperature of the emanating smoke are standard data. See table:
Swedish calculation method
There is the so-called Swedish calculation method, based on the choice of the height of the chimney depending on the ratio of the furnace floor area and the cross-section of the pipe. Typically the method is used for wood-burning fireplaces. Calculate the ratio:
then the ratio is converted into a percentage and the height of the chimney is selected according to the graph, depending on the resulting size and shape of the hog.
When using the Swedish calculation method, it is possible to calculate the cross-sectional area of the bur channel based on the area of the firebox and the height of the pipe, determined in accordance with SP 7.13130.2013 depending on the height of the roof ridge and the location of the chimney relative to the ridge (see below).
How to correctly calculate chimney parameters
After the calculations, the height of the pipe is checked against the requirements of the regulatory document - it must be no lower than that required by SP 7.13130.2013. In any case, take a larger length value.
The best chimney diameter is the one corresponding to the heating unit’s passport.
Determining the height of the chimney
The height of the chimney is determined at the design stage of the house, depending on the location of the pipe on the roof, the distance to the ridge or to neighboring tall objects (houses, large trees).
Why is this parameter needed?
The draft depends on the height of the chimney, which means the reliability and safety of the heating unit and the safety of the residents of the house. If flue gases are not removed sufficiently, the efficiency of the boiler, stove or fireplace decreases, and there is a danger of poisoning residents with carbon monoxide or carbon monoxide and the danger of fire.
SNiP requirements
In accordance with SP 7.13130.2013, the minimum height of the smoke exhaust pipe from the head to the grate of the heating unit must be at least 5 m. When the pipe exits to flat roof its height must be at least 0.5 m. The maximum length of the horizontal section should not exceed 1 m.
To create the required vacuum when the chimney exits the roof, the height of the pipe head above the roof is also specified (see table below).
There is also the concept of wind support. If the chimney is located near a tall building, a tree, or a low free-standing chimney is located close to the house, then these objects can block the chimney from the wind and reduce draft. To determine whether the pipe does not fall into the wind pressure zone, it is necessary to draw an inclined line through the top point of the house or tree at an angle of 45° to the horizontal. The pipe head should be above this line.
Of the two height values, choose the larger one. If for compliance regulatory requirements If you have to build a very high chimney (much higher than 5 m), you will have to install a smoke exhauster that provides sufficient draft for any length of the pipe.
Table of the height of the chimney above the ridge
The height of the chimney above the ridge must comply with SP 7.13130.2013. The information is given in the table:
The distance from the ridge to the pipe is measured strictly horizontally.
Calculation of the cross section of the smoke channel
SP 7.13130.2013 determines the minimum cross-sections of the chimney duct of a brick or concrete chimney depending on the power of the heating unit:
- up to 3.5 kW – 140×140 mm (Ø no less than 158 mm);
- 3.5-5.2 kW – 140×200 mm (Ø no less than 189 mm);
- 5.2-7.0 kW – 140×270 mm (Ø no less than 219 mm).
When upgrading stoves with unknown power, either leave a smoke pipe of the existing cross-section, or insert a round stainless steel liner of suitable diameter inside. If modern ceramic or sandwich pipes are installed on an old furnace, then the internal diameter of the modules must be no less than the diameter of the liner that would fit when installed in the old pipe. Theoretically, you can look for a design for an old stove, but in most cases this is not possible.
Most often, a stove maker was invited, and he assembled stoves and chimneys of 2-3 mastered standard sizes without any designs or calculations. It is possible to determine all the features of a brick stove by appearance is impossible, and the only guideline when choosing the cross-section of a liner or a new chimney is the cross-section of the hog of a pre-existing chimney (provided that it worked well). If the chimney is, then when replacing or modernizing it, it is better to invite a heating engineer and entrust him with the appropriate calculations.
The diameter of pipes with an internal surface made of smooth ceramic or stainless steel may be slightly smaller (by 30%). The valves of solid fuel furnaces must have an unobstructed hole with a cross-section of at least 15×15 mm.
Internal section of the chimney: which is better?
In a square or rectangular chimney, there will be smoke turbulence in the corners, slightly inhibiting the flow of gases and facilitating the settling of condensation and soot. For the same reason, the corners of the pipe practically do not participate in the flow of gases. In addition, square and rectangular hogs have a larger wall surface area than round hogs - this increases the friction of rising gases against the walls and slows them down more.
Therefore, it is advisable to choose a chimney of a round shape, and modern pipes made of sandwich or ceramics are produced with a circular cross-section, just like pipes made of stainless steel and asbestos cement. The square or rectangular shape is found in older brick chimneys. But for modern boilers, stoves and fireplaces, a round stainless steel liner must be inserted into the brick box.
What to do if the chimney cross-section is square
But it is also possible to use a chimney pipe with a square cross-section - a square or rectangular shape does not have a very significant effect on the draft, and a complete rework of the pipe is complex and expensive. Brick chimneys traditional ovens from time immemorial they had a square or rectangular shape; if necessary, you can use the old pipe even now.
It should be borne in mind that with a modern heating unit brick pipe Without a liner it will not work for a long time - it will be destroyed by chemically aggressive condensate.
The brick pipe should have a cross-section approximately one and a half times larger than that of a round chimney with smooth steel walls. Therefore, installing a liner does not impair the smoke removal ability of a square brick pipe.
What should be the diameter of the chimney for the stove?
Ideally, the diameter of the chimney should be equal to the diameter of the boiler outlet pipe or slightly larger (if the pipe has a non-standard cross-section).
The diameter of the chimney for the stove is determined from the cross-sectional area using the eternal school formula: s=πd2/4. The cross-sectional area should be equal to or slightly less than the cross-sectional area of a square hog, determined by regulatory documents depending on the boiler power.
How does the diameter of the chimney affect its height?
When installing a chimney, there is an immutable dependence: the narrower the cross-section of the hog, the higher the pipe should be. A taller chimney increases the draft and rising speed of the flue gases. But an excessively high one begins to slow down the draft - the hot gases cool down, and they are also slowed down by friction against the walls of the hog, especially a rough brick one.
Calculation of the optimal traction indicator
Draft – reduction of pressure in the chimney. Calculation of draft is needed to check whether the height and cross-section of the chimney hood are correctly determined.
where ∆P is the pressure difference at the inlet and outlet of the pipe, Pa;
a – atmospheric pressure, Pa;
h – pipe height, m;
Ti – average indoor temperature, K;
To – external temperature, K.
For a non-specialist, the draft indicator does not provide sufficient information, so the amount of exhaust smoke that is provided by the presence of draft in the chimney is usually calculated. It is the volume of exhaust gases that shows how much the pipe ensures complete removal of exhaust gases with toxic substances and the safety of residents.
The formula is accurate only for air; for smoke the formula is approximate. But even by an approximate value it is possible to assess whether the chimney ensures the safety of people. The calculated value of the removed gases must be greater than or equal to the calculated value of the outgoing gases (see above).
where Vcalc is the flow of removed gases, m/s3;
S – cross-sectional area of the hog, m2;
C – coefficient taking into account friction losses; usually taken equal to 0.65-0.7;
g – free fall acceleration, 9.807 m/s2;
h – pipe height, m;
Ti – average indoor temperature, K (temperature in Kelvin is 273 units higher than the temperature in °C);
To – external temperature, K.
What causes natural draft in the furnace?
Natural draft in the chimney occurs due to the rise of hot and less dense smoke upward. Dense cold air is sucked into the firebox from below, oxygen reacts with the fuel, the air with smoke heats up in the firebox, rises, and so on.
Calculation example
As an example, I will show the calculation of the volume of air removed for a pipe with an internal diameter of 0.125 m, an area of 0.012266 m2 and a height of, for example, 6 m, an internal temperature of 20 °C and a minimum external temperature of -20 °C.
Conclusion: such a pipe ensures complete removal of flue gases from the firebox. Vcalc significantly exceeds Vout, so to reduce the cost of the chimney it is worth calculating Vcalc for a shorter or narrower pipe and consider the possibility of installing them.
When choosing a chimney of a smaller cross-section and height, it is necessary to calculate the draft at 10 °C - it will be less than at low temperatures, but Vout must be equal to or greater than Vcal at positive temperatures, otherwise complete evacuation of carbon monoxide and carbon monoxide from the room will not be ensured in warm weather.
In this case, Vcalc is significantly larger than Vout, since in the calculations we took a structure with a length of 6 m (and not the optimal 5 m) and rounded the calculated diameter of 0.118 m up.
Online calculators
Ready-made chimney parameters greatly facilitate calculations of chimney parameters. online calculators.
Conclusion
I say goodbye to my dear reader. When you build a new cottage or modernize an old house, pay special attention to the design of the chimney - this is the key to the safety of your family. Receive new articles from the site, bring friends to the site, share interesting and useful information with subscribers on social networks.
Before proceeding with the installation of the chimney, it is necessary to carry out design work, including calculation of the chimney and selection of the material from which the chimney will be made. On an industrial scale, it is more appropriate to entrust all calculations to professionals, but on a private construction scale, you can limit yourself to your own efforts.
Types of chimneys
Chimneys are designed to remove smoke and combustion products harmful to humans from a stove or other heating device outside the room. In any chimney, the draft of the chimney formed during the process of filling the latter with gases must be produced naturally, that is, without the use of additional devices.
Currently, chimneys are manufactured:
- made of brick. For such a chimney, a solid foundation is additionally built. It is advisable to add lime to the composition of the connecting mixture used for bricklaying. This will avoid excessive accumulation of condensate, which can destroy the walls of the product;
- made of sandwich pipes made from two layers of metal, between which insulation is laid. In most cases, stainless steel is used to make sandwich pipes, and basalt is used as insulation;
- from ceramics. Such chimneys are highly durable, but also very expensive. Therefore, they are used for arranging industrial chimneys. Due to its heavy weight, a ceramic chimney, like a brick chimney, requires the manufacture of an additional foundation;
- made of polymer. Such a chimney cannot be exposed to too high temperatures, so it can be used to remove harmful substances from geysers and small boiler houses. A polymer chimney is highly durable, low cost and easy to install.
In some cases, materials for the manufacture of chimneys can be combined. For example, a polymer chimney is lined with bricks.
The choice of material for making a chimney depends on the intended heating device.
Basic parameters for calculating a chimney
To calculate the dimensions of chimneys, consisting of the height of the chimney and the cross-sectional diameter, you need to know the basic parameters of the heating device used. You can find the required values in the accompanying documentation of the purchased equipment.
Calculation of chimney height
The height of the chimney affects the performance of heating devices. The minimum size of the chimney (according to the fundamental documents - SNiPs) is 5 m. If the chimney is smaller than the specified size, then the device will not generate the necessary natural draft. However, it is not recommended to make chimneys too high. In this case, the draft of the chimney is also reduced due to the slow passage and cooling of the smoke.
Calculation of chimney draft is used for the construction of industrial chimneys, and involves a very complex calculation system. For a private small building this figure is negligible.
In private homes, calculating the height of the chimney is based on the following rules:
- the total length of chimneys must be more than 5 m from the base to the final fungus;
- if the chimney goes to flat roof, then it should rise above it by 500 mm;
- if on pitched roof the chimney is led out no further than 1.5 m from the roof ridge, or if there is an additional fence on the roof, then the final section of the pipe must be brought out 500 mm beyond the level of the highest structure;
- if on a pitched roof the chimney is located within 1.5 - 3 m from the roof arch, then the height of the smoke exhaust pipe should be at the same level with it;
- if on a pitched roof the chimney is installed at a distance of more than 3 m from the roof ridge, then the height of the pipe should be calculated so that the horizontal level line of the roof ridge and the line connecting the roof ridge to the chimney form an angle approximately equal to 10º.
It should be noted that the chimney pipe should not be located close to skylights, doors and so on. This may cause sparks to enter the structure, especially when strong wind and lead to fire.
The height of the chimney located on the roof of the building is calculated in compliance with existing fire safety rules.
Calculation of the cross-section of the chimney pipe
- determine the amount of fuel burned in the heating device in 1 hour. In most cases, this parameter is indicated in the specification attached to the heating equipment upon purchase. An independent calculation of this indicator is presented below;
- temperature indicator of the gas located at the entrance to the chimney. The parameter can be found in the additional equipment characteristics. In most cases, it is taken equal to 150ºС – 200ºС;
- the gas passage speed in the chimney is 2 m/s;
- chimney height;
- the natural draft indicator is assumed to be 4 Pa per 1 m of chimney.
Thus, the cross-section of the chimney depends solely on the amount of fuel burned during operation of the equipment.
When calculating the diameter of the chimney, you must use the formula for the area of a circle (where π is the number “pi”):
F=(π*d²)/4
Based on this formula, we get:
d²=4*F/π
To calculate the cross-sectional area of the pipe, it is necessary to determine the volume of gases located at the entrance to the chimney pipe. This parameter depends on the amount of fuel consumed and is calculated by the formula:
Vgas=B*Vfuel*(1+t/273)/3600, Where
- B – the amount of fuel in kilograms that is burned in 1 hour (kg/hour);
- Vfuel is a tabular coefficient depending on the type of fuel (you can find it in GOST No. 2127 or in the table below);
- t is the gas temperature recorded at the outlet of the pipe (also shown in the table).
The cross-sectional area (F) is found as the ratio of gas V to the speed of gas movement in the pipe (W), that is
F=Vgas/W
d²=(4*Vgas)/π*W
For example, heating a building for an hour requires 10 kg of firewood, the humidity of which is approximately 25%. The correction factor for this type of fuel, based on the above table, is equal to 10. The gas temperature recorded at the exit to the chimney pipe is assumed to be 150ºС.
Then the calculation of the boiler room chimney will be as follows:
- 1+150/273=1,55
- Vgas=10*10*1.55/3600=0.043
- d²=(4*0/043)/3.14*2=0.027
- d=0.165
That is, when certain conditions the diameter of the chimney pipe must be at least 165 mm.
In order not to complicate the installation of a chimney with complex calculations, you can use the standards developed by specialists:
- for heating equipment whose power is less than 3.5 kW, a chimney with dimensions of at least 0.14 * 0.14 m is suitable;
- if the declared power of the heating boiler is within the range of 3.5 kW - 5 kW, then the parameters of the chimney must be at least 0.14 * 0.20 m;
- if appliances with a power from 5 kW to 7 kW are used to heat the room, then the cross-section of the chimney pipe used should not be less than 0.14 * 0.27 m.
If the power of the heating device is known during the construction of the chimney, then the data calculated by experts can be used to determine the cross-section of the pipe. If the power of the heating equipment is unknown, then it is necessary to carry out calculations according to the specified scheme.
Thus, when choosing a material for making a chimney, as well as when calculating the parameters necessary for its construction, one must rely on the power of the heating device. The greater its power, the larger and more reliable the chimney should be. If the parameters of the device are unknown, and it is not possible to calculate the required values on your own, then you must seek the help of qualified specialists. Any error in the calculations can lead to the chimney not working or to incomplete removal of harmful substances from the living space.
Thrust is a separate aerodynamic process in which combustion products move from the zone with increased level pressure to an area with a lower indicator. In this regard, the chimney draft is the most important parameter when calculating the entire heating system of a private house. If the calculation is incorrect, reverse draft occurs, in which combustion products are not removed, but enter into living space. What factors determine the level of traction? How to correctly calculate the parameter? In what ways can you increase cravings if there is insufficiency? Read on.
What is traction
The process of formation of natural draft in a chimney can be described from the point of view of a science such as physics as follows:
- gases heated by the combustion of wood or their analogues in a stove (boiler) have a temperature of about 1000ºC;
- according to the laws of physics, heated air always rises;
- rising through a special pipe (at a speed of approximately 2 m/s), the gases create an area of low pressure;
- the pressure is stabilized due to the influx of fresh air, which enters the furnace (boiler) through special blowers, grates and other similar equipment.
Based on the physical processes of the formation of natural draft in a pipe, it is possible to determine a list of factors influencing its value. These include:
- chimney length. In accordance with current standards, the length of the chimney duct should not be less than 5 m. The optimal length can be determined based on the location of the chimney on the roof;
- the material from which it is made. To make the passage of gases as easy as possible, it is recommended to make the chimney from materials with a smooth inner surface. And if this rule is not followed, then resort to periodic cleaning of the channel to remove soot and other deposits;
- presence/absence of insulation. If the smoke channel is not insulated from the outside, then when the gases cool, a large amount of condensate will form, which negatively affects the draft level;
Chimney insulation can only be done with non-combustible materials.
- pipe section. The level of draft depends on the correctly selected diameter of the chimney.
The draft in the smoke exhaust duct is also determined by natural factors:
- temperature and humidity inside the living space;
- weather conditions (wind, precipitation, low temperature, etc.);
- number of residents;
- frequency of ventilation and so on.
How to check the presence/absence of traction? You can check the draft level in the pipe using a lit match, candle or paper.
Thrust calculation
So, draft calculation is the calculation of the chimney cross-section for gas boiler, fireplace, stove or other heating equipment. How to calculate the cross section? To do this you need to define:
- the volume of gas that will pass through the chimney in 1 hour;
- cross-sectional area of the chimney;
- section diameter.
Calculation of gas volume
To calculate the volume of gas passing through the smoke channel, the following formula is used:
V = B x V1 x (1 + T/273) /3600 , Where
B – mass of fuel burned per 1 hour of operation of heating equipment;
V1 – correction factor, which depends on the type of fuel used for heating;
T – gas temperature determined at the exit from the chimney.
Indicators V1 and T can be obtained from the table available in GOST 2127 - 47.
Calculation of cross-sectional area
After determining the volume of gases passing through the smoke channel, you can calculate the cross-sectional size of the pipe:
S = V/W, Where
V – volume calculated earlier;
W is the speed of passage of gases through the smoke channel (this value is constant and equal to 2 m/s).
Diameter determination
The next step is to directly determine the diameter of the chimney. The following formula is used for this:
D = √4 * S/π, Where
S – cross-sectional area of the smoke exhaust channel;
π is a constant equal to 3.14.
Example
For example, let’s carry out the calculation using the specified formulas with the following parameters:
- 10 kg of wood is burned every hour in the stove installed in the bathhouse;
- the temperature of the gases at the outlet of the pipe is 130ºС.
Let's calculate the volume of gases:
V = 10x10x(1+130/273)/3600 = 0.041 (m³/hour)
Let's determine the cross-section of the smoke exhaust pipe:
S = 0.041/02 = 0.0205 (m²)
Let's find the most suitable pipe diameter according to the given parameters:
D = √ 4 * 0.0205 / 3.14 = 0.162 (m)
This means that for the stove used in the example, it is enough to install a chimney with a diameter of 165 - 170 mm.
How to make calculations and install a chimney yourself, watch the video.
How to increase traction in an already installed channel
The above calculations make it possible to build a chimney with optimal parameters to obtain a normal level of natural draft. What to do if there is backdraft? Is it possible to increase the indicator and how to increase traction on your own? There are several ways:
- cleaning the smoke exhaust duct. When soot and other types of deposits settle, the working diameter of the pipe decreases significantly, which leads to a decrease in traction. You can clean it:
- using a metal brush - brush. To do this, you need to tie the ruff to a strong rope and add a weight to the structure. Cleaning is done from the roof;
- special means such as a “Chimney Sweep” log;
When using special means You must strictly follow the instructions provided on the product packaging or special insert.
- folk remedies. For example, the peel from raw potatoes, aspen firewood and so on;
- elimination of design defects made during the construction of the pipe (elimination of cracks, lengthening or shortening, elimination of unnecessary bends, insulation, and so on);
- installation of additional equipment.
The following can be used as additional equipment to enhance traction:
- regulator The device is installed on the pipe and by opening/closing the damper allows you to adjust the traction force in ;
- deflector-amplifier. The increase in thrust occurs due to the redirection of air flows formed due to an increase in the diameter of the device;
- vane. The draft stabilizer, like the deflector, is installed at the end of the chimney and serves to enhance draft due to the streamlining of air flows. In addition, the weather vane helps stabilize the draft level during strong gusts of wind;
- rotary turbine. When exposed to wind, the device begins to rotate, creating an area of low pressure around itself, which increases traction.
Unlike other devices, a rotary turbine performs its functions exclusively in the presence of wind. In addition, the device does not protect the chimney from clogging with leaves, small birds and other pollutants.
All additional devices require periodic maintenance: cleaning in the warm season and clearing of ice in winter. If cleaning is not carried out in a timely manner, the performance of the device will be reduced to a minimum and the required effect will not be achieved.